In order to have sexual reproduction, you need 2 sexes In many organisms, males and females are in conflict over their investment of reproductive effort - males invest in mating opportunities - females invest directly in offspring This conflict manifests in many traits associated with sex - Males and females often express very different traits to maximize their respective success at reproduction Conflict between the sexes

In animals with internal fertilization (pregnancies) and/or parental care for offspring, females nearly always invest more time and energy per offspring than males Orangutans: - males invest 15 minutes in copulation - females invest 8 months in pregnancy, 3 years in nursing, and 8 years in protecting and nurturing her young Asymmetrical investment in offspring

Males need only contribute one haploid genome (their genes) to their offspring Females generally invest far more energy per offspring Some animals release sperm and eggs directly into the environment, and provide no parental care - in these organisms, eggs are larger and contain energetically expensive yolk - females can make fewer gametes (egg cells) than males Asymmetrical investment in offspring

Because of this asymmetry in reproductive investment, males and females face different challenges in maximizing their fitness (making the most offspring) Once a female’s eggs have been fertilized, she generally cannot mate again until she’s done caring for that batch of offspring A male can mate as often as he can find a receptive female Asymmetrical investment in offspring

Consider a population w/ one male fish and 10 females: Asymmetrical investment in offspring one male can mate with, and fertilize, all 10 females Male reproductive success is limited by access to mates

Now, consider a population w/ 10 male fish and one female: Asymmetrical investment in offspring Only one male can ultimately father her offspring Female will (1) choose her mate, or (2) mate with the male who can fend off all his competitors Female reproductive success is not limited by access to mates

In many species, males and females are extremely different in their appearance or behavior: sexual dimorphism Sexual dimorphism Fiddler crabs male Peacocks

Darwin’s “sequel,” The Descent of Man and Selection in Relation to Sex, was dedicated to this phenomenon: How could traits involved in male display or competition persist in the face of natural selection? - shouldn’t natural selection tend to weed out brightly colored individuals (by predation), or those wasting energy on big tails or hard-to-build body structures? Sexual dimorphism

Darwin’s answer: a distinct form of selective pressure exists that tends to maximize reproductive success: sexual selection Whereas natural selection makes a population more adapted to its environment, sexual selection does not - it makes one sex more successful in mating with the other - increases fitness, by increasing reproductive success Sexual selection

Males compete for territory, control of females, or opportunities to fertilize; females just go with the winner (1) Combat is a common form of pre-mating competition, resulting in evolution of crazybig male features for fighting - antlers of deer - body size: in iguanas & elephant seals, big males larger than the ideal size don’t survive for long, but farther many children due to holding better & larger territories Intrasexual selection

Males compete for territory, control of females, or opportunities to fertilize; females just go with the winner (2) Sperm competition is a form of post-mating competition - males can make larger ejaculates when there’s competition for females - can be triggered by scent of nearby competitor males - can be modified depending on male’s assessment of a female’s promiscuity - multiple kinds of sperm: sprinters, blockers and killers - extreme case: testicular hijacking in bugs Intrasexual selection

When males cannot monopolize territory, females may be able to “choose” among the males (or are they?..) This introduces a novel form of evolutionary pressure: males versus females in a continual struggle, each seeking to maximize their different reproductive goals Many aspects of this conflict are not a matter of “choice,” but depend on a continuous back-and-forth evolutionary tug of war Intersexual selection

Sexual selection and female “choice” Males have traits important in mating displays (color, tail, claws) whereas females lack such ornamentation Fiddler crabs male, w/ large claw Peacocks

Sexual selection and female “choice” Moller (1988) studied female choice in barn swallows - males have long tails - hypothesis: females prefer males with longer tails - hence, sexual selection has driven the evolution of this male display trait

Sexual selection and female “choice” Data from Moller (1988): distribution of tail length in males

Sexual selection and female “choice” Experiment: manipulate tail length of 44 males, divided into 4 groups of 11  Shortened tail: clip 2 cm out of tail feathers  Control 1: clipped 2 cm off, glued back on (control for gluing)  Control 2: captured and banded birds, didn’t touch feathers  Enlongated tail: glued the 2 cm cut off of shortened tails onto the end of the tail of these birds

pre-mating period % second clutches # of offspring Result: males with elongated tails: (1) got mates the fastest (2) frequently produced 2nd clutches (3) had the most # of offspring

pre-mating period % second clutches # of offspring Conclusion: females choose males based on tail length, which is presumed to indicate the relative fitness of a male

What is a potential flaw in this experiment? Shortened tail treatment: clip 2 cm out of tail feathers

Data from Andersson (1982): female choice for tail length in the widowbird Experiment: alter tail length by 25 cm Result: elongated males had more nests on their territories; shortened males had fewer nests Conclusion: male ornaments are favored by female choice, and may have evolved because of it

Male display vs. female resistance Female Choice theory: males with big tails are advertising their good genes/health, are therefore attractive to females - may be genetic (carry good alleles) - may be phenotypic (conditional: no parasites, healthy) Run-away selection theory: male alleles super-stimulate females with dramatic mating displays and exaggerated features  females counter-adapt by reducing their interest in the exaggerated trait

Run-away evolution of male display Female attraction to a male trait that is absent (pre-existing bias) Mutation produces a rudimentary male display trait (exploitation) 

Run-away evolution of male display Female attraction to a male trait that is absent (pre-existing bias) Mutation produces a rudimentary male display trait (exploitation) Female fitness declines Male attraction declines Female resistance  

Run-away evolution of male display Female fitness declines Male attraction declines Female resistance Exaggeration of male display

Male display vs. female resistance Run-away evolution of male display features, to overwhelm female disinterest (which increases every generation) Eventually, males end up stuck with huge display features that do nothing, but are necessary to get any attention at all - females appear to win this evolutionary contest

Receiver bias in female choice Evidence for pre-existing sensory bias (“receiver bias”): (1) foraging cues: - female stickleback fish prefer red objects when foraging, also prefer males with red throats + bellies - female bowerbirds are attracted to food objects that match the color of objects males use in their displays (2) female sailfin mollies prefer males with more surface area (3) predator avoidance: female wax moths avoid bat sonar, but are attracted to higher frequency vibrations from male moths (4) general attraction towards symmetry

Female choice based on fitness cues Presumption of many studies: all exaggerated traits somehow convey to females which males are the most fit - may or may not: some traits overwhelm female sensory systems, promoting bad mating decisions and not indicating male fitness - other traits may be honest indicators of male fitness, and females that prefer these traits will produce fitter offspring - combination: flaws in big display traits may indicate a male’s lack of resistance to parasites, so may tell a female something useful

Female choice based on fitness cues 1 Female grey tree frogs prefer males who produce longer, faster mating calls: Do these males have higher fitness? Experiment: fertilize eggs with sperm from long-calling vs. short-calling males - fitness of offspring was then judged by 5 criteria: how fast offspring grew and matured, their size, survival Results: longer-calling males fathered offspring that were the same or more fit than kids of short-calling males

Fitness cues 2: male choice of females Female baboons produce sexual swellings during ovulation - also advertize her reproductive value, as predicted by theory of “honest signaling” - females with larger swellings attained reproductive maturity at at younger age, had more offspring, and more surviving kids Result: males expend more effort fighting over, and spend more time grooming, females with larger swellings Domb & Pagel 2001, Nature 410:

Female choice can operate after mating - female birds can be coerced into mating by “inferior” (socially less dominant) males - however, when this happens, they can lure dominant competitors to displace sperm of their rivals - can also selectively eject sperm of inferior males - female ducks lay larger eggs after mating with preferred males - no effects of paternity on offspring condition