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The behavioralecology ofanimalreproduction
I. Introduction to BehavioralEcology
Behavioral ecologists testhypotheses about theadaptive function of abehavior
Behavioral Ecology
Behavioral ecologists testhypotheses about theadaptive function of abehavior
Adaptive function?
Behavioral Ecology
Behavioral ecologists testshypotheses about the adaptivefunction of a behavior
Adaptive function?
Contribution of a behavior to thefitness of an organism
Behavioral Ecology
Adaptive function: thecontribution of a behavior tothe fitness of an organism
Fitness?
Behavioral Ecology
Adaptive function: thecontribution of a behavior tothe fitness of an organism
Fitness?The relative contribution of an
individual to the gene pool ofthe following generation
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II. Introduction to sexualselection
What’s sexual selection?
What’s sexual selection?
Genetic variation amongindividuals in mating success
Intrasexual vs. Intersexualselection
Intrasexual selection?
Variation in ability to competewithin the same sex for theopportunity to mate
Intrasexual vs. Intersexualselection
Intersexual selection?
Variation in mate attractionand mating success
Intrasexual vs. Intersexualselection
Intersexual selection
“The power to charm” CharlesDarwin
Intrasexual vs. Intersexualselection
Sexual selection is part ofnatural selection
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Intrasexual vs. Intersexualselection
Sexual selection is part ofnatural selection
III. Mate choice
Which sex is more particularabout mates?
Mate choice
Which sex is more particularabout mates?
It depends...
I I I . Mate choice
Trivers’ hypothesis:
The sex that invests the mostresources in reproductionshould be the choosiestabout mates
II I . Mate choice
Trivers ’ hypothesis:
The sex that invests the mostresources in reproductionshould be the choosiestabout mates
Why?
III . Mate choice
Trivers’ hypothesis
If little investment per mating -maximize the number ofmatings
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I I I . Mate choice
Trivers’ hypothesis
If little investment per mating -maximize the number ofmatings
If large investment per matingmaximize the success of eachmating
Mate choice
Trivers’ hypothesisIf little investment per mating -
maximize the number ofmatings
If large investment per matingmaximize the success of eachmating
-so individuals should choosemates carefully!
I I I . Mate choice
What kinds of resources areinvested in reproduction?
II I . Mate choice
What kinds of resources areinvested in reproduction?
stored nutrition
Mate choice
What kinds of resources areinvested in reproduction?
stored nutritiontime and energy
II I . Mate choice
How would you test Trivers’hypothesis (that the sex thatinvests most is choosier)?
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I I I . Mate choiceExample: “Nuptial gifts” in mormon
crickets
Males deliver huge sperm packet (upto 30% of male body weight) tofemales. Females consume theexternal portion.
Mate choice
Example: “Nuptial gifts” inmormon crickets
Which sex of mormon cricketwould you predict is choosierabout mates?
II I . Mate choice
Example: “Nuptial gifts” inmormon crickets
Males choose females on basisof body size
Mea
n w
eigh
t of f
emal
es
Rejected Accepted
I I I . Mate choice
Males choosefemales onbasis of bodysize
Heavier femalescarry moreeggs - soincrease malereproductivesuccess
Mea
n w
eigh
t of f
emal
es Rejected Accepted
I I I . Mate choice
What’s the basis of thechoice?
Mate choice
What’s the basis of the choice?
i. Direct estimates ofreproductive potential (e.g.female body size in mormoncrickets)
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I I I . Mate choice
What’s the basis of the choice?
i.Direct estimates ofreproductive potential
ii. Material benefits (e.g.territories)
I I I . Mate choice
What’s the basis of the choice?
i.Direct estimates ofreproductive potential
ii. Material benefits (e.g.territories)
I I I . Mate choice
What’s the basis of the choice?
i. Direct estimates ofreproductive potential
ii. Material benefits (e.g.territories)
iii. Indirect estimates ofreproductive potential
E.g. bright plumage, elaboratecalls, songs etc.
Mate choice
iii. Indirect estimates ofreproductive potential
Do female birds actually choosemales that can do silly dances,sing elaborate songs or havebrilliant feathers?
Mate choice
Yes they do...
Aver
age
no. o
f nes
ts p
er m
ale
Tailmanipulationexperiment inlong-tailedwidow birds Artificially Normal Artificially
lengthened shortened
I I I . Mate choice
Why don’ twidowbirdshave tailsas long asthe ones‘made’ by theexperimenters?
Aver
age
no. o
f nes
ts p
er m
ale
-why Darwinmade a distinction between sexualselection and selection for viability
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Why do females choose on thebasis of ornament and display?
Two nonexclusive hypotheses
Why do females choose on thebasis of ornament and display?
Two nonexclusive hypotheses
1) The “good genes” hypothesis:Showiness is correlated with the
presence of genetic alleles inmales that lead to greateroffspring fitness
Why do females choose on thebasis of ornament and display?
Two nonexclusive hypotheses
1) The “good genes” hypothesis:One aspect : males with inherited
resistance to parasites tend tohave brighter plumage, etc.
Why do females choose on thebasis of ornament and display?
Two nonexclusive hypotheses
1) The “good genes” hypothesis:One aspect : males with inherited
resistance to parasites tend tobe brighter colored etc.
Example: turkey snoods!
Males with inherited resistance toparasites tend to be brightercolored etc.
Example: turkey snoods
Femaleturkeysprefermaleswith
bigsnoods
Males with inherited resistance toparasites tend to be brightercolored etc.
Example: Turkey snoods
And males with big snoods are moreresistant to parasites
snood
Snoo
d le
ngth
Log Coccidea no. per gram feces
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Why do females choose on thebasis of ornament and display?
Two nonexclusive hypotheses
1) The “good genes” hypothesis
2) “Runaway sexual selection”
2) Runaway sexual selectionoccurs when
a) a slight preference by femalesfor showy trait leads to greaterreproductive success of maleswith trait (even if the trait is notcorrelated with any other ‘goodtrait’ e.g. size, agility,resistance to parasites)
2) Runaway sexual selectionoccurs when
a) a slight preference by femalesfor showy trait leads to greaterreproductive success of maleswith trait (even if the traitdoesn’t directly increase thefitness of the offspring)
b) a genetic association ofpreference by females for showytrait and showiness of malesdevelops
Why do females choose on thebasis of ornament and display?
1) The “good genes” hypothesis
2) “Runaway sexual selection”
What’s the difference betweenthese hypotheses?
Mating systems
Mating systems are defined by therelative maximum number ofmates of males and females
Mating systems
I. Polygyny - the dominant matingsystem in animals with internalfertil ization
poly - manygyn-female
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Mating systems
I. PolygynyThe maximum no. of mates for
males is greater than themaximum no. of mates for females
Mating systems
I. PolygynyThe maximum no. of mates for
males is greater than themaximum no. of mates for females
Why not the average?
Mating systems
I. Polygyny
The maximum no. of mates formales is greater than themaximum no. of mates for females
Why not the average?When there are equal numbers of
each sex, the average number ofmates must be equal
Mating systems
I. Polygyny
The maximum no. of mates formales is greater than themaximum no. of mates for females
Why not the average?When there are equal numbers of
each sex, the average number ofmates must be equal
So is the variation in matingsuccess greater for one sex thanthe other? Which one?
III. Mating systems
i. PolygynyIn polygynous systems, the
variation in reproductive successis greater in males
Mating systems
I. Polygyny
Why is polygyny the most commonmating system?
Because females often invest morein reproduction than males
Fewer gametes - sometimes eggsbig relative to body size, and inmammals, both gestation andlactation performed by female
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Mating systems
I. Polygyny
So how does greater femaleinvestment lead to polygyny?-same logic as for mate choice
Mating systems
I. Polygyny
So how does greater femaleinvestment lead to polygyny? -same logic as for mate choice
If little investment per mating -maximize the number of matings
If large investment per matingmaximize the success of eachmating
Mating systems
II. Monogamy (or monogyny)
Maximum number of mates formales = maximum number ofmates for females = 1
Mating systems
II. Monogamy (or monogyny)
Maximum number of mates formales = maximum number ofmates for females = 1
Monogamy may evolve whenrearing offspring requires morethan one individual
Mating systems
II. Monogamy (or monogyny)
Monogamy may evolve whenrearing offspring requires morethan one individualCommon in birdsHumans generallymonogamous
An aside: What about humanbeings - how much ofbehavioral ecological theorycan be applied to us?
No question that humanbehavior molded in part byevolutionary history
But humans unique in animalsin having social and culturalconventions that oftenoverride evolutionary fitnessconsiderations
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Mating systems
III. Polyandry
Poly - many, andr- maleMaximum number of mates for
females is greater than themaximum number of mates formales
Mating systems
III. Polyandry
A likely example from earlier in thislecture?
Mating systems
III. Polyandry
Examples?Mormon cricket,In some birds, females defend
territories, some with manymales, males build nests andraise the young
Sexual cannibalism
An observation that needsexplaining (at least to us)...
Sexual cannibalism
In some animals, especiallyspiders, females eat mates,during or after mating...
Sexual cannibalism
What’s the hypothesis the narratorproposes?
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Sexual cannibalism
What’s the hypothesis the narratorproposes?
Males are victims of the predatoryinstinct of females
This hypothesis suggests anevolutionary constraint - malesare not able to devise effectivestrategies to avoid being eaten
Sexual cannibalism
evolutionary constraint - theoptimal trait or behavior (e.g.being able to always avoid gettingeaten) cannot be selectedbecause there is no geneticvariation for it
Sexual cannibalism
What’s the hypothesis the narratorproposes?
Males are victims of the predatoryinstinct of females
Sexual cannibalism
What’s the hypothesis the narratorproposes?
Males are victims of the predatoryinstinct of females
In other systems, it appears beingeaten is adaptive for the male
Sexual cannibalism
In other systems, itappears beingeaten is adaptivefor the male
Example: theAustralian redback spider (closerelative of theblack widow)
Males summersaultinto the ‘jaws’ offemales
chelicerae Sexual cannibalism
Example theAustralian red backspider (closerelative of the blackwidow)
Males only likely tofind one mate perlifetime - if they’r eeaten more of theirsperm istransferred
