11.1 sexual selection favors characteristics that enhance reproductive success primary sexual...

From Nordell and Valone, Animal Behavior: Concepts, Methods, and Applications, © 2014 by Oxford University Press

11.1 Sexual selection favors characteristics that enhance reproductive success

• Primary sexual characteristics– Genitalia and organs of reproduction

• Secondary sexual characteristics– Morphological differences between the sexes

that are not directly involved in reproduction


Bateman’s hypothesis

• Bateman’s hypothesis:– Female

reproductive success is most strongly limited by the number and success of eggs that she can produce, whereas male reproductive success is limited by the number of mates

Figure 11-9

Figure 11-10

Figure 11-11

Figure 11-12

Figure 10.8 Differences between the sexes in sexual behavior may arise from fundamental differences in parental investment that affect the rate at which individuals can produce offspring

Antlers as weapons in red deer

• Research question: Are antlers weapons used in mate competition? (Kruuk et al. 2002)


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• Methods:– Red deer (Cervus

elaphas)– Collected and

weighed antlers shed

– Used blood samples to determine reproductive success (paternity)

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• Results:– Males with larger

antlers had higher reproductive success

• Conclusion:– Sexual selection

has influenced antlers as an exaggerated trait in mate choice

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Weapon size and mating success in dung beetles

• Research question: What is the role of the male’s horn-like projection? (Pomfret & Knell 2006)

• Hypothesis: Large horns provide an advantage in mate competition

• Prediction: Males with the larger horn will win most fights


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• Methods:– Dung beetles (Euoniticellus intermedius)– Staged fights between males matched

for body size but not horn length– Recorded outcome of interactions and

mating success

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• Results:– Males with larger

horns won more fights

• Conclusion:– The maintenance of

long horns can be explained by sexual selection driven by male-male competition

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Ornaments and mate choice in peafowl

• Research question: Are peacock tails an ornament used in mate choice? (Loyau, Saint Jalme, & Sorci 2005)

• Males aggregate on leks, where they display to females


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• Methods:– Peacocks (Pavo cristatus)– Measured:

• Tail length and male body size• Number and duration of tail displays• Number of vocalizations and copulations

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• Results:– Larger males with

longer tails were most successful in defending a display site

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• Results:– Males performing the

most displays and with the greatest number of ocelli obtained the most copulations

• Conclusion:– Mate competition and

mate choice are important in evolution of peacock tail

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The origin of sexually selected traits: the sensory bias hypothesis in guppies

• Research question: How does a male trait become selected in female mate preference? (Rodd et al. 2002)

• Hypothesis: Sensory bias hypothesis – female mating preferences are a byproduct of preexisting biases in a female’s sensory system

• Prediction: Males and females should be attracted to orange-colored objects because orange is associated with carotenoids


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• Methods:– Guppies (Poecilia

reticulata)– Several populations– Placed small color

discs on leaf in water– Recorded all

approaches and pecks

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• Results:– Males and females were

attracted to orange and red discs

• Conclusion:– Female mating

preference could be linked to preexisting preference for orange food objects

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Jones and Hunter 1998


11.2 Females select males to obtain direct material benefits

• Direct material benefits– Material resources obtained by a female

from mating with a particular male

• Nuptial gift– A physical resource such as a food item

that a male provides to a female to enhance his mating success

Female choice and nuptial gifts in fireflies

• Research question: How does the variation in the duration of male flashes affect female choice and fitness? (Lewis, Cratsley, & Rooney 2004)


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• Methods:– Fireflies (Photinus

ignitus)– Measured male flash

duration and spermatophore size

– Created flashes of different lengths and measured female response

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• Results:– Males with

longer flash duration had larger spermatophores

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• Results:– Females responded more

to longer flashes

• Conclusion:– Females prefer to mate

with males with the longest flash duration and thus the largest spermatophores, which should increase fitness

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Female choice and territory quality in lizards

• Research question: How does territory quality affect female choice and fitness? (Calsbeek & Sinervo 2002)

• Territory quality is based on rockiness (microclimates)


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• Methods:– Side-blotched lizards (Uta stansburiana)– Larger males have territories with more

rocks – Manipulated territory quality

• Added rocks to small males’ territories• Removed rocks from large males’ territories

– Observed female mate choice– Measured egg-laying date and egg mass

of females

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• Results:– Females preferred improved

territories– Females on improved territories

laid eggs sooner and produced larger egg masses

• Conclusion:– Females select males based on

territory quality and derive a significant fitness gain by doing so

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11.3 Female mate choice can evolve via indirect benefits to offspring

• Indirect genetic benefits– Genetic benefits females can obtain for

their offspring by mating with males that have high genetic quality


Fisherian runaway and good genes

• Runaway process– An evolutionary process in which a male

trait co-evolves with a female preference for it and becomes increasingly exaggerated

• Handicap principle– Well-developed secondary sexual

characteristics are costly to survival but reliable signals of fitness

• Good genes– The alleles of high-quality individuals

Figure 11-32

Figure 16-14

Figure 16-15b

Sexual dimorphism

And behavior!

Long-tailed widowbird (male on left)

https://www.youtube.com/watch?v=koSvAh6za7Y

Mate choice for good genes in frogs

• Research question: How do females assess male quality? (Jacquiéry et al. 2009)

• Hypothesis: Females select males for genetic benefits based on their vocalizations

• Prediction: The most attractive males provide the best gene alleles (fitness) to offspring


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• Methods:– European tree frogs (Hyla arborea)– Captured and identified males

from four ponds– Observed number of matings– Collected egg masses and reared

in lab– Measured tadpole growth rate and

survival– Genotyped tadpoles to determine

parentage

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SONG video

https://www.youtube.com/watch?v=N7Zh4HGJJQU



• Results:– Only 10 of 15 males

sired offspring– Males that sired more

offspring produced tadpoles with higher growth rates

• Conclusion:– Females select males

for genetic benefits

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Good genes and immune system function in birds

• Research question: How does parasite load affect mate choice? (Zuk et al. 1990)

• Hypothesis: Hamilton-Zuk hypothesis– Parasites and pathogens play an important role in

sexual selection when secondary sexual traits are costly and condition-dependent

• Predictions:1. Females should prefer to mate with males that have

the greatest expression of secondary sexual traits2. High parasite loads will reduce that expression in

males


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• Methods:– Red jungle fowl (Gallus

gallus)– Infected half the males with

intestinal nematode– Measured size and color

intensity of comb – Conducted mate choice trials

with infected and controls (uninfected) males

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• Results:– Parasitized males had

smaller, duller combs than controls

– Females preferred control males

• Conclusion:– Support for Hamilton-Zuk

hypothesis

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Mate choice fitness benefits in spiders

• Research question: What is the importance of pheromones as signals of male quality in mate choice? (Koh et al. 2009)


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• Methods:– Spitting spider (Scytodes sp.)– Mate choice experiment in Y

test chamber• Phase 1: Two males at end of

arms (chemical cues only)• Phase 2: Pheromones of males

only at end of arms (on filter paper)

– Mating experiment• Half of females mated with

preferred male, half with nonpreferred male

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• Results:– In Phase 2, almost

all females chose the pheromone of the male they chose in Phase 1

– Females who mated with preferred males had larger egg sacs, more and heavier eggs, and a higher hatch rate

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• Conclusion:– Male pheromones may function to

convey information about male quality

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11.4 Sexual selection can also occur after mating

• Mate guarding– When a male follows his mate to prevent

her from mating with rivals – beetle video; fly video

• Extra-pair young– Offspring of a pair-bonded female

produced outside the pair bond by a third-party male

https://www.youtube.com/watch?v=YSXRWb_LjFk

https://vimeo.com/31026739

Mate guarding in warblers

• Research question: How effective is mate guarding? (Chuang-Dobbs, Webster, & Holmes 2001)


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• Methods:– Black-throated blue

warblers (Dendroica caerulescens)

– Observations:• Followed focal males and

measured distance between them and their mates

– Experiment:• Removed some males for

one hour during female’s fertile period

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• Results:– Males who spent more time

mate guarding had fewer extra-pair offspring

– Experimentally removed males had more extra-pair offspring

• Conclusion:– Mate guarding can be an

effective strategy for increasing paternity assurance

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Sexual selection after mating

• Sperm competition– Competition between sperm of different

males to fertilize eggs

• Cryptic female choice– When female influences the fertilization

success of sperm from one male over that of others

• Inbreeding depression– A reduction in fitness as a result of mating

with close relatives

Sperm competition in tree swallows

• Research question: How can males increase paternity assurance? (Crowe et al. 2009)

• Hypothesis: Paternity assurance can be enhanced through frequent copulations

• Prediction: An increase in copulation rate should result in an increase in paternity for a male


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• Methods:– Tree swallows

(Tachycineta bicolor)– Uniquely marked birds

and collected blood samples for genetic analysis

– Observed focal pairs for three hours per day for two weeks

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• Results:– Increases in copulation

rate resulted in decrease of extra- pair offspring

• Conclusion:– Males can reduce

paternity loss from sperm competition by increasing their copulation frequency

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Inbreeding avoidance via cryptic female choice in spiders

• Research question: Do females avoid inbreeding depression by cryptic female choice? (Welke & Schneider 2009)


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• Methods:– Orb spiders (Argiope lobata)– Experiment 1– Mating trials in which a

female mated sequentially with:• 1) Two sibling males (SS)• 2) Two nonsibling males (NN)• 3) Sibling and nonsibling male (SN or NS)

– Experiment 2 – Mating trials• One male of pair was made sterile

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• Results:– For second male:

• Females stored more sperm from nonsibling than from sibling

• More eggs fertilized

• Conclusion:– Females display cryptic female

choice in regard to matings with siblings, which should reduce inbreeding depression

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11.5 Mate choice by females favors alternative reproductive tactics in males

• Alternative mating tactics– Multiple behavioral mating phenotypes in a population

• Satellite male– Alternative, parasitic mating tactic in which a male

remains near a bourgeois male to intercept females that are attracted to the bourgeois male

• Sneaker male– Alternative, parasitic mating tactic in which a male

attempts to avoid detection so that he can quickly enter a bourgeois territory to fertilize eggs being deposited in a nest


Evolution of alternative reproductive tactics

• Conditional strategy– Use of a particular strategy based on an

individual’s condition

• Evolutionary stable strategy (ESS)– A strategy that, if adopted by individuals

in a population, cannot be trumped by another strategy because it yields the highest fitness

Conditional satellite males in tree frogs

• Research question: Does male tree frog mating behavior represent a conditional reproductive strategy? (Humfeld 2008)

• Hypothesis: Males will adopt reproductive strategies based on their condition

• Predictions:1. Males differ in attractiveness to females;

unattractive males will be satellites2. Satellite males should have the same call

preferences as females (lower-frequency calls)


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• Methods:– Green tree frogs (Hyla

cinerea)– Recorded calls of bourgeois

and satellite males– Created synthesized calls

(lowest frequencies 700 Hz and 840 Hz or 840 Hz and 1120 Hz)

– Conducted choice tests for calls with females and satellite males

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• Results:– Bourgeois males were

larger and produced lower-frequency calls than satellite males

– Females and satellite males preferred lowest-frequency calls

• Conclusion:– Males adopt a conditional

strategy

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ESS and sunfish sneaker males

• Research question: What is the reproductive success of parental and sneaker male pumpkinseed sunfish? (Rio-Cardenas & Webster 2008)


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• Methods:– Pumpkinseed sunfish

(Lepomis gibbosus)– Mapped all nest

locations– Captured nesting males

(parentals), females, and sneakers at nest and all developing eggs

– Conducted genotype analysis

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• Results:– Parentals larger and older than

sneakers– Parentals sired about 85% of

offspring in their nest– Sneakers are very rare in

population, and their reproductive success is proportional to their abundance

• Conclusion:– The two strategies have equal

fitness

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Two examples of alternative mating tactics.

Bryan D. Neff, and Erik I. Svensson Phil. Trans. R. Soc. B 2013;368:20120045

©2013 by The Royal Society

Roughgarden 2004


11.6 Learning and mating

• Mate choice copying– When one individual observes and

copies the mating decisions of another individual

• Sexual imprinting –– mate choice influenced by traits seen

in ‘sensitive’ period (e.g., influenced by parents’ traits)

Mate copying in guppies

• Research question: Do females copy the mate choice of other females? (Dugatkin 1992)


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• Methods:– Focal female mate choice test– Model female then placed next to

nonchosen male so that focal female could observe

– Focal female mate choice test

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• Results:– 17 out of 20 females spent more time

near the male that had been near the model female

• Conclusion:– Females switched their mate choice

preference to copy the mate choice of the model female

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The benefit of mate copying

• Mate choice copying should only occur when discrimination among potential mates is difficult– Males are very similar– Females are inexperienced

Nonindependent mate choice by male mosquitofish

• Research question: Does the risk of sperm competition affect mate choice? (Wong & McCarthy 2009)


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Nonindependent mate choice by male mosquitofish• Methods:

– Focal male mate choice test– Rival male then placed next to

each female– Focal male mate choice test– Experiment 1 – focal male

cannot see rival– Experiment 2 – focal male

observes rival near chosen female

– Experiment 3 – focal male observes rival near nonchosen female

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Nonindependent mate choice by male mosquitofish

• Results:– Experiments 1 and 3 –

males preferred larger female

– Experiment 2 – focal male spent less time with chosen female during the last stage

• Conclusion:– Male mate choice is affected

by risk of sperm competition

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Megalagrion calliphya Beautiful Hawaiian Damselfly

Individual Type

Proportion Sexual InteractionsProportion Sexual Interaction Time

MaleGynomorphAndromorphMaleGynomorphAndromorph

1.0

0.8

0.6

0.4

0.2

0.0

Figure 6. The proportion of sexual interactions between Megalagrion calliphya focal andromorphs, gynomorphs, and males from Upper Mountain House Road (UMHR) (Wilcoxon Each Pair test, n=9 gynomorphs, n=8 males, n=8 andromorphs, p=0.0305 between males and andromorphs, p>0.05 for all other comparisons). In addition, the proportion of the total time spent sexually interacting between the three individual types at UMHR (Wilcoxon Each Pair test, n=10 gynomorphs, n=11 males, n=10 andromorphs, p=0.0365). The quartile method was used to calculate endpoints. Outliers are signified by the * symbol.

R. Rasmussen MAP Summer 2016

11.1 sexual selection favors characteristics that enhance reproductive success primary sexual...

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