Hindawi Publishing CorporationISRN EcologyVolume 2013 Article ID 948915 6 pageshttpdxdoiorg1011552013948915

Research ArticleFactors Influencing Aggression Levels in Root Vole Populationsunder the Effect of Food Supply and Predation

Haiyan Nie1 Mingcan Yao1 and Jike Liu2

1 College of Life Science and Technology Central South University of Forestry and Technology Changsha 410004 China2 College of Life Sciences Zhejiang University Hang Zhou Zhejiang 310029 China

Correspondence should be addressed to Haiyan Nie niehaiyan126com

Received 27 June 2013 Accepted 13 August 2013

Academic Editors C Jarnevich M Perez-Fernandez and D Pimentel

Copyright copy 2013 Haiyan Nie et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Which factor determines animal aggressivity Wynn-Edwards proposed the hypothesis that aggressive level increases withpopulation density Adams andMesterton-Gibbons proposed the hypothesis that bodyweight is an indicator of animal aggressivityhowever Smith and Price hypothesized that aggression level varied with external conditions that is the population lived in themost unfavorable environment demonstrated the highest average aggression level and the population that lived under the mostfavorable external conditions demonstrated the lowest average aggression level In this paper we tested these three hypotheses bymanipulating enclosed root vole (Microtus oeconomus) populations under different food and predation treatments and observedtheir aggressive behavior Aggressive behavior was measured by matching mice in a neutral arena The experimental resultssupported Smith and Pricersquos hypothesis and Adams and Mesterton-Gibbonsrsquos hypothesis however they did not support Wynn-Edwardsrsquo hypothesis Moreover we found that reproductively active individuals were more aggressive We concluded that thegrowth of population density did not cause or otherwise bring about increased aggressive behavior of root voles but the externalfactors (predation and food supply) and physical factors (body weight and reproductive condition) were significantly correlatedwith aggression levels in a root vole population

1 Introduction

Wynn-Edwardsrsquo hypothesis proposed that animals adjusttheir population density to available resources throughsocial behavior and that aggressive behavior increases withpopulation density [1] Some subsequent field experimentssupported this hypothesis [2ndash8] However Vale et al demon-strated another aspect of the relationship between agonisticbehavior and population density [9] They showed that withdifferent genetic strains of mice at equal densities one strainwas aggressive and the other was not Lidicker examined pat-terns of wounding in population of California vole (Microtuscalifornicus Peale 1848) and concluded that season wasmoreimportant than density in determining levels of aggression[10]

Aggression levels of fluctuating arvicoline populationshave been assessed by observation of agonistic behaviorduring dyadic encounters in neutral arenas for five species

meadow vole Microtus pennsylvanicus (Ord 1815) [5 11ndash13] prairie vole Microtus ochrogaster (Wagner 1842) [1213] beach vole Microtus breweri (Baird 1858) [14] red-backed vole Clethrionomys gapperi (Vigors 1830) [15] andTownsendrsquos vole Microtus townsendii (Bachman 1839) [16]Only Krebs found evidence that aggression level increasedwith population density [5] In general the relationshipbetween agonistic behavior and population density remainsuncertain

Smith and Price proposed the hypothesis that aggressionlevel varied with external conditions that is the populationlived in the most unfavorable environment demonstratedthe highest average aggression level and the populationthat lived under the most favorable external conditionsdemonstrated the lowest average aggression level [17] Smithand Price analyzed population aggression by ESS gamemodel which predicts that when the value of the resource isgreater winning a ldquowarrdquo is more important for foraging food

2 ISRN Ecology

defending territory protecting offspring and so forth andthe proportion of ldquohawksrdquo in the population is higher [17]

However Adams and Mesterton-Gibbons proposed thebody weight assessment hypothesis that is body weight isan indicator of strength [18] Each animal compares its ownstrength to that of its opponent and withdraws when it judgesitself to be the probable loser For weaker opponents strongaggressiveness may mean wasting energy needlessly againstmuch stronger opponents who would win in any case

During the course of studying the effects of food supplypredation and the interaction between them on the popu-lation dynamics of root voles Microtus oeconomus (Pallas1776) by adopting factorial experiments in field enclosures[19] we obtained data of aggressive behavior from rootvole populations In this paper we analyzed the relationshipbetween aggression level and population density weight andexternal conditions in root vole populations to test the abovethree hypotheses

2 Materials and Methods

21 Study Area The fieldwork was conducted from Mayto October in 1990 at Haibei Alpine Meadow EcosystemResearch Station of Chinese Academy of Sciences Theresearch station is located at northeast of Tibet in a largevalley oriented NW-SE surrounded on all sides by the QilianMountains with N latitude 37∘291015840 sim 37∘451015840 and E longitude101∘

121015840

sim 101∘

231015840 The average altitude of mountain area is

4000m a s l and 2900sim3500m in the valley The DatongRiver passes the south of the areaThe landscape is character-ized by large mountain ranges with steep valleys and gorgesinterspersed with relatively level and wide intermountaingrassland basins The natural conditions vegetation and soilstructure of the study area has been reported elsewhere [20]

The study site was located at an old field of Elymus nutansmeadow a secondary vegetation type after the original onehad been destroyed The majority of plants were Elymusnutans Poa spThalictrum alpinum Kobresia humilis Poten-tilla fruticosa and so forth The soil was loose moist andfertile with a dense and high plant leaf layer which wasthe natural habitat for root voles The dominant plant Enutans was soft and nutritive and preferred strongly by rootvoles [21] Besides root voles other rodents inhabited the Enutans meadows such as zokor Myospalax baileyi (Thomas1911) pikas Ochotona cansus (Lyon 1907) and O curzoniae(Hodgson 1858)

The avian and mammalian predators observed in thestudy area were the falcon Falco tinnunculus (Linnaeus 1758)buzzard Buteo hemilasius (Temminck and Schlegel 1844)weaselsMustela altaica (Pallas 1811)M eversmanni (Lesson1827) the wolf Canis lupus (Linnaeus 1758) foxes Vulpesvulpes (Linnaeus 1758) and V ferrilatus (Hodgson 1842)The dominant species among them were the buzzard thefalcon and the weasels

We constructed the enclosures with 2 times 1m120575 05 gal-vanized steel panels that extended 05m below and 1mabove ground Four treatments (2 times 2 factorial design) withtwo replicates were designed as follows no predator food

supplemented (minusP +F) predator access food supplemented(+P +F) no-predator nonsupplemented (minusP minusF) the con-trol predator access nonsupplemented (+PminusF)Within eachenclosure 42 trapping stations were staked in a 5 times 7m arrayA wooden cage trap was placed at each trapping station and3 trapping cages were placed in each removal areaThere werea total of 45 cages in every enclosure The minusP +F and minusP minusFenclosures were covered by fishing netting with a 3 times 3 cmgrid to prevent access by avian predators and the roof of thenet was kept in place with a Φ10 times 250 cm pillar The +P +Fand +P minusF enclosures were not covered with netting and hada series of low panels (030m high) every 10m except on sidesshared with no-predator enclosures Thus predators couldeasily enter these enclosures Live traps were set all alongoutside the enclosures with low panels to detect escapees

Supplemental food was granulated rabbit chow of TK-10 type (made in Shanghai Feed Processing Plant) a highquality food onwhichmicrotine animals grow and reproducewell [21] In +P +F and minusP +F enclosures food were keptin 500mL glass jars which were placed near every odd-numbered trap station in every row There were 22 food jarsin every enclosure and foodwas replenished weekly to ensurefreshness and availability

22 PopulationDensity After removal of the original residentroot voles and nontarget species five pairs of root voles froma laboratory colony were randomly assigned as populationfounders in each enclosure which represented a moderatedensity compared to the natural population around the studyarea The founder voles were nonsiblings of 3ndash5 months oldand born in a laboratory The animals had 1-2 weeks tobecome accustomed to the enclosures before trapping began[19]

There were three trapping days every two weeks in atrapping session At dusk the day before a trapping sessionthe traps were set open bedded with clean cotton and baitedwith cracked corn We examined the cages once every twohours each day during every trapping session All cageswere covered with hardboards of 2 cm thickness to reducetrap mortality caused by high temperature and were closedbetween trapping sessions None of the animals died in thetraps during the trapping sessions

Upon first capture each animal was marked with anumbered aluminum Style 1005-1s Monel ear tag producedby the National Band amp Tag Company The whole markingprocedure followed the Act on Animal Experimentationand Management released by the Ministry of Science andTechnology of the Peoplersquos Republic of China in 1988 We didnot punch holes in the ears of animals but only clipped thetags onto their ears The whole marking procedure did notappear to cause any distress or discomfort to the animalsTherecaptured individuals were released at the point of captureafter the following data were recorded tag number sexweight station number reproductive condition and generalcondition Male reproductive condition could be determinedby the size of scrotum and the location of testis and theindividuals with enlarged scrotum and descended testis werereproductively active Whether a female was pregnant or

ISRN Ecology 3

lactating could be judged by the width of vaginal opening andthe prominence of the nipples [22]

We classified root voles into two age classes based on size[23] males gt25 g and females gt22 g were adults others werejuveniles Population densities for every enclosure withineach trapping session were estimated separately using thejackknife estimator for model Mh (including heterogeneityin capture rates) in the program CAPTURE [24]

23 Aggressive Behavior Testing Aggression of adult rodent isstrongly influenced by food quality during early developmen-tal periods [25] In the current study population founderswere fed with food of equal quality during infancy and hadnever been utilized for aggressive behavior tests Aggressivebehavior tests were conducted (1) 5 weeks after the foundershad been placed in the enclosures (2) when the number ofindividuals in each enclosure had reached a fair size and(3) when there were adequate numbers of individuals forbehavioral observation Individuals selected for aggressivebehavior testing were born in the enclosure

It is difficult to accurately test the aggression levels ofsmall mammals directly in the field [26] so our aggressiontests were conducted in the behavior laboratory near theenclosure The apparatus for aggressive behavior testing wasa glass aquarium partitioned by a movable wooden board atthe center The box was cushioned with a 1 cm layer of sawdust and crushed corn The box was placed in a dark roomin the laboratory A 40w fluorescent lamp was suspendedabout 1m above the boxThe observer watched and recordedaggression behavior through a hole in a green curtain hung50 cm distance from the box

Every individual was observed nomore than twice withina trapping period and was tested only against others fromthe same enclosureThe encounters were different every timeThe period between two observations was 30 minutes Beforelifting the partition board 3minutes were given for root volesto acclimatize to the new space The board was then raisedand the voles watched for ten minutes The root voles wereput back into cages after observation and fed with adequatecrushed corns and water and then taken back to the siteswhere they had been trapped within 1-2 hours The glass boxwas cleaned with soapwater to eliminate odor gathered in thebox

The following behaviors were recorded following theclassification standard of Hofmann et al [12] as follows(1) threatmdashraising forelegs and stretching head shakingforelegs teeth baring and screaming (2) uprightmdashhind legsstanding straight body extending and the two individualsfacing each other closely during mutual upright (3) lungemdashone individual stretching head toward the other leapingfrom ground attacking and biting each other (4) boxingmdashthe upright individual striking head and shoulders of theother one (5) wrestlemdashthis behavior occurred rapidly and itwas difficult to tell the launcher (6) chasemdashone individualpursuing the other often ended with one jumping on to theotherrsquos rear violently (7) retreatmdashone individual fleeing awayfrom the approaching or attacking individual (8) approachmdashone individual moving toward the other to a distance shorter

than 5 cm Agonistic behavior 1 to 7 occurred frequentlywhen both individuals werewithin approach distance amongwhich threat lunge and chase were initiative and nondefen-sive or obviously aggressive Therefore the total number ofthreat lunge and chase was calculated as to stand for therelative aggression level of dyadic encounters andwas definedas an aggression count [27]

24 Game Model We used ESS (Evolutionary Stable Strat-egy) game models [28] to analyze the aggressive behaviorof root voles An ESS is a state of game dynamics wherein a very large population of competitors another mutantstrategy cannot successfully enter the population to disturbthe existing dynamic which in itself is population mixdependent In the ESS model a root vole population consistsof hawks and doves Let 119901 be the proportion of hawks in apopulation and (1 minus 119901) the proportion of doves If an animalwon a contest the benefit is 119881 units If an animal incurreda serious injury the cost is 119882 units And if it is involvedin a long display the cost is 119879 units According to Parker119901 = 119881119882 or in other words the greater the value of theresource the higher the proportion of hawks Conversely thegreater the cost of injury the greater the proportion of thedove [28]

Among the agonistic behavior sequences recorded in theroot vole population lunge and chasewere initiative and non-defensive or obviously aggressive so the individual whichexhibited these two behaviours in a dyadic encounter wasdefined as ldquoa hawkrdquo otherwise ldquoa doverdquo

25 Statistical Analysis Linear correlation coefficient be-tween body weight and aggression level was calculated inorder to determine the relation between individual sizeand its aggression level correlation coefficient betweenaggression counts and population density was calculated bySpearman rank correlation analysis in order to determine therelation between aggression level and population density

Kruskal-Wallis 119867-test was used to analyze the differencebetween average aggression levels under different treatmentsand also used to analyze aggression difference betweenmales and females under different treatments Differencebetween average aggression counts of reproductive individu-als and non-reproductive individuals was analyzed by 119905-testIndependent effects and interaction of food and predationon aggressiveness of root vole populations under differenttreatment were analyzed by two-way ANOVAs

3 Results

31 Aggression Level and Population Density Wemarked andrecaptured root vole populations under four treatments for20 trapping sessions in total [19] There was no significantlinear regression between aggression counts and populationdensity (Figure 1) According to the results of a Spearmanrank correlation analysis correlation coefficients betweenaggression counts and population density under all fourtreatments were not significant (minusP +F 119903 = minus0237 119899 = 18119875 = 0328 +P +F 119903 = minus0329 119899 = 20 119875 = 0151

4 ISRN Ecology

Table 1119867-test of Kruskal-Wallis for aggression levels of root vole populations under different treatments

Treatment+P minusF minusP +F minusP minusF +P minusF

Experimentdata Rank Experiment

data Rank Experimentdata Rank Experiment

data Rank

Aggressionaccounts

300 400 3200 4100 6500 4900 3600 45002500 3750 5300 4700 6000 4800 3500 43502000 3050 1300 1950 7800 5100 7400 5000200 300 2500 3750 2500 3750 3300 4200500 600 1100 1600 2400 3500 3700 4600100 200 3500 4350 2100 32001300 1950 1000 1400 2200 3300800 1050 1400 2150 2300 3400700 850 1600 2550 2600 4000600 700 1600 25502500 3750 700 8509000 5200 1500 23501000 1400 1200 1750000 100 1000 14001200 1750 1800 2800400 500 1900 2900800 1050 2000 3050900 1200 1700 27001500 23501400 2150

119877119894

= 32300 46900 35950 22000119877119894is the sum of sample rank under i treatmentminusP +F no predator food-supplemented enclosures+P +F predator access food-supplemented enclosuresminusP minusF no predator nonsupplemented enclosures+P minusF predator access nonsupplemented enclosures the control

minusP minusF 119903 = minus0183 119899 = 9 119875 = 0606 +P minusF 119903 = minus0112119899 = 5 119875 = 0823)

32 Food Predation and Aggression Level The differencebetween average aggression levels under different treatmentswas significant (119867 = 2412 df = 3 119875 lt 0001) according tothe Kruskal-Wallis119867-test (Table 1)

33 Sex and Aggression Level After observing 35 dyadicencounters between males and females we found extremelyweak aggression between different sexes which could beneglected in the aggressive behavior analysis Therefore onlythe aggressive behavior between the same sex dyads wasanalyzed in this paper

The difference of aggression level between malesand females under different treatments was inconsistent(Figure 2) The aggression level of males was significantlyhigher than that of females in minusP minusF and +P minusF population(the control) but the aggression level of females wassignificantly higher than that of males in +P +F accordingto 119867-test (+P +F 119867 = 5023 df = 1 119875 lt 0025 minusP minusF119867 = 14438 df = 1 119875 lt 0005 +P minusF 119867 = 6818 df = 1119875 lt 001) However the difference of aggression levelbetween males and females was not significant in minusP +Fpopulation (119867 = 2029 df = 1 119875 lt 025)

0

10

20

30

40

50

60

70

80

0 10 20 30Population density

Agg

ress

ion

coun

t

+F +P+F minusP

minusF minusPminusF +P

Figure 1 Aggression level and population density

34 Reproductive Activity and Aggression Level Individualsthat were reproductively active showed higher incidencesof aggressive acts than nonreproductively active individuals

ISRN Ecology 5

0

5

10

15

20

25

30

35

40

45

50Av

erag

e agg

ress

ion

coun

ts

Treatments

+F+

Pminus

+F+

Pminus

+Fminus

Pminus

+Fminus

Pminus

minusFminus

Pminus

minusFminus

Pminus

minusF+

Pminus

minusF+

Pminus

Figure 2 Difference of aggression level between males and females

Reproductively active voles especially lactating females wereevidently more aggressive usually attacking the opponentto escape in a hurry The difference between the aggres-sion counts of reproductive individuals and those of non-reproductive individuals was found to be significant (119905 =11885 df = 50 119875 = 00026)

35 Body Weight and Aggression Level Heavy voles showedhigh incidences of acts of aggression The smaller oneinvariably gives up first The heavier individual in a dyadicencounter usually became the dominant and the smallerone became the subordinate Body weight was significantlypositively correlated with aggression counts (119903 = 0375737119899 = 52 119875 = 001)

36 Aggressive Behavior Strategies under Different TreatmentsThe aggressive behavior pattern of root voles under differenttreatments (Table 2) was consistent with the prediction froman ESS hawk-dove model that is the proportion of ldquohawksrdquoin root vole populations under unfavorable conditions (+PminusF)wasmuch higher than that of root vole populations underfavorable conditions (minusP +F) and the proportions of ldquohawksrdquowere at intermediate level under mediate environments (minusPminusF +P +F)The aggressiveness of root vole populations underdifferent treatment was different significantly (119865 = 79909119899 = 20 119875 = 001)

4 Discussion

Theexperimental results supported Smith andPricersquos hypoth-esis and Adams and Mesterton-Gibbonsrsquos hypothesis how-ever did not support Wynn-Edwardsrsquo hypothesis Moreoverwe found that reproductively active individuals were moreaggressive

Table 2 Proportion of hawks in the populations of root voles underdifferent treatments

Treatments minusP +F +P +F minusP minusF +P minusFProportion of hawks 017 018 033 040

In our factorial experiment bodyweight was significantlypositively correlatedwith aggression counts with heavier andreproductively active root voles showing significantly higherincidences of aggressive acts (119903 = 0376 119899 = 52 119875 = 001)which supportedAdams andMesterton-Gibbonsrsquo hypothesis

The conventional agonistic behavior of root voles wasritualized noninjurious behavior which was consistent withthe conclusion of Smith and Price [17] and Lendrem [29]An ESS is always a mix of hawks and doves [17] ESSgame model predicts that when the value of the resource isgreater the proportion of ldquohawksrdquo in the population is higherOur experimental results supported this hypothesis Underthe most unfavorable conditions (+P minusF) when resourcesare most valuable winning a ldquowarrdquo is most important forforaging food defending territory protecting offspring andso forth and hence the highest proportion of ldquohawksrdquo inthe population on the other hand under the most favorableconditions (minusP +F) the lowest proportion of ldquohawksrdquo in thepopulation (Table 2) and the difference were significant

The difference in the aggression counts between repro-ductively active individuals and those of non-reproductivelyactive individuals was found to be significant (119905 = 11885df = 50 119875 = 00026) which was consistent with the resultsof Turner and Iverson [11] Fairbairn [30] and Reich etal [14] that is hormone level in an animal influencing itsaggressiveness

The experimental results showed that the aggression levelof dispersers was significantly lower than that of residentswhich agreed with the results of Fairbairn [30] but differedfrom that observed in the beach vole (Microtus breweri)[14] a species related to the root vole In a wild beachvole population dispersers and residents exhibited similaraggressive behavior patterns The cause of this phenomenonmight be that beach vole dispersers were a random sample ofthe resident population in terms of sex ratio and age structurewhereas in our study and the findings of Fairbairn [30]almost all dispersers were subadults which were subordinateindividuals in the population Subordinate males were lessaggressive and they disperse in response to social pressureduring the breeding season [30]

To summarize external environmental factors such aspredation and food treatment as well as internal conditionssuch as body weight and reproductive activity were moreimportant than density and gender in determining aggressionlevels of root vole populations

Acknowledgments

The authors wish to express their sincere appreciation to DrJianping Su Dr Meicai Wei Dr Wei Liu and Mr YumingLi for their help in the course of field experiment anddata analysis and to Dr Ben Bravery for his help in paper

6 ISRN Ecology

editingThe research is completed under the financial supportof the National Natural Science Foundation no 3870345and 30270242 and the Central South Forestry UniversityFoundation for the Introduction of High-Level-Talents no101-0630

References

[1] V C Wynn-Edwards Animal Dispersion in Relation To SocialBehavior Oliver and Boyd London UK 1962

[2] R M F Sadleir ldquoThe relationship between agonistic behaviorand population changes in the deermouse Peromyscus manic-ulatus (Wagner)rdquo Journal of Animal Ecology vol 34 no 2 pp331ndash352 1965

[3] M C Healey ldquoAggression and self-regulation of population sizein deer micerdquo Ecology vol 48 no 3 pp 377ndash392 1967

[4] C J Krebs B L Keller andRH Tamarin ldquoMicrotuspopulationbiology demographic changes in fluctuating populations ofMochrogaster and M pennsylvanicusrdquo Ecology vol 50 no 4 pp587ndash607 1969

[5] C J Krebs ldquoMicrotus population biology behavioral changesassociated with the population cycle in M ochrogaster and Mpennsylvanicusrdquo Ecology vol 51 no 1 pp 34ndash52 1970

[6] B L Keller and C J Krebs ldquoMicrotus population biologyIII Reproductive changes in fluctuating populations of Mochrogaster and M pennsylvanicus in southern Indiana 1965ndash67rdquo Ecological Monographs vol 40 no 3 pp 263ndash294 1970

[7] W H Conley Behavior demography and competition in Micro-tus longicaudus and M mexicanus [PhD thesis] Texas TechUniversity Lubbock Tex USA 1971

[8] W H Conley ldquoCompetition between Microtus a behavioralhypothesisrdquo Ecology vol 57 no 2 pp 224ndash237 1976

[9] J R Vale C A Vale and J P Harley ldquoInteraction of genotypeand population number with regard to aggressive behaviorsocial grooming and adrenal and gonadal weight in male micerdquoCommunications in Behavioral Biology vol 6 no 2 pp 209ndash2211971

[10] W Z Jr Lidicker ldquoRegulation of numbers in small mammalpopulationsmdashHistorical reflections and a synthesisrdquo in Popu-lations of Small Mammals Under Natural Conditions vol 5pp 122ndash141 PymatunIng Laboratory of Ecology University ofPittsburgh Pittsburgh Pa USA 1978

[11] B N Turner and S L Iverson ldquoThe annual cycle of aggressionin male Microtus pennsylvanicus and its relation to populationparametersrdquo Ecology vol 54 no 5 pp 967ndash981 1973

[12] J E Hofmann L L Getz and B J Klatt ldquoLevels of maleaggressiveness in fluctuating population ofMicrotus ochrogasterandMicrotus pennsylvanicusrdquo Canadian Journal of Zoology vol60 no 5 pp 898ndash912 1982

[13] J E Hofmann and L L Getz ldquoLevels of female aggressivenessin fluctuating populations of Microtus ochrogaster and Mpennsylvanicusrdquo ActaTheriologica Sinica vol 24 no 2 pp 125ndash131 2004

[14] L M Reich K M Wood B E Rothstein and R H TamarinldquoAggressive behaviour of male Microtus breweri and its demo-graphic implicationsrdquo Animal Behaviour vol 30 no 1 pp 117ndash122 1982

[15] S Mihok ldquoChittyrsquos hypothesis and behavior in subarctic red-back volesClethrionomys gapperirdquoOikos vol 36 no 3 pp 281ndash295 1981

[16] M J Taitt and C J Krebs ldquoManipulation of female behaviourin field populations of Microtus townsendiirdquo Journal of AnimalEcology vol 51 no 2 pp 681ndash690 1982

[17] J M Smith and G R Price ldquoThe logic of animal conflictrdquoNature vol 246 no 5427 pp 15ndash18 1973

[18] E S Adams and M Mesterton-Gibbons ldquoAnimal contests asevolutionary gamesrdquo American Scientist vol 86 no 4 pp 334ndash341 1998

[19] H Nie and J Liu ldquoRegulation of root vole population dynamicsby food supply and predation a two-factor experimentrdquo Oikosvol 109 no 2 pp 387ndash395 2005

[20] W Xia Alpine Meadow Ecosystem Gansu Peoplersquos PublishingHouse Lanzhou China 1982

[21] J Liu X Wang and W Liu ldquoStudies on the nutritional ecologyof small herbivorous mammals patterns of food selection andresource utilization for root voles and Gansu pikasrdquo in AlpineMeadow Ecosystem Fasc 3 pp 111ndash124 Science Press BeijingChina 1991 (Chinese)

[22] L J Liu S Su L W Liu W X Wang N H Nie and L YLi ldquoField experimental studies on the multifactorial hypothesisof population system regulation for small rodents an analysisof effects of food availability and predation on populationdynamics of root volerdquo Acta Theriologica Sinica vol 14 no 2pp 117ndash129 1994 (Chinese)

[23] J Liang J Zeng ZWang and YHan ldquoStudy on the growth anddevelopment of root volesrdquo Acta Plateau Biology Sinica vol 1no 2 pp 195ndash206 1982

[24] D L Otis K P Burnham G C White and D R AndersonldquoStatistical inference from capture data on closed animal popu-lationsrdquoWildlife Monographs vol 62 no 1 pp 1ndash135 1978

[25] J L Smart ldquoUndernutrition and aggressionrdquo in Multidisci-plinary Approaches To Aggression Research pp 179ndash191 Ams-terdam Elsevier North-Holland Biomedical Press AmsterdamThe Netherlands 1981

[26] M J Taitt and C J Krebs ldquoPopulation dynamics and cyclesrdquoin Biology of New World Microtus pp 567ndash620 The AmericanSociety of Mammalogists Shippensburg Pa USA 1985

[27] E ADesy GO Batzli and J Liu ldquoEffects of food and predationon behaviour of prairie voles a field experimentrdquoOikos vol 58no 2 pp 159ndash168 1990

[28] G A Parker ldquoEvolutionarily Stable Strategiesrdquo in BehaviouralEcology An Evolutionary Approach pp 30ndash61 Sinauer Asso-ciates Sunderland Mass USA 1984

[29] D W LendremModeling in Behavior Ecology An IntroductoryText Columbia University Press New York NY USA 1984

[30] D J Fairbairn ldquoBehaviour of dispersing deer mice (Peromyscusmaniculatus)rdquo Behavioral Ecology and Sociobiology vol 3 no 3pp 265ndash282 1978

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2 ISRN Ecology

defending territory protecting offspring and so forth andthe proportion of ldquohawksrdquo in the population is higher [17]

However Adams and Mesterton-Gibbons proposed thebody weight assessment hypothesis that is body weight isan indicator of strength [18] Each animal compares its ownstrength to that of its opponent and withdraws when it judgesitself to be the probable loser For weaker opponents strongaggressiveness may mean wasting energy needlessly againstmuch stronger opponents who would win in any case

During the course of studying the effects of food supplypredation and the interaction between them on the popu-lation dynamics of root voles Microtus oeconomus (Pallas1776) by adopting factorial experiments in field enclosures[19] we obtained data of aggressive behavior from rootvole populations In this paper we analyzed the relationshipbetween aggression level and population density weight andexternal conditions in root vole populations to test the abovethree hypotheses

2 Materials and Methods

21 Study Area The fieldwork was conducted from Mayto October in 1990 at Haibei Alpine Meadow EcosystemResearch Station of Chinese Academy of Sciences Theresearch station is located at northeast of Tibet in a largevalley oriented NW-SE surrounded on all sides by the QilianMountains with N latitude 37∘291015840 sim 37∘451015840 and E longitude101∘

121015840

sim 101∘

231015840 The average altitude of mountain area is

4000m a s l and 2900sim3500m in the valley The DatongRiver passes the south of the areaThe landscape is character-ized by large mountain ranges with steep valleys and gorgesinterspersed with relatively level and wide intermountaingrassland basins The natural conditions vegetation and soilstructure of the study area has been reported elsewhere [20]

The study site was located at an old field of Elymus nutansmeadow a secondary vegetation type after the original onehad been destroyed The majority of plants were Elymusnutans Poa spThalictrum alpinum Kobresia humilis Poten-tilla fruticosa and so forth The soil was loose moist andfertile with a dense and high plant leaf layer which wasthe natural habitat for root voles The dominant plant Enutans was soft and nutritive and preferred strongly by rootvoles [21] Besides root voles other rodents inhabited the Enutans meadows such as zokor Myospalax baileyi (Thomas1911) pikas Ochotona cansus (Lyon 1907) and O curzoniae(Hodgson 1858)

The avian and mammalian predators observed in thestudy area were the falcon Falco tinnunculus (Linnaeus 1758)buzzard Buteo hemilasius (Temminck and Schlegel 1844)weaselsMustela altaica (Pallas 1811)M eversmanni (Lesson1827) the wolf Canis lupus (Linnaeus 1758) foxes Vulpesvulpes (Linnaeus 1758) and V ferrilatus (Hodgson 1842)The dominant species among them were the buzzard thefalcon and the weasels

We constructed the enclosures with 2 times 1m120575 05 gal-vanized steel panels that extended 05m below and 1mabove ground Four treatments (2 times 2 factorial design) withtwo replicates were designed as follows no predator food

supplemented (minusP +F) predator access food supplemented(+P +F) no-predator nonsupplemented (minusP minusF) the con-trol predator access nonsupplemented (+PminusF)Within eachenclosure 42 trapping stations were staked in a 5 times 7m arrayA wooden cage trap was placed at each trapping station and3 trapping cages were placed in each removal areaThere werea total of 45 cages in every enclosure The minusP +F and minusP minusFenclosures were covered by fishing netting with a 3 times 3 cmgrid to prevent access by avian predators and the roof of thenet was kept in place with a Φ10 times 250 cm pillar The +P +Fand +P minusF enclosures were not covered with netting and hada series of low panels (030m high) every 10m except on sidesshared with no-predator enclosures Thus predators couldeasily enter these enclosures Live traps were set all alongoutside the enclosures with low panels to detect escapees

Supplemental food was granulated rabbit chow of TK-10 type (made in Shanghai Feed Processing Plant) a highquality food onwhichmicrotine animals grow and reproducewell [21] In +P +F and minusP +F enclosures food were keptin 500mL glass jars which were placed near every odd-numbered trap station in every row There were 22 food jarsin every enclosure and foodwas replenished weekly to ensurefreshness and availability

22 PopulationDensity After removal of the original residentroot voles and nontarget species five pairs of root voles froma laboratory colony were randomly assigned as populationfounders in each enclosure which represented a moderatedensity compared to the natural population around the studyarea The founder voles were nonsiblings of 3ndash5 months oldand born in a laboratory The animals had 1-2 weeks tobecome accustomed to the enclosures before trapping began[19]

There were three trapping days every two weeks in atrapping session At dusk the day before a trapping sessionthe traps were set open bedded with clean cotton and baitedwith cracked corn We examined the cages once every twohours each day during every trapping session All cageswere covered with hardboards of 2 cm thickness to reducetrap mortality caused by high temperature and were closedbetween trapping sessions None of the animals died in thetraps during the trapping sessions

Upon first capture each animal was marked with anumbered aluminum Style 1005-1s Monel ear tag producedby the National Band amp Tag Company The whole markingprocedure followed the Act on Animal Experimentationand Management released by the Ministry of Science andTechnology of the Peoplersquos Republic of China in 1988 We didnot punch holes in the ears of animals but only clipped thetags onto their ears The whole marking procedure did notappear to cause any distress or discomfort to the animalsTherecaptured individuals were released at the point of captureafter the following data were recorded tag number sexweight station number reproductive condition and generalcondition Male reproductive condition could be determinedby the size of scrotum and the location of testis and theindividuals with enlarged scrotum and descended testis werereproductively active Whether a female was pregnant or

ISRN Ecology 3

lactating could be judged by the width of vaginal opening andthe prominence of the nipples [22]

We classified root voles into two age classes based on size[23] males gt25 g and females gt22 g were adults others werejuveniles Population densities for every enclosure withineach trapping session were estimated separately using thejackknife estimator for model Mh (including heterogeneityin capture rates) in the program CAPTURE [24]

23 Aggressive Behavior Testing Aggression of adult rodent isstrongly influenced by food quality during early developmen-tal periods [25] In the current study population founderswere fed with food of equal quality during infancy and hadnever been utilized for aggressive behavior tests Aggressivebehavior tests were conducted (1) 5 weeks after the foundershad been placed in the enclosures (2) when the number ofindividuals in each enclosure had reached a fair size and(3) when there were adequate numbers of individuals forbehavioral observation Individuals selected for aggressivebehavior testing were born in the enclosure

It is difficult to accurately test the aggression levels ofsmall mammals directly in the field [26] so our aggressiontests were conducted in the behavior laboratory near theenclosure The apparatus for aggressive behavior testing wasa glass aquarium partitioned by a movable wooden board atthe center The box was cushioned with a 1 cm layer of sawdust and crushed corn The box was placed in a dark roomin the laboratory A 40w fluorescent lamp was suspendedabout 1m above the boxThe observer watched and recordedaggression behavior through a hole in a green curtain hung50 cm distance from the box

Every individual was observed nomore than twice withina trapping period and was tested only against others fromthe same enclosureThe encounters were different every timeThe period between two observations was 30 minutes Beforelifting the partition board 3minutes were given for root volesto acclimatize to the new space The board was then raisedand the voles watched for ten minutes The root voles wereput back into cages after observation and fed with adequatecrushed corns and water and then taken back to the siteswhere they had been trapped within 1-2 hours The glass boxwas cleaned with soapwater to eliminate odor gathered in thebox

The following behaviors were recorded following theclassification standard of Hofmann et al [12] as follows(1) threatmdashraising forelegs and stretching head shakingforelegs teeth baring and screaming (2) uprightmdashhind legsstanding straight body extending and the two individualsfacing each other closely during mutual upright (3) lungemdashone individual stretching head toward the other leapingfrom ground attacking and biting each other (4) boxingmdashthe upright individual striking head and shoulders of theother one (5) wrestlemdashthis behavior occurred rapidly and itwas difficult to tell the launcher (6) chasemdashone individualpursuing the other often ended with one jumping on to theotherrsquos rear violently (7) retreatmdashone individual fleeing awayfrom the approaching or attacking individual (8) approachmdashone individual moving toward the other to a distance shorter

than 5 cm Agonistic behavior 1 to 7 occurred frequentlywhen both individuals werewithin approach distance amongwhich threat lunge and chase were initiative and nondefen-sive or obviously aggressive Therefore the total number ofthreat lunge and chase was calculated as to stand for therelative aggression level of dyadic encounters andwas definedas an aggression count [27]

24 Game Model We used ESS (Evolutionary Stable Strat-egy) game models [28] to analyze the aggressive behaviorof root voles An ESS is a state of game dynamics wherein a very large population of competitors another mutantstrategy cannot successfully enter the population to disturbthe existing dynamic which in itself is population mixdependent In the ESS model a root vole population consistsof hawks and doves Let 119901 be the proportion of hawks in apopulation and (1 minus 119901) the proportion of doves If an animalwon a contest the benefit is 119881 units If an animal incurreda serious injury the cost is 119882 units And if it is involvedin a long display the cost is 119879 units According to Parker119901 = 119881119882 or in other words the greater the value of theresource the higher the proportion of hawks Conversely thegreater the cost of injury the greater the proportion of thedove [28]

Among the agonistic behavior sequences recorded in theroot vole population lunge and chasewere initiative and non-defensive or obviously aggressive so the individual whichexhibited these two behaviours in a dyadic encounter wasdefined as ldquoa hawkrdquo otherwise ldquoa doverdquo

25 Statistical Analysis Linear correlation coefficient be-tween body weight and aggression level was calculated inorder to determine the relation between individual sizeand its aggression level correlation coefficient betweenaggression counts and population density was calculated bySpearman rank correlation analysis in order to determine therelation between aggression level and population density

Kruskal-Wallis 119867-test was used to analyze the differencebetween average aggression levels under different treatmentsand also used to analyze aggression difference betweenmales and females under different treatments Differencebetween average aggression counts of reproductive individu-als and non-reproductive individuals was analyzed by 119905-testIndependent effects and interaction of food and predationon aggressiveness of root vole populations under differenttreatment were analyzed by two-way ANOVAs

3 Results

31 Aggression Level and Population Density Wemarked andrecaptured root vole populations under four treatments for20 trapping sessions in total [19] There was no significantlinear regression between aggression counts and populationdensity (Figure 1) According to the results of a Spearmanrank correlation analysis correlation coefficients betweenaggression counts and population density under all fourtreatments were not significant (minusP +F 119903 = minus0237 119899 = 18119875 = 0328 +P +F 119903 = minus0329 119899 = 20 119875 = 0151

4 ISRN Ecology

Table 1119867-test of Kruskal-Wallis for aggression levels of root vole populations under different treatments

Treatment+P minusF minusP +F minusP minusF +P minusF

Experimentdata Rank Experiment

data Rank Experimentdata Rank Experiment

data Rank

Aggressionaccounts

300 400 3200 4100 6500 4900 3600 45002500 3750 5300 4700 6000 4800 3500 43502000 3050 1300 1950 7800 5100 7400 5000200 300 2500 3750 2500 3750 3300 4200500 600 1100 1600 2400 3500 3700 4600100 200 3500 4350 2100 32001300 1950 1000 1400 2200 3300800 1050 1400 2150 2300 3400700 850 1600 2550 2600 4000600 700 1600 25502500 3750 700 8509000 5200 1500 23501000 1400 1200 1750000 100 1000 14001200 1750 1800 2800400 500 1900 2900800 1050 2000 3050900 1200 1700 27001500 23501400 2150

119877119894

= 32300 46900 35950 22000119877119894is the sum of sample rank under i treatmentminusP +F no predator food-supplemented enclosures+P +F predator access food-supplemented enclosuresminusP minusF no predator nonsupplemented enclosures+P minusF predator access nonsupplemented enclosures the control

minusP minusF 119903 = minus0183 119899 = 9 119875 = 0606 +P minusF 119903 = minus0112119899 = 5 119875 = 0823)

32 Food Predation and Aggression Level The differencebetween average aggression levels under different treatmentswas significant (119867 = 2412 df = 3 119875 lt 0001) according tothe Kruskal-Wallis119867-test (Table 1)

33 Sex and Aggression Level After observing 35 dyadicencounters between males and females we found extremelyweak aggression between different sexes which could beneglected in the aggressive behavior analysis Therefore onlythe aggressive behavior between the same sex dyads wasanalyzed in this paper

The difference of aggression level between malesand females under different treatments was inconsistent(Figure 2) The aggression level of males was significantlyhigher than that of females in minusP minusF and +P minusF population(the control) but the aggression level of females wassignificantly higher than that of males in +P +F accordingto 119867-test (+P +F 119867 = 5023 df = 1 119875 lt 0025 minusP minusF119867 = 14438 df = 1 119875 lt 0005 +P minusF 119867 = 6818 df = 1119875 lt 001) However the difference of aggression levelbetween males and females was not significant in minusP +Fpopulation (119867 = 2029 df = 1 119875 lt 025)

0

10

20

30

40

50

60

70

80

0 10 20 30Population density

Agg

ress

ion

coun

t

+F +P+F minusP

minusF minusPminusF +P

Figure 1 Aggression level and population density

34 Reproductive Activity and Aggression Level Individualsthat were reproductively active showed higher incidencesof aggressive acts than nonreproductively active individuals

ISRN Ecology 5

0

5

10

15

20

25

30

35

40

45

50Av

erag

e agg

ress

ion

coun

ts

Treatments

+F+

Pminus

+F+

Pminus

+Fminus

Pminus

+Fminus

Pminus

minusFminus

Pminus

minusFminus

Pminus

minusF+

Pminus

minusF+

Pminus

Figure 2 Difference of aggression level between males and females

Reproductively active voles especially lactating females wereevidently more aggressive usually attacking the opponentto escape in a hurry The difference between the aggres-sion counts of reproductive individuals and those of non-reproductive individuals was found to be significant (119905 =11885 df = 50 119875 = 00026)

35 Body Weight and Aggression Level Heavy voles showedhigh incidences of acts of aggression The smaller oneinvariably gives up first The heavier individual in a dyadicencounter usually became the dominant and the smallerone became the subordinate Body weight was significantlypositively correlated with aggression counts (119903 = 0375737119899 = 52 119875 = 001)

36 Aggressive Behavior Strategies under Different TreatmentsThe aggressive behavior pattern of root voles under differenttreatments (Table 2) was consistent with the prediction froman ESS hawk-dove model that is the proportion of ldquohawksrdquoin root vole populations under unfavorable conditions (+PminusF)wasmuch higher than that of root vole populations underfavorable conditions (minusP +F) and the proportions of ldquohawksrdquowere at intermediate level under mediate environments (minusPminusF +P +F)The aggressiveness of root vole populations underdifferent treatment was different significantly (119865 = 79909119899 = 20 119875 = 001)

4 Discussion

Theexperimental results supported Smith andPricersquos hypoth-esis and Adams and Mesterton-Gibbonsrsquos hypothesis how-ever did not support Wynn-Edwardsrsquo hypothesis Moreoverwe found that reproductively active individuals were moreaggressive

Table 2 Proportion of hawks in the populations of root voles underdifferent treatments

Treatments minusP +F +P +F minusP minusF +P minusFProportion of hawks 017 018 033 040

In our factorial experiment bodyweight was significantlypositively correlatedwith aggression counts with heavier andreproductively active root voles showing significantly higherincidences of aggressive acts (119903 = 0376 119899 = 52 119875 = 001)which supportedAdams andMesterton-Gibbonsrsquo hypothesis

The conventional agonistic behavior of root voles wasritualized noninjurious behavior which was consistent withthe conclusion of Smith and Price [17] and Lendrem [29]An ESS is always a mix of hawks and doves [17] ESSgame model predicts that when the value of the resource isgreater the proportion of ldquohawksrdquo in the population is higherOur experimental results supported this hypothesis Underthe most unfavorable conditions (+P minusF) when resourcesare most valuable winning a ldquowarrdquo is most important forforaging food defending territory protecting offspring andso forth and hence the highest proportion of ldquohawksrdquo inthe population on the other hand under the most favorableconditions (minusP +F) the lowest proportion of ldquohawksrdquo in thepopulation (Table 2) and the difference were significant

The difference in the aggression counts between repro-ductively active individuals and those of non-reproductivelyactive individuals was found to be significant (119905 = 11885df = 50 119875 = 00026) which was consistent with the resultsof Turner and Iverson [11] Fairbairn [30] and Reich etal [14] that is hormone level in an animal influencing itsaggressiveness

The experimental results showed that the aggression levelof dispersers was significantly lower than that of residentswhich agreed with the results of Fairbairn [30] but differedfrom that observed in the beach vole (Microtus breweri)[14] a species related to the root vole In a wild beachvole population dispersers and residents exhibited similaraggressive behavior patterns The cause of this phenomenonmight be that beach vole dispersers were a random sample ofthe resident population in terms of sex ratio and age structurewhereas in our study and the findings of Fairbairn [30]almost all dispersers were subadults which were subordinateindividuals in the population Subordinate males were lessaggressive and they disperse in response to social pressureduring the breeding season [30]

To summarize external environmental factors such aspredation and food treatment as well as internal conditionssuch as body weight and reproductive activity were moreimportant than density and gender in determining aggressionlevels of root vole populations

Acknowledgments

The authors wish to express their sincere appreciation to DrJianping Su Dr Meicai Wei Dr Wei Liu and Mr YumingLi for their help in the course of field experiment anddata analysis and to Dr Ben Bravery for his help in paper

6 ISRN Ecology

editingThe research is completed under the financial supportof the National Natural Science Foundation no 3870345and 30270242 and the Central South Forestry UniversityFoundation for the Introduction of High-Level-Talents no101-0630

References

[1] V C Wynn-Edwards Animal Dispersion in Relation To SocialBehavior Oliver and Boyd London UK 1962

[2] R M F Sadleir ldquoThe relationship between agonistic behaviorand population changes in the deermouse Peromyscus manic-ulatus (Wagner)rdquo Journal of Animal Ecology vol 34 no 2 pp331ndash352 1965

[3] M C Healey ldquoAggression and self-regulation of population sizein deer micerdquo Ecology vol 48 no 3 pp 377ndash392 1967

[4] C J Krebs B L Keller andRH Tamarin ldquoMicrotuspopulationbiology demographic changes in fluctuating populations ofMochrogaster and M pennsylvanicusrdquo Ecology vol 50 no 4 pp587ndash607 1969

[5] C J Krebs ldquoMicrotus population biology behavioral changesassociated with the population cycle in M ochrogaster and Mpennsylvanicusrdquo Ecology vol 51 no 1 pp 34ndash52 1970

[6] B L Keller and C J Krebs ldquoMicrotus population biologyIII Reproductive changes in fluctuating populations of Mochrogaster and M pennsylvanicus in southern Indiana 1965ndash67rdquo Ecological Monographs vol 40 no 3 pp 263ndash294 1970

[7] W H Conley Behavior demography and competition in Micro-tus longicaudus and M mexicanus [PhD thesis] Texas TechUniversity Lubbock Tex USA 1971

[8] W H Conley ldquoCompetition between Microtus a behavioralhypothesisrdquo Ecology vol 57 no 2 pp 224ndash237 1976

[9] J R Vale C A Vale and J P Harley ldquoInteraction of genotypeand population number with regard to aggressive behaviorsocial grooming and adrenal and gonadal weight in male micerdquoCommunications in Behavioral Biology vol 6 no 2 pp 209ndash2211971

[10] W Z Jr Lidicker ldquoRegulation of numbers in small mammalpopulationsmdashHistorical reflections and a synthesisrdquo in Popu-lations of Small Mammals Under Natural Conditions vol 5pp 122ndash141 PymatunIng Laboratory of Ecology University ofPittsburgh Pittsburgh Pa USA 1978

[11] B N Turner and S L Iverson ldquoThe annual cycle of aggressionin male Microtus pennsylvanicus and its relation to populationparametersrdquo Ecology vol 54 no 5 pp 967ndash981 1973

[12] J E Hofmann L L Getz and B J Klatt ldquoLevels of maleaggressiveness in fluctuating population ofMicrotus ochrogasterandMicrotus pennsylvanicusrdquo Canadian Journal of Zoology vol60 no 5 pp 898ndash912 1982

[13] J E Hofmann and L L Getz ldquoLevels of female aggressivenessin fluctuating populations of Microtus ochrogaster and Mpennsylvanicusrdquo ActaTheriologica Sinica vol 24 no 2 pp 125ndash131 2004

[14] L M Reich K M Wood B E Rothstein and R H TamarinldquoAggressive behaviour of male Microtus breweri and its demo-graphic implicationsrdquo Animal Behaviour vol 30 no 1 pp 117ndash122 1982

[15] S Mihok ldquoChittyrsquos hypothesis and behavior in subarctic red-back volesClethrionomys gapperirdquoOikos vol 36 no 3 pp 281ndash295 1981

[16] M J Taitt and C J Krebs ldquoManipulation of female behaviourin field populations of Microtus townsendiirdquo Journal of AnimalEcology vol 51 no 2 pp 681ndash690 1982

[17] J M Smith and G R Price ldquoThe logic of animal conflictrdquoNature vol 246 no 5427 pp 15ndash18 1973

[18] E S Adams and M Mesterton-Gibbons ldquoAnimal contests asevolutionary gamesrdquo American Scientist vol 86 no 4 pp 334ndash341 1998

[19] H Nie and J Liu ldquoRegulation of root vole population dynamicsby food supply and predation a two-factor experimentrdquo Oikosvol 109 no 2 pp 387ndash395 2005

[20] W Xia Alpine Meadow Ecosystem Gansu Peoplersquos PublishingHouse Lanzhou China 1982

[21] J Liu X Wang and W Liu ldquoStudies on the nutritional ecologyof small herbivorous mammals patterns of food selection andresource utilization for root voles and Gansu pikasrdquo in AlpineMeadow Ecosystem Fasc 3 pp 111ndash124 Science Press BeijingChina 1991 (Chinese)

[22] L J Liu S Su L W Liu W X Wang N H Nie and L YLi ldquoField experimental studies on the multifactorial hypothesisof population system regulation for small rodents an analysisof effects of food availability and predation on populationdynamics of root volerdquo Acta Theriologica Sinica vol 14 no 2pp 117ndash129 1994 (Chinese)

[23] J Liang J Zeng ZWang and YHan ldquoStudy on the growth anddevelopment of root volesrdquo Acta Plateau Biology Sinica vol 1no 2 pp 195ndash206 1982

[24] D L Otis K P Burnham G C White and D R AndersonldquoStatistical inference from capture data on closed animal popu-lationsrdquoWildlife Monographs vol 62 no 1 pp 1ndash135 1978

[25] J L Smart ldquoUndernutrition and aggressionrdquo in Multidisci-plinary Approaches To Aggression Research pp 179ndash191 Ams-terdam Elsevier North-Holland Biomedical Press AmsterdamThe Netherlands 1981

[26] M J Taitt and C J Krebs ldquoPopulation dynamics and cyclesrdquoin Biology of New World Microtus pp 567ndash620 The AmericanSociety of Mammalogists Shippensburg Pa USA 1985

[27] E ADesy GO Batzli and J Liu ldquoEffects of food and predationon behaviour of prairie voles a field experimentrdquoOikos vol 58no 2 pp 159ndash168 1990

[28] G A Parker ldquoEvolutionarily Stable Strategiesrdquo in BehaviouralEcology An Evolutionary Approach pp 30ndash61 Sinauer Asso-ciates Sunderland Mass USA 1984

[29] D W LendremModeling in Behavior Ecology An IntroductoryText Columbia University Press New York NY USA 1984

[30] D J Fairbairn ldquoBehaviour of dispersing deer mice (Peromyscusmaniculatus)rdquo Behavioral Ecology and Sociobiology vol 3 no 3pp 265ndash282 1978

Submit your manuscripts athttpwwwhindawicom

Forestry ResearchInternational Journal of

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EcosystemsJournal of

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Applied ampEnvironmentalSoil Science

Volume 2014

Advances in

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Environmental Chemistry

Atmospheric SciencesInternational Journal of

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Geophysics

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OceanographyInternational Journal of

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

ISRN Ecology 3

lactating could be judged by the width of vaginal opening andthe prominence of the nipples [22]

We classified root voles into two age classes based on size[23] males gt25 g and females gt22 g were adults others werejuveniles Population densities for every enclosure withineach trapping session were estimated separately using thejackknife estimator for model Mh (including heterogeneityin capture rates) in the program CAPTURE [24]

23 Aggressive Behavior Testing Aggression of adult rodent isstrongly influenced by food quality during early developmen-tal periods [25] In the current study population founderswere fed with food of equal quality during infancy and hadnever been utilized for aggressive behavior tests Aggressivebehavior tests were conducted (1) 5 weeks after the foundershad been placed in the enclosures (2) when the number ofindividuals in each enclosure had reached a fair size and(3) when there were adequate numbers of individuals forbehavioral observation Individuals selected for aggressivebehavior testing were born in the enclosure

It is difficult to accurately test the aggression levels ofsmall mammals directly in the field [26] so our aggressiontests were conducted in the behavior laboratory near theenclosure The apparatus for aggressive behavior testing wasa glass aquarium partitioned by a movable wooden board atthe center The box was cushioned with a 1 cm layer of sawdust and crushed corn The box was placed in a dark roomin the laboratory A 40w fluorescent lamp was suspendedabout 1m above the boxThe observer watched and recordedaggression behavior through a hole in a green curtain hung50 cm distance from the box

Every individual was observed nomore than twice withina trapping period and was tested only against others fromthe same enclosureThe encounters were different every timeThe period between two observations was 30 minutes Beforelifting the partition board 3minutes were given for root volesto acclimatize to the new space The board was then raisedand the voles watched for ten minutes The root voles wereput back into cages after observation and fed with adequatecrushed corns and water and then taken back to the siteswhere they had been trapped within 1-2 hours The glass boxwas cleaned with soapwater to eliminate odor gathered in thebox

The following behaviors were recorded following theclassification standard of Hofmann et al [12] as follows(1) threatmdashraising forelegs and stretching head shakingforelegs teeth baring and screaming (2) uprightmdashhind legsstanding straight body extending and the two individualsfacing each other closely during mutual upright (3) lungemdashone individual stretching head toward the other leapingfrom ground attacking and biting each other (4) boxingmdashthe upright individual striking head and shoulders of theother one (5) wrestlemdashthis behavior occurred rapidly and itwas difficult to tell the launcher (6) chasemdashone individualpursuing the other often ended with one jumping on to theotherrsquos rear violently (7) retreatmdashone individual fleeing awayfrom the approaching or attacking individual (8) approachmdashone individual moving toward the other to a distance shorter

than 5 cm Agonistic behavior 1 to 7 occurred frequentlywhen both individuals werewithin approach distance amongwhich threat lunge and chase were initiative and nondefen-sive or obviously aggressive Therefore the total number ofthreat lunge and chase was calculated as to stand for therelative aggression level of dyadic encounters andwas definedas an aggression count [27]

24 Game Model We used ESS (Evolutionary Stable Strat-egy) game models [28] to analyze the aggressive behaviorof root voles An ESS is a state of game dynamics wherein a very large population of competitors another mutantstrategy cannot successfully enter the population to disturbthe existing dynamic which in itself is population mixdependent In the ESS model a root vole population consistsof hawks and doves Let 119901 be the proportion of hawks in apopulation and (1 minus 119901) the proportion of doves If an animalwon a contest the benefit is 119881 units If an animal incurreda serious injury the cost is 119882 units And if it is involvedin a long display the cost is 119879 units According to Parker119901 = 119881119882 or in other words the greater the value of theresource the higher the proportion of hawks Conversely thegreater the cost of injury the greater the proportion of thedove [28]

Among the agonistic behavior sequences recorded in theroot vole population lunge and chasewere initiative and non-defensive or obviously aggressive so the individual whichexhibited these two behaviours in a dyadic encounter wasdefined as ldquoa hawkrdquo otherwise ldquoa doverdquo

25 Statistical Analysis Linear correlation coefficient be-tween body weight and aggression level was calculated inorder to determine the relation between individual sizeand its aggression level correlation coefficient betweenaggression counts and population density was calculated bySpearman rank correlation analysis in order to determine therelation between aggression level and population density

Kruskal-Wallis 119867-test was used to analyze the differencebetween average aggression levels under different treatmentsand also used to analyze aggression difference betweenmales and females under different treatments Differencebetween average aggression counts of reproductive individu-als and non-reproductive individuals was analyzed by 119905-testIndependent effects and interaction of food and predationon aggressiveness of root vole populations under differenttreatment were analyzed by two-way ANOVAs

3 Results

31 Aggression Level and Population Density Wemarked andrecaptured root vole populations under four treatments for20 trapping sessions in total [19] There was no significantlinear regression between aggression counts and populationdensity (Figure 1) According to the results of a Spearmanrank correlation analysis correlation coefficients betweenaggression counts and population density under all fourtreatments were not significant (minusP +F 119903 = minus0237 119899 = 18119875 = 0328 +P +F 119903 = minus0329 119899 = 20 119875 = 0151

4 ISRN Ecology

Table 1119867-test of Kruskal-Wallis for aggression levels of root vole populations under different treatments

Treatment+P minusF minusP +F minusP minusF +P minusF

Experimentdata Rank Experiment

data Rank Experimentdata Rank Experiment

data Rank

Aggressionaccounts

300 400 3200 4100 6500 4900 3600 45002500 3750 5300 4700 6000 4800 3500 43502000 3050 1300 1950 7800 5100 7400 5000200 300 2500 3750 2500 3750 3300 4200500 600 1100 1600 2400 3500 3700 4600100 200 3500 4350 2100 32001300 1950 1000 1400 2200 3300800 1050 1400 2150 2300 3400700 850 1600 2550 2600 4000600 700 1600 25502500 3750 700 8509000 5200 1500 23501000 1400 1200 1750000 100 1000 14001200 1750 1800 2800400 500 1900 2900800 1050 2000 3050900 1200 1700 27001500 23501400 2150

119877119894

= 32300 46900 35950 22000119877119894is the sum of sample rank under i treatmentminusP +F no predator food-supplemented enclosures+P +F predator access food-supplemented enclosuresminusP minusF no predator nonsupplemented enclosures+P minusF predator access nonsupplemented enclosures the control

minusP minusF 119903 = minus0183 119899 = 9 119875 = 0606 +P minusF 119903 = minus0112119899 = 5 119875 = 0823)

32 Food Predation and Aggression Level The differencebetween average aggression levels under different treatmentswas significant (119867 = 2412 df = 3 119875 lt 0001) according tothe Kruskal-Wallis119867-test (Table 1)

33 Sex and Aggression Level After observing 35 dyadicencounters between males and females we found extremelyweak aggression between different sexes which could beneglected in the aggressive behavior analysis Therefore onlythe aggressive behavior between the same sex dyads wasanalyzed in this paper

The difference of aggression level between malesand females under different treatments was inconsistent(Figure 2) The aggression level of males was significantlyhigher than that of females in minusP minusF and +P minusF population(the control) but the aggression level of females wassignificantly higher than that of males in +P +F accordingto 119867-test (+P +F 119867 = 5023 df = 1 119875 lt 0025 minusP minusF119867 = 14438 df = 1 119875 lt 0005 +P minusF 119867 = 6818 df = 1119875 lt 001) However the difference of aggression levelbetween males and females was not significant in minusP +Fpopulation (119867 = 2029 df = 1 119875 lt 025)

0

10

20

30

40

50

60

70

80

0 10 20 30Population density

Agg

ress

ion

coun

t

+F +P+F minusP

minusF minusPminusF +P

Figure 1 Aggression level and population density

34 Reproductive Activity and Aggression Level Individualsthat were reproductively active showed higher incidencesof aggressive acts than nonreproductively active individuals

ISRN Ecology 5

0

5

10

15

20

25

30

35

40

45

50Av

erag

e agg

ress

ion

coun

ts

Treatments

+F+

Pminus

+F+

Pminus

+Fminus

Pminus

+Fminus

Pminus

minusFminus

Pminus

minusFminus

Pminus

minusF+

Pminus

minusF+

Pminus

Figure 2 Difference of aggression level between males and females

Reproductively active voles especially lactating females wereevidently more aggressive usually attacking the opponentto escape in a hurry The difference between the aggres-sion counts of reproductive individuals and those of non-reproductive individuals was found to be significant (119905 =11885 df = 50 119875 = 00026)

35 Body Weight and Aggression Level Heavy voles showedhigh incidences of acts of aggression The smaller oneinvariably gives up first The heavier individual in a dyadicencounter usually became the dominant and the smallerone became the subordinate Body weight was significantlypositively correlated with aggression counts (119903 = 0375737119899 = 52 119875 = 001)

36 Aggressive Behavior Strategies under Different TreatmentsThe aggressive behavior pattern of root voles under differenttreatments (Table 2) was consistent with the prediction froman ESS hawk-dove model that is the proportion of ldquohawksrdquoin root vole populations under unfavorable conditions (+PminusF)wasmuch higher than that of root vole populations underfavorable conditions (minusP +F) and the proportions of ldquohawksrdquowere at intermediate level under mediate environments (minusPminusF +P +F)The aggressiveness of root vole populations underdifferent treatment was different significantly (119865 = 79909119899 = 20 119875 = 001)

4 Discussion

Theexperimental results supported Smith andPricersquos hypoth-esis and Adams and Mesterton-Gibbonsrsquos hypothesis how-ever did not support Wynn-Edwardsrsquo hypothesis Moreoverwe found that reproductively active individuals were moreaggressive

Table 2 Proportion of hawks in the populations of root voles underdifferent treatments

Treatments minusP +F +P +F minusP minusF +P minusFProportion of hawks 017 018 033 040

In our factorial experiment bodyweight was significantlypositively correlatedwith aggression counts with heavier andreproductively active root voles showing significantly higherincidences of aggressive acts (119903 = 0376 119899 = 52 119875 = 001)which supportedAdams andMesterton-Gibbonsrsquo hypothesis

The conventional agonistic behavior of root voles wasritualized noninjurious behavior which was consistent withthe conclusion of Smith and Price [17] and Lendrem [29]An ESS is always a mix of hawks and doves [17] ESSgame model predicts that when the value of the resource isgreater the proportion of ldquohawksrdquo in the population is higherOur experimental results supported this hypothesis Underthe most unfavorable conditions (+P minusF) when resourcesare most valuable winning a ldquowarrdquo is most important forforaging food defending territory protecting offspring andso forth and hence the highest proportion of ldquohawksrdquo inthe population on the other hand under the most favorableconditions (minusP +F) the lowest proportion of ldquohawksrdquo in thepopulation (Table 2) and the difference were significant

The difference in the aggression counts between repro-ductively active individuals and those of non-reproductivelyactive individuals was found to be significant (119905 = 11885df = 50 119875 = 00026) which was consistent with the resultsof Turner and Iverson [11] Fairbairn [30] and Reich etal [14] that is hormone level in an animal influencing itsaggressiveness

The experimental results showed that the aggression levelof dispersers was significantly lower than that of residentswhich agreed with the results of Fairbairn [30] but differedfrom that observed in the beach vole (Microtus breweri)[14] a species related to the root vole In a wild beachvole population dispersers and residents exhibited similaraggressive behavior patterns The cause of this phenomenonmight be that beach vole dispersers were a random sample ofthe resident population in terms of sex ratio and age structurewhereas in our study and the findings of Fairbairn [30]almost all dispersers were subadults which were subordinateindividuals in the population Subordinate males were lessaggressive and they disperse in response to social pressureduring the breeding season [30]

To summarize external environmental factors such aspredation and food treatment as well as internal conditionssuch as body weight and reproductive activity were moreimportant than density and gender in determining aggressionlevels of root vole populations

Acknowledgments

The authors wish to express their sincere appreciation to DrJianping Su Dr Meicai Wei Dr Wei Liu and Mr YumingLi for their help in the course of field experiment anddata analysis and to Dr Ben Bravery for his help in paper

6 ISRN Ecology

editingThe research is completed under the financial supportof the National Natural Science Foundation no 3870345and 30270242 and the Central South Forestry UniversityFoundation for the Introduction of High-Level-Talents no101-0630

References

[1] V C Wynn-Edwards Animal Dispersion in Relation To SocialBehavior Oliver and Boyd London UK 1962

[2] R M F Sadleir ldquoThe relationship between agonistic behaviorand population changes in the deermouse Peromyscus manic-ulatus (Wagner)rdquo Journal of Animal Ecology vol 34 no 2 pp331ndash352 1965

[3] M C Healey ldquoAggression and self-regulation of population sizein deer micerdquo Ecology vol 48 no 3 pp 377ndash392 1967

[4] C J Krebs B L Keller andRH Tamarin ldquoMicrotuspopulationbiology demographic changes in fluctuating populations ofMochrogaster and M pennsylvanicusrdquo Ecology vol 50 no 4 pp587ndash607 1969

[5] C J Krebs ldquoMicrotus population biology behavioral changesassociated with the population cycle in M ochrogaster and Mpennsylvanicusrdquo Ecology vol 51 no 1 pp 34ndash52 1970

[6] B L Keller and C J Krebs ldquoMicrotus population biologyIII Reproductive changes in fluctuating populations of Mochrogaster and M pennsylvanicus in southern Indiana 1965ndash67rdquo Ecological Monographs vol 40 no 3 pp 263ndash294 1970

[7] W H Conley Behavior demography and competition in Micro-tus longicaudus and M mexicanus [PhD thesis] Texas TechUniversity Lubbock Tex USA 1971

[8] W H Conley ldquoCompetition between Microtus a behavioralhypothesisrdquo Ecology vol 57 no 2 pp 224ndash237 1976

[9] J R Vale C A Vale and J P Harley ldquoInteraction of genotypeand population number with regard to aggressive behaviorsocial grooming and adrenal and gonadal weight in male micerdquoCommunications in Behavioral Biology vol 6 no 2 pp 209ndash2211971

[10] W Z Jr Lidicker ldquoRegulation of numbers in small mammalpopulationsmdashHistorical reflections and a synthesisrdquo in Popu-lations of Small Mammals Under Natural Conditions vol 5pp 122ndash141 PymatunIng Laboratory of Ecology University ofPittsburgh Pittsburgh Pa USA 1978

[11] B N Turner and S L Iverson ldquoThe annual cycle of aggressionin male Microtus pennsylvanicus and its relation to populationparametersrdquo Ecology vol 54 no 5 pp 967ndash981 1973

[12] J E Hofmann L L Getz and B J Klatt ldquoLevels of maleaggressiveness in fluctuating population ofMicrotus ochrogasterandMicrotus pennsylvanicusrdquo Canadian Journal of Zoology vol60 no 5 pp 898ndash912 1982

[13] J E Hofmann and L L Getz ldquoLevels of female aggressivenessin fluctuating populations of Microtus ochrogaster and Mpennsylvanicusrdquo ActaTheriologica Sinica vol 24 no 2 pp 125ndash131 2004

[14] L M Reich K M Wood B E Rothstein and R H TamarinldquoAggressive behaviour of male Microtus breweri and its demo-graphic implicationsrdquo Animal Behaviour vol 30 no 1 pp 117ndash122 1982

[15] S Mihok ldquoChittyrsquos hypothesis and behavior in subarctic red-back volesClethrionomys gapperirdquoOikos vol 36 no 3 pp 281ndash295 1981

[16] M J Taitt and C J Krebs ldquoManipulation of female behaviourin field populations of Microtus townsendiirdquo Journal of AnimalEcology vol 51 no 2 pp 681ndash690 1982

[17] J M Smith and G R Price ldquoThe logic of animal conflictrdquoNature vol 246 no 5427 pp 15ndash18 1973

[18] E S Adams and M Mesterton-Gibbons ldquoAnimal contests asevolutionary gamesrdquo American Scientist vol 86 no 4 pp 334ndash341 1998

[19] H Nie and J Liu ldquoRegulation of root vole population dynamicsby food supply and predation a two-factor experimentrdquo Oikosvol 109 no 2 pp 387ndash395 2005

[20] W Xia Alpine Meadow Ecosystem Gansu Peoplersquos PublishingHouse Lanzhou China 1982

[21] J Liu X Wang and W Liu ldquoStudies on the nutritional ecologyof small herbivorous mammals patterns of food selection andresource utilization for root voles and Gansu pikasrdquo in AlpineMeadow Ecosystem Fasc 3 pp 111ndash124 Science Press BeijingChina 1991 (Chinese)

[22] L J Liu S Su L W Liu W X Wang N H Nie and L YLi ldquoField experimental studies on the multifactorial hypothesisof population system regulation for small rodents an analysisof effects of food availability and predation on populationdynamics of root volerdquo Acta Theriologica Sinica vol 14 no 2pp 117ndash129 1994 (Chinese)

[23] J Liang J Zeng ZWang and YHan ldquoStudy on the growth anddevelopment of root volesrdquo Acta Plateau Biology Sinica vol 1no 2 pp 195ndash206 1982

[24] D L Otis K P Burnham G C White and D R AndersonldquoStatistical inference from capture data on closed animal popu-lationsrdquoWildlife Monographs vol 62 no 1 pp 1ndash135 1978

[25] J L Smart ldquoUndernutrition and aggressionrdquo in Multidisci-plinary Approaches To Aggression Research pp 179ndash191 Ams-terdam Elsevier North-Holland Biomedical Press AmsterdamThe Netherlands 1981

[26] M J Taitt and C J Krebs ldquoPopulation dynamics and cyclesrdquoin Biology of New World Microtus pp 567ndash620 The AmericanSociety of Mammalogists Shippensburg Pa USA 1985

[27] E ADesy GO Batzli and J Liu ldquoEffects of food and predationon behaviour of prairie voles a field experimentrdquoOikos vol 58no 2 pp 159ndash168 1990

[28] G A Parker ldquoEvolutionarily Stable Strategiesrdquo in BehaviouralEcology An Evolutionary Approach pp 30ndash61 Sinauer Asso-ciates Sunderland Mass USA 1984

[29] D W LendremModeling in Behavior Ecology An IntroductoryText Columbia University Press New York NY USA 1984

[30] D J Fairbairn ldquoBehaviour of dispersing deer mice (Peromyscusmaniculatus)rdquo Behavioral Ecology and Sociobiology vol 3 no 3pp 265ndash282 1978

Submit your manuscripts athttpwwwhindawicom

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental and Public Health

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcosystemsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Marine BiologyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental Chemistry

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Waste ManagementJournal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

4 ISRN Ecology

Table 1119867-test of Kruskal-Wallis for aggression levels of root vole populations under different treatments

Treatment+P minusF minusP +F minusP minusF +P minusF

Experimentdata Rank Experiment

data Rank Experimentdata Rank Experiment

data Rank

Aggressionaccounts

300 400 3200 4100 6500 4900 3600 45002500 3750 5300 4700 6000 4800 3500 43502000 3050 1300 1950 7800 5100 7400 5000200 300 2500 3750 2500 3750 3300 4200500 600 1100 1600 2400 3500 3700 4600100 200 3500 4350 2100 32001300 1950 1000 1400 2200 3300800 1050 1400 2150 2300 3400700 850 1600 2550 2600 4000600 700 1600 25502500 3750 700 8509000 5200 1500 23501000 1400 1200 1750000 100 1000 14001200 1750 1800 2800400 500 1900 2900800 1050 2000 3050900 1200 1700 27001500 23501400 2150

119877119894

= 32300 46900 35950 22000119877119894is the sum of sample rank under i treatmentminusP +F no predator food-supplemented enclosures+P +F predator access food-supplemented enclosuresminusP minusF no predator nonsupplemented enclosures+P minusF predator access nonsupplemented enclosures the control

minusP minusF 119903 = minus0183 119899 = 9 119875 = 0606 +P minusF 119903 = minus0112119899 = 5 119875 = 0823)

32 Food Predation and Aggression Level The differencebetween average aggression levels under different treatmentswas significant (119867 = 2412 df = 3 119875 lt 0001) according tothe Kruskal-Wallis119867-test (Table 1)

33 Sex and Aggression Level After observing 35 dyadicencounters between males and females we found extremelyweak aggression between different sexes which could beneglected in the aggressive behavior analysis Therefore onlythe aggressive behavior between the same sex dyads wasanalyzed in this paper

The difference of aggression level between malesand females under different treatments was inconsistent(Figure 2) The aggression level of males was significantlyhigher than that of females in minusP minusF and +P minusF population(the control) but the aggression level of females wassignificantly higher than that of males in +P +F accordingto 119867-test (+P +F 119867 = 5023 df = 1 119875 lt 0025 minusP minusF119867 = 14438 df = 1 119875 lt 0005 +P minusF 119867 = 6818 df = 1119875 lt 001) However the difference of aggression levelbetween males and females was not significant in minusP +Fpopulation (119867 = 2029 df = 1 119875 lt 025)

0

10

20

30

40

50

60

70

80

0 10 20 30Population density

Agg

ress

ion

coun

t

+F +P+F minusP

minusF minusPminusF +P

Figure 1 Aggression level and population density

34 Reproductive Activity and Aggression Level Individualsthat were reproductively active showed higher incidencesof aggressive acts than nonreproductively active individuals

ISRN Ecology 5

0

5

10

15

20

25

30

35

40

45

50Av

erag

e agg

ress

ion

coun

ts

Treatments

+F+

Pminus

+F+

Pminus

+Fminus

Pminus

+Fminus

Pminus

minusFminus

Pminus

minusFminus

Pminus

minusF+

Pminus

minusF+

Pminus

Figure 2 Difference of aggression level between males and females

Reproductively active voles especially lactating females wereevidently more aggressive usually attacking the opponentto escape in a hurry The difference between the aggres-sion counts of reproductive individuals and those of non-reproductive individuals was found to be significant (119905 =11885 df = 50 119875 = 00026)

35 Body Weight and Aggression Level Heavy voles showedhigh incidences of acts of aggression The smaller oneinvariably gives up first The heavier individual in a dyadicencounter usually became the dominant and the smallerone became the subordinate Body weight was significantlypositively correlated with aggression counts (119903 = 0375737119899 = 52 119875 = 001)

36 Aggressive Behavior Strategies under Different TreatmentsThe aggressive behavior pattern of root voles under differenttreatments (Table 2) was consistent with the prediction froman ESS hawk-dove model that is the proportion of ldquohawksrdquoin root vole populations under unfavorable conditions (+PminusF)wasmuch higher than that of root vole populations underfavorable conditions (minusP +F) and the proportions of ldquohawksrdquowere at intermediate level under mediate environments (minusPminusF +P +F)The aggressiveness of root vole populations underdifferent treatment was different significantly (119865 = 79909119899 = 20 119875 = 001)

4 Discussion

Theexperimental results supported Smith andPricersquos hypoth-esis and Adams and Mesterton-Gibbonsrsquos hypothesis how-ever did not support Wynn-Edwardsrsquo hypothesis Moreoverwe found that reproductively active individuals were moreaggressive

Table 2 Proportion of hawks in the populations of root voles underdifferent treatments

Treatments minusP +F +P +F minusP minusF +P minusFProportion of hawks 017 018 033 040

In our factorial experiment bodyweight was significantlypositively correlatedwith aggression counts with heavier andreproductively active root voles showing significantly higherincidences of aggressive acts (119903 = 0376 119899 = 52 119875 = 001)which supportedAdams andMesterton-Gibbonsrsquo hypothesis

The conventional agonistic behavior of root voles wasritualized noninjurious behavior which was consistent withthe conclusion of Smith and Price [17] and Lendrem [29]An ESS is always a mix of hawks and doves [17] ESSgame model predicts that when the value of the resource isgreater the proportion of ldquohawksrdquo in the population is higherOur experimental results supported this hypothesis Underthe most unfavorable conditions (+P minusF) when resourcesare most valuable winning a ldquowarrdquo is most important forforaging food defending territory protecting offspring andso forth and hence the highest proportion of ldquohawksrdquo inthe population on the other hand under the most favorableconditions (minusP +F) the lowest proportion of ldquohawksrdquo in thepopulation (Table 2) and the difference were significant

The difference in the aggression counts between repro-ductively active individuals and those of non-reproductivelyactive individuals was found to be significant (119905 = 11885df = 50 119875 = 00026) which was consistent with the resultsof Turner and Iverson [11] Fairbairn [30] and Reich etal [14] that is hormone level in an animal influencing itsaggressiveness

The experimental results showed that the aggression levelof dispersers was significantly lower than that of residentswhich agreed with the results of Fairbairn [30] but differedfrom that observed in the beach vole (Microtus breweri)[14] a species related to the root vole In a wild beachvole population dispersers and residents exhibited similaraggressive behavior patterns The cause of this phenomenonmight be that beach vole dispersers were a random sample ofthe resident population in terms of sex ratio and age structurewhereas in our study and the findings of Fairbairn [30]almost all dispersers were subadults which were subordinateindividuals in the population Subordinate males were lessaggressive and they disperse in response to social pressureduring the breeding season [30]

To summarize external environmental factors such aspredation and food treatment as well as internal conditionssuch as body weight and reproductive activity were moreimportant than density and gender in determining aggressionlevels of root vole populations

Acknowledgments

The authors wish to express their sincere appreciation to DrJianping Su Dr Meicai Wei Dr Wei Liu and Mr YumingLi for their help in the course of field experiment anddata analysis and to Dr Ben Bravery for his help in paper

6 ISRN Ecology

editingThe research is completed under the financial supportof the National Natural Science Foundation no 3870345and 30270242 and the Central South Forestry UniversityFoundation for the Introduction of High-Level-Talents no101-0630

References

[1] V C Wynn-Edwards Animal Dispersion in Relation To SocialBehavior Oliver and Boyd London UK 1962

[2] R M F Sadleir ldquoThe relationship between agonistic behaviorand population changes in the deermouse Peromyscus manic-ulatus (Wagner)rdquo Journal of Animal Ecology vol 34 no 2 pp331ndash352 1965

[3] M C Healey ldquoAggression and self-regulation of population sizein deer micerdquo Ecology vol 48 no 3 pp 377ndash392 1967

[4] C J Krebs B L Keller andRH Tamarin ldquoMicrotuspopulationbiology demographic changes in fluctuating populations ofMochrogaster and M pennsylvanicusrdquo Ecology vol 50 no 4 pp587ndash607 1969

[5] C J Krebs ldquoMicrotus population biology behavioral changesassociated with the population cycle in M ochrogaster and Mpennsylvanicusrdquo Ecology vol 51 no 1 pp 34ndash52 1970

[6] B L Keller and C J Krebs ldquoMicrotus population biologyIII Reproductive changes in fluctuating populations of Mochrogaster and M pennsylvanicus in southern Indiana 1965ndash67rdquo Ecological Monographs vol 40 no 3 pp 263ndash294 1970

[7] W H Conley Behavior demography and competition in Micro-tus longicaudus and M mexicanus [PhD thesis] Texas TechUniversity Lubbock Tex USA 1971

[8] W H Conley ldquoCompetition between Microtus a behavioralhypothesisrdquo Ecology vol 57 no 2 pp 224ndash237 1976

[9] J R Vale C A Vale and J P Harley ldquoInteraction of genotypeand population number with regard to aggressive behaviorsocial grooming and adrenal and gonadal weight in male micerdquoCommunications in Behavioral Biology vol 6 no 2 pp 209ndash2211971

[10] W Z Jr Lidicker ldquoRegulation of numbers in small mammalpopulationsmdashHistorical reflections and a synthesisrdquo in Popu-lations of Small Mammals Under Natural Conditions vol 5pp 122ndash141 PymatunIng Laboratory of Ecology University ofPittsburgh Pittsburgh Pa USA 1978

[11] B N Turner and S L Iverson ldquoThe annual cycle of aggressionin male Microtus pennsylvanicus and its relation to populationparametersrdquo Ecology vol 54 no 5 pp 967ndash981 1973

[12] J E Hofmann L L Getz and B J Klatt ldquoLevels of maleaggressiveness in fluctuating population ofMicrotus ochrogasterandMicrotus pennsylvanicusrdquo Canadian Journal of Zoology vol60 no 5 pp 898ndash912 1982

[13] J E Hofmann and L L Getz ldquoLevels of female aggressivenessin fluctuating populations of Microtus ochrogaster and Mpennsylvanicusrdquo ActaTheriologica Sinica vol 24 no 2 pp 125ndash131 2004

[14] L M Reich K M Wood B E Rothstein and R H TamarinldquoAggressive behaviour of male Microtus breweri and its demo-graphic implicationsrdquo Animal Behaviour vol 30 no 1 pp 117ndash122 1982

[15] S Mihok ldquoChittyrsquos hypothesis and behavior in subarctic red-back volesClethrionomys gapperirdquoOikos vol 36 no 3 pp 281ndash295 1981

[16] M J Taitt and C J Krebs ldquoManipulation of female behaviourin field populations of Microtus townsendiirdquo Journal of AnimalEcology vol 51 no 2 pp 681ndash690 1982

[17] J M Smith and G R Price ldquoThe logic of animal conflictrdquoNature vol 246 no 5427 pp 15ndash18 1973

[18] E S Adams and M Mesterton-Gibbons ldquoAnimal contests asevolutionary gamesrdquo American Scientist vol 86 no 4 pp 334ndash341 1998

[19] H Nie and J Liu ldquoRegulation of root vole population dynamicsby food supply and predation a two-factor experimentrdquo Oikosvol 109 no 2 pp 387ndash395 2005

[20] W Xia Alpine Meadow Ecosystem Gansu Peoplersquos PublishingHouse Lanzhou China 1982

[21] J Liu X Wang and W Liu ldquoStudies on the nutritional ecologyof small herbivorous mammals patterns of food selection andresource utilization for root voles and Gansu pikasrdquo in AlpineMeadow Ecosystem Fasc 3 pp 111ndash124 Science Press BeijingChina 1991 (Chinese)

[22] L J Liu S Su L W Liu W X Wang N H Nie and L YLi ldquoField experimental studies on the multifactorial hypothesisof population system regulation for small rodents an analysisof effects of food availability and predation on populationdynamics of root volerdquo Acta Theriologica Sinica vol 14 no 2pp 117ndash129 1994 (Chinese)

[23] J Liang J Zeng ZWang and YHan ldquoStudy on the growth anddevelopment of root volesrdquo Acta Plateau Biology Sinica vol 1no 2 pp 195ndash206 1982

[24] D L Otis K P Burnham G C White and D R AndersonldquoStatistical inference from capture data on closed animal popu-lationsrdquoWildlife Monographs vol 62 no 1 pp 1ndash135 1978

[25] J L Smart ldquoUndernutrition and aggressionrdquo in Multidisci-plinary Approaches To Aggression Research pp 179ndash191 Ams-terdam Elsevier North-Holland Biomedical Press AmsterdamThe Netherlands 1981

[26] M J Taitt and C J Krebs ldquoPopulation dynamics and cyclesrdquoin Biology of New World Microtus pp 567ndash620 The AmericanSociety of Mammalogists Shippensburg Pa USA 1985

[27] E ADesy GO Batzli and J Liu ldquoEffects of food and predationon behaviour of prairie voles a field experimentrdquoOikos vol 58no 2 pp 159ndash168 1990

[28] G A Parker ldquoEvolutionarily Stable Strategiesrdquo in BehaviouralEcology An Evolutionary Approach pp 30ndash61 Sinauer Asso-ciates Sunderland Mass USA 1984

[29] D W LendremModeling in Behavior Ecology An IntroductoryText Columbia University Press New York NY USA 1984

[30] D J Fairbairn ldquoBehaviour of dispersing deer mice (Peromyscusmaniculatus)rdquo Behavioral Ecology and Sociobiology vol 3 no 3pp 265ndash282 1978

Submit your manuscripts athttpwwwhindawicom

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental and Public Health

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcosystemsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Marine BiologyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental Chemistry

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Waste ManagementJournal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

ISRN Ecology 5

0

5

10

15

20

25

30

35

40

45

50Av

erag

e agg

ress

ion

coun

ts

Treatments

+F+

Pminus

+F+

Pminus

+Fminus

Pminus

+Fminus

Pminus

minusFminus

Pminus

minusFminus

Pminus

minusF+

Pminus

minusF+

Pminus

Figure 2 Difference of aggression level between males and females

Reproductively active voles especially lactating females wereevidently more aggressive usually attacking the opponentto escape in a hurry The difference between the aggres-sion counts of reproductive individuals and those of non-reproductive individuals was found to be significant (119905 =11885 df = 50 119875 = 00026)

35 Body Weight and Aggression Level Heavy voles showedhigh incidences of acts of aggression The smaller oneinvariably gives up first The heavier individual in a dyadicencounter usually became the dominant and the smallerone became the subordinate Body weight was significantlypositively correlated with aggression counts (119903 = 0375737119899 = 52 119875 = 001)

36 Aggressive Behavior Strategies under Different TreatmentsThe aggressive behavior pattern of root voles under differenttreatments (Table 2) was consistent with the prediction froman ESS hawk-dove model that is the proportion of ldquohawksrdquoin root vole populations under unfavorable conditions (+PminusF)wasmuch higher than that of root vole populations underfavorable conditions (minusP +F) and the proportions of ldquohawksrdquowere at intermediate level under mediate environments (minusPminusF +P +F)The aggressiveness of root vole populations underdifferent treatment was different significantly (119865 = 79909119899 = 20 119875 = 001)

4 Discussion

Theexperimental results supported Smith andPricersquos hypoth-esis and Adams and Mesterton-Gibbonsrsquos hypothesis how-ever did not support Wynn-Edwardsrsquo hypothesis Moreoverwe found that reproductively active individuals were moreaggressive

Table 2 Proportion of hawks in the populations of root voles underdifferent treatments

Treatments minusP +F +P +F minusP minusF +P minusFProportion of hawks 017 018 033 040

In our factorial experiment bodyweight was significantlypositively correlatedwith aggression counts with heavier andreproductively active root voles showing significantly higherincidences of aggressive acts (119903 = 0376 119899 = 52 119875 = 001)which supportedAdams andMesterton-Gibbonsrsquo hypothesis

The conventional agonistic behavior of root voles wasritualized noninjurious behavior which was consistent withthe conclusion of Smith and Price [17] and Lendrem [29]An ESS is always a mix of hawks and doves [17] ESSgame model predicts that when the value of the resource isgreater the proportion of ldquohawksrdquo in the population is higherOur experimental results supported this hypothesis Underthe most unfavorable conditions (+P minusF) when resourcesare most valuable winning a ldquowarrdquo is most important forforaging food defending territory protecting offspring andso forth and hence the highest proportion of ldquohawksrdquo inthe population on the other hand under the most favorableconditions (minusP +F) the lowest proportion of ldquohawksrdquo in thepopulation (Table 2) and the difference were significant

The difference in the aggression counts between repro-ductively active individuals and those of non-reproductivelyactive individuals was found to be significant (119905 = 11885df = 50 119875 = 00026) which was consistent with the resultsof Turner and Iverson [11] Fairbairn [30] and Reich etal [14] that is hormone level in an animal influencing itsaggressiveness

The experimental results showed that the aggression levelof dispersers was significantly lower than that of residentswhich agreed with the results of Fairbairn [30] but differedfrom that observed in the beach vole (Microtus breweri)[14] a species related to the root vole In a wild beachvole population dispersers and residents exhibited similaraggressive behavior patterns The cause of this phenomenonmight be that beach vole dispersers were a random sample ofthe resident population in terms of sex ratio and age structurewhereas in our study and the findings of Fairbairn [30]almost all dispersers were subadults which were subordinateindividuals in the population Subordinate males were lessaggressive and they disperse in response to social pressureduring the breeding season [30]

To summarize external environmental factors such aspredation and food treatment as well as internal conditionssuch as body weight and reproductive activity were moreimportant than density and gender in determining aggressionlevels of root vole populations

Acknowledgments

The authors wish to express their sincere appreciation to DrJianping Su Dr Meicai Wei Dr Wei Liu and Mr YumingLi for their help in the course of field experiment anddata analysis and to Dr Ben Bravery for his help in paper

6 ISRN Ecology

editingThe research is completed under the financial supportof the National Natural Science Foundation no 3870345and 30270242 and the Central South Forestry UniversityFoundation for the Introduction of High-Level-Talents no101-0630

References

[1] V C Wynn-Edwards Animal Dispersion in Relation To SocialBehavior Oliver and Boyd London UK 1962

[2] R M F Sadleir ldquoThe relationship between agonistic behaviorand population changes in the deermouse Peromyscus manic-ulatus (Wagner)rdquo Journal of Animal Ecology vol 34 no 2 pp331ndash352 1965

[3] M C Healey ldquoAggression and self-regulation of population sizein deer micerdquo Ecology vol 48 no 3 pp 377ndash392 1967

[4] C J Krebs B L Keller andRH Tamarin ldquoMicrotuspopulationbiology demographic changes in fluctuating populations ofMochrogaster and M pennsylvanicusrdquo Ecology vol 50 no 4 pp587ndash607 1969

[5] C J Krebs ldquoMicrotus population biology behavioral changesassociated with the population cycle in M ochrogaster and Mpennsylvanicusrdquo Ecology vol 51 no 1 pp 34ndash52 1970

[6] B L Keller and C J Krebs ldquoMicrotus population biologyIII Reproductive changes in fluctuating populations of Mochrogaster and M pennsylvanicus in southern Indiana 1965ndash67rdquo Ecological Monographs vol 40 no 3 pp 263ndash294 1970

[7] W H Conley Behavior demography and competition in Micro-tus longicaudus and M mexicanus [PhD thesis] Texas TechUniversity Lubbock Tex USA 1971

[8] W H Conley ldquoCompetition between Microtus a behavioralhypothesisrdquo Ecology vol 57 no 2 pp 224ndash237 1976

[9] J R Vale C A Vale and J P Harley ldquoInteraction of genotypeand population number with regard to aggressive behaviorsocial grooming and adrenal and gonadal weight in male micerdquoCommunications in Behavioral Biology vol 6 no 2 pp 209ndash2211971

[10] W Z Jr Lidicker ldquoRegulation of numbers in small mammalpopulationsmdashHistorical reflections and a synthesisrdquo in Popu-lations of Small Mammals Under Natural Conditions vol 5pp 122ndash141 PymatunIng Laboratory of Ecology University ofPittsburgh Pittsburgh Pa USA 1978

[11] B N Turner and S L Iverson ldquoThe annual cycle of aggressionin male Microtus pennsylvanicus and its relation to populationparametersrdquo Ecology vol 54 no 5 pp 967ndash981 1973

[12] J E Hofmann L L Getz and B J Klatt ldquoLevels of maleaggressiveness in fluctuating population ofMicrotus ochrogasterandMicrotus pennsylvanicusrdquo Canadian Journal of Zoology vol60 no 5 pp 898ndash912 1982

[13] J E Hofmann and L L Getz ldquoLevels of female aggressivenessin fluctuating populations of Microtus ochrogaster and Mpennsylvanicusrdquo ActaTheriologica Sinica vol 24 no 2 pp 125ndash131 2004

[14] L M Reich K M Wood B E Rothstein and R H TamarinldquoAggressive behaviour of male Microtus breweri and its demo-graphic implicationsrdquo Animal Behaviour vol 30 no 1 pp 117ndash122 1982

[15] S Mihok ldquoChittyrsquos hypothesis and behavior in subarctic red-back volesClethrionomys gapperirdquoOikos vol 36 no 3 pp 281ndash295 1981

[16] M J Taitt and C J Krebs ldquoManipulation of female behaviourin field populations of Microtus townsendiirdquo Journal of AnimalEcology vol 51 no 2 pp 681ndash690 1982

[17] J M Smith and G R Price ldquoThe logic of animal conflictrdquoNature vol 246 no 5427 pp 15ndash18 1973

[18] E S Adams and M Mesterton-Gibbons ldquoAnimal contests asevolutionary gamesrdquo American Scientist vol 86 no 4 pp 334ndash341 1998

[19] H Nie and J Liu ldquoRegulation of root vole population dynamicsby food supply and predation a two-factor experimentrdquo Oikosvol 109 no 2 pp 387ndash395 2005

[20] W Xia Alpine Meadow Ecosystem Gansu Peoplersquos PublishingHouse Lanzhou China 1982

[21] J Liu X Wang and W Liu ldquoStudies on the nutritional ecologyof small herbivorous mammals patterns of food selection andresource utilization for root voles and Gansu pikasrdquo in AlpineMeadow Ecosystem Fasc 3 pp 111ndash124 Science Press BeijingChina 1991 (Chinese)

[22] L J Liu S Su L W Liu W X Wang N H Nie and L YLi ldquoField experimental studies on the multifactorial hypothesisof population system regulation for small rodents an analysisof effects of food availability and predation on populationdynamics of root volerdquo Acta Theriologica Sinica vol 14 no 2pp 117ndash129 1994 (Chinese)

[23] J Liang J Zeng ZWang and YHan ldquoStudy on the growth anddevelopment of root volesrdquo Acta Plateau Biology Sinica vol 1no 2 pp 195ndash206 1982

[24] D L Otis K P Burnham G C White and D R AndersonldquoStatistical inference from capture data on closed animal popu-lationsrdquoWildlife Monographs vol 62 no 1 pp 1ndash135 1978

[25] J L Smart ldquoUndernutrition and aggressionrdquo in Multidisci-plinary Approaches To Aggression Research pp 179ndash191 Ams-terdam Elsevier North-Holland Biomedical Press AmsterdamThe Netherlands 1981

[26] M J Taitt and C J Krebs ldquoPopulation dynamics and cyclesrdquoin Biology of New World Microtus pp 567ndash620 The AmericanSociety of Mammalogists Shippensburg Pa USA 1985

[27] E ADesy GO Batzli and J Liu ldquoEffects of food and predationon behaviour of prairie voles a field experimentrdquoOikos vol 58no 2 pp 159ndash168 1990

[28] G A Parker ldquoEvolutionarily Stable Strategiesrdquo in BehaviouralEcology An Evolutionary Approach pp 30ndash61 Sinauer Asso-ciates Sunderland Mass USA 1984

[29] D W LendremModeling in Behavior Ecology An IntroductoryText Columbia University Press New York NY USA 1984

[30] D J Fairbairn ldquoBehaviour of dispersing deer mice (Peromyscusmaniculatus)rdquo Behavioral Ecology and Sociobiology vol 3 no 3pp 265ndash282 1978

Submit your manuscripts athttpwwwhindawicom

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental and Public Health

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcosystemsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Marine BiologyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental Chemistry

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Waste ManagementJournal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

6 ISRN Ecology

editingThe research is completed under the financial supportof the National Natural Science Foundation no 3870345and 30270242 and the Central South Forestry UniversityFoundation for the Introduction of High-Level-Talents no101-0630

References

[1] V C Wynn-Edwards Animal Dispersion in Relation To SocialBehavior Oliver and Boyd London UK 1962

[2] R M F Sadleir ldquoThe relationship between agonistic behaviorand population changes in the deermouse Peromyscus manic-ulatus (Wagner)rdquo Journal of Animal Ecology vol 34 no 2 pp331ndash352 1965

[3] M C Healey ldquoAggression and self-regulation of population sizein deer micerdquo Ecology vol 48 no 3 pp 377ndash392 1967

[4] C J Krebs B L Keller andRH Tamarin ldquoMicrotuspopulationbiology demographic changes in fluctuating populations ofMochrogaster and M pennsylvanicusrdquo Ecology vol 50 no 4 pp587ndash607 1969

[5] C J Krebs ldquoMicrotus population biology behavioral changesassociated with the population cycle in M ochrogaster and Mpennsylvanicusrdquo Ecology vol 51 no 1 pp 34ndash52 1970

[6] B L Keller and C J Krebs ldquoMicrotus population biologyIII Reproductive changes in fluctuating populations of Mochrogaster and M pennsylvanicus in southern Indiana 1965ndash67rdquo Ecological Monographs vol 40 no 3 pp 263ndash294 1970

[7] W H Conley Behavior demography and competition in Micro-tus longicaudus and M mexicanus [PhD thesis] Texas TechUniversity Lubbock Tex USA 1971

[8] W H Conley ldquoCompetition between Microtus a behavioralhypothesisrdquo Ecology vol 57 no 2 pp 224ndash237 1976

[9] J R Vale C A Vale and J P Harley ldquoInteraction of genotypeand population number with regard to aggressive behaviorsocial grooming and adrenal and gonadal weight in male micerdquoCommunications in Behavioral Biology vol 6 no 2 pp 209ndash2211971

[10] W Z Jr Lidicker ldquoRegulation of numbers in small mammalpopulationsmdashHistorical reflections and a synthesisrdquo in Popu-lations of Small Mammals Under Natural Conditions vol 5pp 122ndash141 PymatunIng Laboratory of Ecology University ofPittsburgh Pittsburgh Pa USA 1978

[11] B N Turner and S L Iverson ldquoThe annual cycle of aggressionin male Microtus pennsylvanicus and its relation to populationparametersrdquo Ecology vol 54 no 5 pp 967ndash981 1973

[12] J E Hofmann L L Getz and B J Klatt ldquoLevels of maleaggressiveness in fluctuating population ofMicrotus ochrogasterandMicrotus pennsylvanicusrdquo Canadian Journal of Zoology vol60 no 5 pp 898ndash912 1982

[13] J E Hofmann and L L Getz ldquoLevels of female aggressivenessin fluctuating populations of Microtus ochrogaster and Mpennsylvanicusrdquo ActaTheriologica Sinica vol 24 no 2 pp 125ndash131 2004

[14] L M Reich K M Wood B E Rothstein and R H TamarinldquoAggressive behaviour of male Microtus breweri and its demo-graphic implicationsrdquo Animal Behaviour vol 30 no 1 pp 117ndash122 1982

[15] S Mihok ldquoChittyrsquos hypothesis and behavior in subarctic red-back volesClethrionomys gapperirdquoOikos vol 36 no 3 pp 281ndash295 1981

[16] M J Taitt and C J Krebs ldquoManipulation of female behaviourin field populations of Microtus townsendiirdquo Journal of AnimalEcology vol 51 no 2 pp 681ndash690 1982

[17] J M Smith and G R Price ldquoThe logic of animal conflictrdquoNature vol 246 no 5427 pp 15ndash18 1973

[18] E S Adams and M Mesterton-Gibbons ldquoAnimal contests asevolutionary gamesrdquo American Scientist vol 86 no 4 pp 334ndash341 1998

[19] H Nie and J Liu ldquoRegulation of root vole population dynamicsby food supply and predation a two-factor experimentrdquo Oikosvol 109 no 2 pp 387ndash395 2005

[20] W Xia Alpine Meadow Ecosystem Gansu Peoplersquos PublishingHouse Lanzhou China 1982

[21] J Liu X Wang and W Liu ldquoStudies on the nutritional ecologyof small herbivorous mammals patterns of food selection andresource utilization for root voles and Gansu pikasrdquo in AlpineMeadow Ecosystem Fasc 3 pp 111ndash124 Science Press BeijingChina 1991 (Chinese)

[22] L J Liu S Su L W Liu W X Wang N H Nie and L YLi ldquoField experimental studies on the multifactorial hypothesisof population system regulation for small rodents an analysisof effects of food availability and predation on populationdynamics of root volerdquo Acta Theriologica Sinica vol 14 no 2pp 117ndash129 1994 (Chinese)

[23] J Liang J Zeng ZWang and YHan ldquoStudy on the growth anddevelopment of root volesrdquo Acta Plateau Biology Sinica vol 1no 2 pp 195ndash206 1982

[24] D L Otis K P Burnham G C White and D R AndersonldquoStatistical inference from capture data on closed animal popu-lationsrdquoWildlife Monographs vol 62 no 1 pp 1ndash135 1978

[25] J L Smart ldquoUndernutrition and aggressionrdquo in Multidisci-plinary Approaches To Aggression Research pp 179ndash191 Ams-terdam Elsevier North-Holland Biomedical Press AmsterdamThe Netherlands 1981

[26] M J Taitt and C J Krebs ldquoPopulation dynamics and cyclesrdquoin Biology of New World Microtus pp 567ndash620 The AmericanSociety of Mammalogists Shippensburg Pa USA 1985

[27] E ADesy GO Batzli and J Liu ldquoEffects of food and predationon behaviour of prairie voles a field experimentrdquoOikos vol 58no 2 pp 159ndash168 1990

[28] G A Parker ldquoEvolutionarily Stable Strategiesrdquo in BehaviouralEcology An Evolutionary Approach pp 30ndash61 Sinauer Asso-ciates Sunderland Mass USA 1984

[29] D W LendremModeling in Behavior Ecology An IntroductoryText Columbia University Press New York NY USA 1984

[30] D J Fairbairn ldquoBehaviour of dispersing deer mice (Peromyscusmaniculatus)rdquo Behavioral Ecology and Sociobiology vol 3 no 3pp 265ndash282 1978

Submit your manuscripts athttpwwwhindawicom

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental and Public Health

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcosystemsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Marine BiologyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental Chemistry

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Waste ManagementJournal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

Submit your manuscripts athttpwwwhindawicom

Forestry ResearchInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental and Public Health

Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EcosystemsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Marine BiologyJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Environmental Chemistry

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Waste ManagementJournal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

BiodiversityInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of