SHORT COMMUNICATIONS 965

Serpell, J. A. 1981. Duets, greetings and tr iumph ceremonies: analogous displays in the parrot genus Trichoglossus. Z. Tierpsychol., 55, 268-283.

Wickler, W. 1980. Vocal dueting and the pair-bond: I. Coyness and partner commitment, a hypothesis. Z. Tierpsychol., 52, 201-209.

Wickler, W. & Seibt, U. 1980. Vocal dueting and the pair- bond: II. Unisono dueting in the African Forest Weaver, Symplectes bicolor. Z. Tierpsychol., 52, 217-226.

Wilkinson, R. & Howse, P. E. 1975. Time resolution of acoustic signals by birds. Nature, Lond., 258, 320-321.

(Received 5 April 1981 ; revised 13 May 1981 ; MS. number: sc-103)

'Sailing' Behaviour in the Fishing Spider, Dolomedes triton (Walckenaer)

When separated from fixed substrata, aquatic arthropods are faced with two immediate problems. First, on moving water the organism must expend energy to manoeuvre against the current or risk detachment from favourable surroundings. Second, without cover, overt swimming and skating movements may increase the probability of de- tection by predators. Here I examine a possible evolution- ary solution to these problems.

Wind-assisted dispersal in spiders is well documented (Comstock 1913; Savory 1977) and has been observed in small adults (e.g. Linyphiidae and Erigonidae) ~ts well as in early instars (van Wingerden & Vugts 1974; Homer 1975; Salmon & Homer 1977). Seasonal trends in aerial activity may occur in many species (Salmon & Homer 1977). Although biotic, density-dependent, and micro- climatic factors for aeronautic behaviour in spiders re- quire clarification, it is apparent from the literature that spiders may respond and orient aeronautically to specific ranges of temperature, air stability, and wind velocity (Duffey 1956; van Wingerden & Vugts 1974). I report that a species of fishing spider, Dolomedes triton (Walckenaer), may use air currents as a source of propulsion across open water surfaces.

Fishing spiders, D. triton, are common among floating foliage and debris of lentic habitats over much of the United States. These spiders are extremely light for their size, with arrangements of tarsal hairs that may contribute to buoyancy and motility on water (Gertsch 1979). Thus adults may exceed body lengths of 3 cm without penetra- tion of surface film. Prey include aquatic invertebrates, tadpoles, and small fish.

Individuals of D. triton were observed in the field and collected at Pendleton Swamp, Pendleton, South Carolina. During afternoons when winds ranged from 4 to 13 mph, I noted that fishing spiders which had been dislodged from substrata or dropped onto open waters frequently elevated their second pairs of legs approxi- mately 70 to 90 ~ to the water surface. Because of pre- vailing air movements, individuals displaying this be- haviour invariably were blown across the water until contact with a substratum was re-established. Appendages then were lowered. The leg-lifting response was not ob- served when spiders were netted and were dropped onto or within a few centimetres of a surface object.

To examine possible causes of the described response, fishing spiders again were netted during windless after- noons and alternately were placed onto floating debris and onto open water. Under both conditions, leg-lifting behaviour was not observed. However, when air was blown through a 5-cm-diameter cardboard cylinder from

a distance of 0.5 m, spiders on open water responded by lifting the second pair of legs and were conveyed across the surface and against existing currents. D. triton o n floating leaves and on debris did not exhibit this behaviour and often responded with body compression against the substratum.

The results obtained in the field were investigated further in the laboratory. Ten spiders were collected, in- cluding two late instar males and eight females of middle instar development with body sizes ranging from 0.7 to 1.6 cm. Spiders wereplaced into filled aquaria, and each spider was positioned alternately onto floating cardboard discs anchored to the tanks, onto unanchored discs, and onto open water. Surface ripples were generated to simu- late water movements in the field. Discs were uniform in colour, in texture, and in diameter (3 cm). Each individual was exposed to low-velocity compressed air of equal force and duration from a distance of 0.5 m. Under uniform lighting, 20 trials were performed for each set of experimental conditions. Every individual w a s tested twice under each substrate condition;

The leg-lifting response was observed in 85 ~ (17 of 20) of the trials with D. triton in open water conditions, in 15 ~ (3 of 20) of the trials with unanchored discs, and in none of the trials with spiders on anchored substrata. Following exposure to air currents, 65 ~ (13 of 20) of the spiders on anchored discs and 45 ~ (9 of 20) of the spiders on unanchored discs exhibited body compression. Statistical tests for equality of proportions indicated that leg-lifting responses occurred significantly more often (P < 0.01, one-tailed) on open water than they occurred on anchored and on unanchored substrata. Frequencies of leg-lifting behaviour were significantly different at the 9 5 ~ level for anchored and unanchored substrata. However, there was no significant difference (P > 0.05) between body compression responses on anchored and on unanchored cardboard discs. No differences existed between sexes or among instars.

Results of these field and laboratory experiments suggest that leg-lifting behaviour in D. triton may be stimulated strongly by simultaneous exposure to open water and to extended air movement. These data further suggest that elevation of the second pair of legs effectively increases the wind-catching capabilities of the organism without altering the spider,s basal area of flotation and, therefore, without impairing its stability on water.

Besides energy-conservative dispersal, the described be- haviour enables fishing spiders to glide rapidly across open water and against currents to the protection of foliage and debris. This movement is accomplished with minimal leg movement and with minimal rippling, both of which could alert predators. By reducing conspicuous- ness and by diminishing time spent on unprotected, un- camouflaged surfaces, susceptibility to predators (i.e. fish and waterfowl) may be minimized. I thus suspect that the reported response in fishing spiders may be a 'sailing' behaviour enabling the organism to use air currents as a source of conveyance in open waters. This behaviour may function as an anti-predator mechanism and to minimize energy expenditures during movements on open water.

I wish to thank the Deshefy-Sekora family. G. SCOTT DESHEFY

Department of Zoology, Clemson University, Clemson, S.C. 29631, U.S.A.

References Comstock, J. H. 1913. The Spider Book, pp. 215-217.

New York: Doubleday, Page & Co.

966 A N I M A L B E H A V I O U R , 2 9 , 3

Duffey, E. 1956. Aerial dispersal in a known spider population. J. Anim. Ecol., 25, 85-111.

Gertsch, W. J. 1979. American Spiders. New York: Van Nostrand Reinhold.

Homer, N. V. 1975. Annual aerial dispersal of jumping spiders in Oklahoma. J. Arachnol., 2, 101-105.

Salmon, J. T. & H o r n e r , N. V. 1977. Aerial dispersion of spiders in North Central Texas. J. ArachnoL, 5, 153-157.

Savory, T. H. 1977. Arachnida, pp. 83-87. London: Academic Press.

van Wingerden, W. K. R. E. & Vugts, Hans F. 1974. Factors influencing aeronautic behaviour of spiders. Bull. Br. Arach. Soc., 3(1), 6-10.

(Received 4 February 1981 ; revised 17 April 1981 ; MS. number: AS-122)

The Possible Communicative Role of Tail-flicking in White-tailed Deer

White-tailed deer (Odocoileus virginianus) possess a white rump patch and underside of tail which is concealed when the tail is down. When the tail is flicked from side to side, the rump patch is momentarily exposed, producing a conspicuous flash of white. During a study of free- ranging white-tailed deer in rural southeastern Indiana (LaGory 1978), the apparently non-random occurrence of tail flicks in behaviour sequences was noted. Tail- flicking may thus be an important visual communicatory signal. The hypothesis that tail flicks occur in a particular behavioural context was tested to elucidate this com- municative role.

The occurrence of tail flicks during and at the end of behaviour bouts (feeding, alert, walking, other as defined by LaGory (1978)) was recorded from 15 March to 13 June 1978 during 60-s focal-animal samples. The end of a bout was arbitrarily defined as the last 2 s of a bout. A Z 2 goodness-of-fit test was used to determine significance; expected number of bouts in which tail-flicking occurred was calculated from the total number of bouts for each behaviour and the total number of tail-flicking bouts observed.

Tail flicks were associated with all behaviour types and occurred during and at the end of bouts. The probability of tail flicks at the end of alert bouts, however, was far greater than expected (Table I; Z 2 = 139.48, df = 6, P < 0.001). Following tail flicks at the end of an alert bout, individuals either fed or walked a short distance before feeding. Flight behaviour was never observed after tail flicks. Other group members could, therefore, use tail- flicking as a reliable cue that the alert animal did not see any nearby disturbance. This cue would be of special value in dense habitats (which are regularly inhabited by white-tailed deer) because the downward movement of the

Table I. Probability of Tail-flicking as Related to Type or Behaviour and Position in Bout

Probability of tail-flicking* Number

Behaviour of bouts During bout End of bout

Alert 172 0.08 (13) 0.41 (71) Feed 161 0.07 (11) 0.09 (15) Walk 92 0.02 (2) 0.01 (1) Other 20 0.00 (0) 0.05 (1)

*Number of bouts in which tail flicks occurred is shown in parentheses.

head after an alert bout may be obscured and therefore unavailable as a cue.

Tail-flicking probably also facilitates group cohesion as animals forage in relatively dense habitats. Hirth & McCullough (1977) argued that North American open- country ungulates maintain group cohesion with the aid of permanently exposed white rump patches. Species in- habiting dense habitats (white-tailed deer; mule deer sub- species O. hemionus eolumbianus, O. h. sitkensis) can cover their rump patches, apparently to allow concealment from predators. If the rump patch remained covered, it could not function in group cohesion. Tail-flicking alleviates this problem and also may be more effective in dense vegetation where movement and flashes of white are more noticeable than a permanently exposed white rump.

I thank M. K. LaGory, D. H. Taylor, D. H. Hirth, and R. H. Yabner for critiques of earlier drafts of this manuscript. Manuscript preparation was supported under contract DE-AC09-76SROO819 between the University of Georgia and the U. S. Department of Energy.

KIRK E. LAGORY* Institute of Environmental Sciences, Miami University, Oxford, Ohio 45056, U.S.A.

*Present address: Savannah River Ecology Laboratory, P.O. Drawer E, Aiken, S.C. 29801.

References Hirth, D. H. & McCullough, D. R. 1977. Evolution of

alarm signals in ungulates with special reference to white-tailed deer. Am. Nat., 111, 31-42.

LaGory, K. E. 1978. White-tailed deer sociality with emphasis on doe-fawn association. M.En. thesis, Miami University.

(Received 2 March 1981 ; revised 8 April 1981 ; MS. number: AS-127)