Zol415, 2004, NatalieDuboisTalkGustafsson & Pärt 1990 Clutch size enlarged control Gustafsson, L.,...

Zol415, 2004, NatalieDuboisTalk

1

Opportunities for polygyny and reproductive investment in house wrens

Natalie S. Dubois

Departments of Zoology &Ecology, Evolutionary Biology and BehaviorKellogg Biological StationMichigan State University

J.J. Audubon

Investment in reproductionBenefits

decreased future reproduction or survival

gameteseggs

young

Costs

increased fitness from current attempt

An individual’s fitness depends on both the number and quality of offspring it is able to produce over its lifetime

INVESTMENT INCURRENT REPRODUCTION

INVESTMENT INFUTURE REPRODUCTIONOR SURVIVAL

TRADE-OFF

Female collared flycatchers with experimentally enlarged broodslay smaller clutches later in life

Gustafsson & Pärt 1990

Clu

tch

size

enlarged

control

Gustafsson, L., and T. Pärt. 1990. Nature 347:279-281.

Experimentally removing second broods increases survival and fecunditythe following year in female great tits

Verhulst 1998

Verhulst, S. 1998. Functional Ecology 12:132-140.

Control RemovedManipulation second clutch

No.

of f

ledg

lings

nex

t yea

r

0

2

4

6

8

Surv

ival

Manipulation of second clutch

Control Removed0

0.4

0.8


2

Under this trade-off we would expect increased opportunities for additional matings to result in decreased investment in the current reproductive attempt

For polygamous or multi-brooded species, future costs can be almost immediate if investment in the current reproductive attempt decreases opportunities for additional matings

Investment in reproductionBenefits

Costs


INVESTMENT INFUTURE REPRODUCTIONOR SURVIVAL

TRADE-OFF

mate attractio

n

opportunity

Smith, H. G. 1995. Proceedings of the Royal Society of London Series B 260:45-51.

mal

e pr

opor

tion

of in

cuba

tion

0

0.2

0.4

single box

doublebox

prop

ortio

n of

tim

e m

ale

sang

du

ring

incu

batio

n pe

riod

0

0.2

0.1

single box

doublebox

Male European starlings with a surplus cavity added to their territories contributed less parental care but increased mate attraction efforts

Smith 1995

investment incurrent reproduction

investment in future reproduction

TRADE-OFF

Previous studies have manipulated female investment in current reproduction and demonstrated trade-offs between current and future reproduction…

…ignoring effects of mate choice and paternal investment on female investment



TRADE-OFF

mate attractio

n

opportunity

And shown that male trade-offs between current and future reproduction can be mediated by mate attraction opportunity…

…ignoring effects on female investment in reproduction


3



TRADE-OFF

mate attractio

n

opportunity

female investment in reproduction

MALES

FEMALES

How do opportunities for polygyny affect

1. male investment in current and future reproduction?

2. female mate choice and investment in reproduction?

Field Guide to the Birds of North America (1994)

HOUSE WRENS

North American distribution

BREEDING SEASON

END APRIL males returnestablish breeding territoriesstart building nests

HOUSE WRENS2° cavitynesters facultatively

polygynous

song to attract matesfemales return

prospect among malesexamine nest siteschoose matecomplete nest

MAY line nestlay clutch (5-7 eggs)incubate

maternal investment

both parents provision young parental investment

JUNE young fledge

second brood

AUGUST end of breeding season


4

Manipulating availability of surplus cavities

pre-arrival

male arrival

first broods

1B

3B

Pre-pairing treatment

rand

omiz

e

available nest box 1B

plugged nest boxes

start of incubation

laying period

FEMALE MATE CHOICEREPRO. INVESTMENT

female arrivalAPRIL AUG

incubation and nestling periods

MALE TRADEOFFSCURRENT VS FUTURE

1B

3B

Post-laying treatment

re-ra

ndom

ize

second broods

MALE INVESTMENT

FEMALE INVESTMENT

1B

3B

Post-laying treatment

re-ra

ndom

ize

Pre-pairing treatment

1B

3B




Conceptual framework: male trade-offs between current and future reproduction

Availability of surplus cavities affects male opportunity to acquire additional mates

Polygyny increases male reproductive success

Male parental care affects reproductive success


INVESTMENT INFUTURE REPRODUCTION


5


Few opportunities foradditional matings

Mate attraction

Parental care




Many opportunities for additional matings

Mate attraction

Parental care



Mate attraction

Parental care




Recorded male song for 30 minutes during incubation and nestling periods

Observed paternal provisioning rates during nestling period

Predictions: Male provisioning and mate attraction

During first broods, males with access to surplus cavities willincrease mate attraction effort and provision at lower rates relative to males with access to single cavities

surplus single

Mal

e pr

ovis

ioni

ng ra

te

Mat

e at

tract

ion

effo

rtsurplus single

Early season nests


6

0

0.1

0.2

0.3

0.4

0.5

IncubationPeriod

Nestling Period

mean ± se

Song

rate

inde

x

surplus

single

Male song rates (early season nests only)

Males with surplus cavities in their territories sang morethan males with single cavities in their territories.

Post-laying treatment P < 0.03Time × treatment P < 0.08

0

0.5

1

4 8 12

Prop

ortio

n of

mal

es p

rovi

sion

ing

nest

lings

Brood day

Male provisioning rates (early season nests only)

There were no differences in the proportion of males provisioningnestlings between males with surplus cavities and single cavities in their territories.

surplussingle

1.5

2

2.5

4 8 12

Mal

e pr

ovis

ioni

ng tr

ips

per n

estli

ng

Brood day

mean ± se

Male provisioning rates (early season nests only)

Of males that provisioned, there were no differences in provisioning ratesbetween males with surplus cavities and single cavities in their territories.

surplus

single

Trade-offs might occur earlier in the nestling period

Male removal studies show that paternal care is most critical when females are brooding young and removal after that period has little effect on reproductive success (Johnson et al. 1992, Bart and Tornes 1989)

Male investment in mate attraction and provisioning effort

Males with surplus cavities added to their territories

Sang more during the nestling periodbut did not provision less

than males with single cavities in their territories

1

2 Provisioning at early season nests might be costly regardless of mate attraction opportunity

Males that care for young after they leave the nest are less likely to rear second broods (Bart 1990)

Provisioning might be costly if paternal care affects probability of rearing a second brood


7

0 0.5 1

Did not provisionon brood day 12

Provisioned on brood day 12

Proportion of males rearing a second brood

P > 0.1

76%

56%

Firs

t bro

od p

rovi

sion

ing

effo

rt

Male second broods

0

0.5

1

4 8 12

Prop

ortio

n of

mal

es p

rovi

sion

ing

nest

lings

Brood day

Male provisioning rates

More males provisioned late season nests for the entire nestling period than early season nests.

late season

early season

BD 12: P < 0.001

surplus

single




If males provide benefits to females access to resourcesgenetic quality

Females should preferentially matewith high quality males mate choice

Female investment

Benefits affect survival or fecundity of females

e.g., territory quality (nest site, food availability),parenting ability (provisioning, incubation)

Female house finches gain direct benefits by mating with brightly colored males

Bright males feed more than dull males

Hill 1991, Hill et al. 1994


8

Females should preferentially matewith high quality males mate choice

Female investment

Benefits affect fitness of offspring

e.g., survival (genetic quality)future reproductive success (attractiveness)

If males provide benefits to females access to resourcesgenetic quality

Female barn swallows gain indirect benefits by mating with long-tailed males

Females mated to long-tailed males have higher reproductive successeven though long-tailed males provide less parental care

Møller 1988, 1994, deLope & Møller 1993

We might expect females mated to preferred males to invest more in reproduction differential access

differential allocationBurley (1986)

Female investment

differential accessHigher quality females might have preferential access to preferred males

differential allocationFemales mated to preferred males might allocate more resources to reproduction

Clutch size Peafowl Petrie & Williams (1993)

Egg size Mallard Cunningham & Russell (2000)

Testosterone Zebra finch Gil et al. (1999)in eggs

Examples of differential allocation by females


Female preferences for males with single cavity territories

Polygyny affects male investment in parental care

Parental care negatively correlated with surplus cavities

For females, cavity availability could signal direct benefits

Conceptual framework:female investment (direct benefits)

Negative trade-off between polygyny and parental care


Competition among males for high quality territories

(more nest sites)

High quality males positively correlated with surplus cavities

For females, cavity availability could

signal male quality

Conceptual framework:female investment (indirect benefits)

Female preferences for males with surplus cavity territories

If there is no trade-off between polygyny and parental care, high quality males might also provide greater parental care


9

Checked nest boxes to determinedate of male territory establishmentfemale settlement (line date as index)

Calculated time unmated for each male= measure of female mate choice

Clutch size as a measure of maternal investment

Female investment Predictions: Female settlement/investment

surplus single

Tim

e m

ale

unm

ated

(fem

ale

mat

e ch

oice

)

surplus single

Clu

tch

size

Direct benefits important to females and negative trade-off between polygyny and parental care

Females will preferentially settle with males defending single cavities and females mated to males with single cavities will invest more in reproduction.

Predictions: Female settlement/investment

Females will preferentially settle with males defending surplus cavities and females mated to males with surplus cavities will invest more in reproduction.

surplus single

Tim

e m

ale

unm

ated

(fem

ale

mat

e ch

oice

)

surplus single

Clu

tch

size

Indirect benefits important to females and/or male quality correlated with parental care

-0.5

-0.25

0

0.25

0.5

surplus single

Stan

dard

ized

log(

days

unm

ated

)

mean ± se

Timing of female settlement (early season nests only)

There was no difference in the time between male arrival and nest lining for males with surplus cavities and single cavities in their territories.


10

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

Early season Late season

Stan

dard

ized

clu

tch

size

mean ± se

surplus

single

P < 0.02

Female investment in clutch size

At early season nests, females mated to males with surplus cavities in their territories laid larger clutchesthan females mated to males with single cavities in their territories.

Female investment in clutch size

Perhaps,Higher quality females pair with males with surplus cavities

Females mated to males with surplus cavities allocate more to reproduction

surplus single

Clu

tch

size Differential access

Differential allocation

Females in better condition might have access to preferred males (or territories)

Females mated to preferred males might invest more in reproduction

Additionally, females mated to preferred males might bias offspring sex ratios

Sex allocation theory predicts that females should bias the sex ratio of their offspring towards the sex with the higher reproductive value.

Differences in reproductive value for male and female offspring might occur when:

•One sex has higher variance in reproductive success •Resource availability differentially affects reproductive success of males and females (e.g., one sex is more costly)

•Sex-specific competition exists between parents and offspring

Female blue tits skew the sex ratio of their offspring in response to the UV plumage ornamentation of their mates, a preferred trait and potential viability indicator.

(Sheldon 1999)

When food availability was restricted, female zebra finches biased sex ratios towards males. (Kilner 1998)[Females are assumed to be the costlier sex]

Thus offspring sex ratios might vary with factors such as parental condition, attractiveness of mates, or sex-specific natal philopatry.

Sex ratio manipulation in house wrens

The Trivers-Willard hypothesis

Sex of the offspring produced should be associated with the quality of the parents’ breeding situation.

In polygynous systems, males in better than average condition as adults might enjoy larger fitness gains than females in better condition as adults.

Reverse might be true under poor conditions.

Females in better condition produce more males. Male bias is due to production of male-biased second broods.

(Whittingham et al. 2003)

Females bias the sex of the last-hatched young towards females. Hatching is asynchronous, but fledging is synchronous. The last-hatched young is at a competitive disadvantage relative to its broodmates. (Albrecht 2000)

Secondary females bias sex ratios towards females. Second-mated females receive little or no paternal assistance and fledge fewer and lower quality young than first-mated females. (Albrecht and Johnson 2002)


11

Sex ratio manipulation in house wrens

Females in higher quality breeding situations bias offspring towards sons

females is lower quality breeding situations bias offspring towards daughters

Prediction: Females mated to males with surplus cavities in their territories will bias brood sex ratios towards males

Offspring sex identification

Collected blood samples from nestlings on brood day 12.

Used a PCR-based technique to amplify homologous sections of the CHD-1 gene on the avian sex chromosomes.

females (ZW) males (ZZ)

[Sexed 247 offspring from 43 early season nests]

1 2 3 4 5 6 7

500

F M

0.25

0.5

0.75

mean ± se

singlesurplus

Prop

ortio

n so

ns

Female investment in offspring sex ratios

At early season nests, females mated to males with surplus cavities in their territories produced clutches with higher sex ratios (proportion males)than females mated to males with single cavities in their territories.

P < 0.02

Suggests that the reproductive value of sons and daughters differs for females mated to males with surplus and single cavities

Within a brood, male nestlings were larger than female nestlings on brood day 12.

33.9 ± 0.3834.5 ± 0.30Wing chord (mm)

19.2 ± 0.0919.5 ± 0.08Tarsus (mm)

10.2 ± 0.1210.5 ± 0.10Mass (g)

Female nestlingsMale nestlingsMORPHOLOGICAL VARIABLE

MANOVA P < 0.03


12

1

1.5

2

2.5

3

3.5

4 8 12

mean ± seFem

ale

prov

isio

ning

trip

s pe

r nes

tling

Brood day

surplussingle

Female investment in provisioning

There were no differences in per-nestling provisioning rates between females that settled in single cavity and surplus cavity territories.

Female investment in offspring sex ratios

Differential accessObserved sex ratio bias could be due to differences in female conditionbeing correlation with cavity availability

• Females in good condition might be able to invest in the costlier sex

• Or good condition sons might have higher reproductive value than good condition daughters

Differential allocationObserved sex ratio bias could be a response to cavity availability (potential signal of male quality)

• Females mated to preferred males might invest in sons if attractiveness or viability is inherited from fathers, and those characters influence sons more than daughters.

1

2

Both suggest that females prefer males with surplus cavities in their territories

Why would females mated to males with surplus cavities invest more in reproduction but not settle earlier?

Nest lining might be a poor index of female settlement date

High search costsmight risk losing primary status by continuing searchsecondary females pay high costs of polygyny (Johnson et al. 1993, 1994)

Multiple cues used in mate choice and reproductive investmentmight use other male and/or territory characters in mate choicebut cavity availability in investment decisions

1

2

3

Summary

Males with surplus cavities added to their territories sang more during the nestling period

Cavity availability had no effect on paternal provisioning

No demonstrated trade-off between mate attraction and paternal provisioning

• High song rates consistent with mate attraction effort

• Provisioning at early season nests could be costly regardless of cavity availability

• Provisioning effort could be related to male quality, which was randomized across cavity availability


13

Summary

First brood females mated to males with surplus cavities laid larger clutches and biased offspring towards males

Suggests higher reproductive investment by females settling in territories with surplus cavities

Females might use cavity availability to assess male quality

• Higher quality males might have greater access to territories containing surplus cavities (female choice for indirect benefits)

• Higher quality males able to secure territories with surplus nest sites under natural conditions might also contribute more paternal care (female choice for direct benefits)

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