Sexual selection Sexual selection -males & females differ in size, appearance, behavior -why sexual dimorphism? -example – human size 10.2 -natural selection cannot produce traits that reduce survival, Darwin looked to other processes -Darwin devoted half a book, Descent of man, to sexual selection -sexual selection – selection that arises from differences in mating success -sexual selection falls into 2 categories, intrasexual, intersexual

A difference between the sexes is called a sexual dimorphism

-mean male height vs mean female height for 200 human societies -males typically taller by 10%

Natural selection cannot always explain sexual dimorphism

Asymmetries in sexual reproduction Asexual reproduction Sexual reproduction live long enough live long enough reproduce reproduce persuade partner to mate -imagine asexual reproduction only have to live long enough & reproduce -sexual population -both of above + persuade member of opposite sex to mate -therefore, individuals vary in success surviving and reproducing and also in success at persuading opposite sex to mate -evolutionary consequences, failing to mate = dying young, no genetic contribution to future generations -sexual selection – differential reproductive success due to variation among individuals in success at getting mates -therefore, evolution by sexual selection equivalent to natural selection: If there is heritable variation in a trait that affects the ability to obtain mates, then variants conducive to success will become more common over time Sexual selection - differential reproductive success due to variation among individuals in success at getting mates

Asymmetries in sexual reproduction Sexual reproduction imposes different selection pressures on females versus males -if sexual selection is to explain differences between the sexes, it must act on the sexes differently

Protects until 7 or 8 yrs old Orangutan mating Females Pregnant for 8 months Nurses for 3 years Protects until 7 or 8 yrs old -compare orangutans -mostly solitary, tolerate each other only for sex -male – 15min of copulation -female – 15 min of copulation, if preg, carries foetus for 8 months, nurses 3 years, protects until 7 or 8 years old -male – a few mL of semen and that’s it, ran regenerate that much in hours or days Males A few mL of semen

Sperm is cheap! -‘sperm is cheap’, eggs are expensive -females make a larger parental investment than males -parental investment, energy + time spent constructing and caring for offspring -parental investment measured in fitness; parental investment increases reproductive success of the offspring receiving it, also decreases remaining reproductive success that parent could invest in offspring -leads to conflict in male and female reprod strategies -males can mate with impunity, female fitness is significantly lowered if she mates with incorrect (poor) male -females are limiting resource for males

Asymmetries in sexual reproduction reproductive success potential realized females small egg (preg) limited males large mate limited -typical of mammals, 90%+ females provide substantial parental care, males little or none -eggs more expensive than sperm -therefore, factors limiting lifetime reproductive success are different for males and females -females potential reproductive success is relatively small and realized reproductive success limited by number of eggs (pregnancies) not by number of males -males potential reproductive success is relatively large and realized reproductive success limited by number of number of females and not by number of ejaculates -therefore, access to mates is a limiting resource for males not but females -therefore predict sexual selection, variation in mating success, more potent force in evolution of males than females

Asymmetries in sexual reproduction male reproductive success limited by access to mates female reproductive success limited by capacity to produce offspring -behavioral consequences of asymmetry limits on fitness -male reproductive success limited by access to mates -female reproductive success limited by capacity to produce offspring -mating on island 1 male & 1000 females vs 1 female & 1000 males

Asymmetries in sexual reproduction Sexual selection theory predicts that males will compete with each other over access to mates and that females will be choosy -therefore males should be competitive for access to females and females should be choosy -large commitment on part of females, can have choice of mates, therefore selective about their mates -confirmed in many species -not hard & fast, some males monogamous, some females promiscuous

Intra-sexual selection - outcome determined within a sex male-male competition Inter-sexual selection - outcome determined by opposite sex female choice -male/male competition – head to head, claw to claw, antler to antler -intrasexual selection – males fight males, female mates with winner; outcome determined within a sex -intersexual selection – male advertise by singing/color etc and female choose male with best display; outcome determined by opposite sex

Intra-sexual selection Male-male competition combat over access to females sperm competition infanticide -male/male competition -occurs when individual males can monopolize access to females -monopolize either female directly, or feeding territory or nesting site -contest – directional selection for greater size, weaponry, disply features can lead to runaway selection

Male-male competition combat - elephant seals -combat -most obvious form of male/male competition -favours large body size, weaponry, armour, clever tactics

Male-male competition sperm competition - damselflies sperm competition -males male competition does not stop after copulation is over -real determinant is fertilization -internal fertile, female mates multiple males, sperm race to the egg -female produce clutches fathered by several males –human twins of different fathers -more sperm, higher probability of fertilization, eg. lottery more tickets -eg. Mediterranean fruit fly -sperm is cheap, but not free -conserve sperm, use minimum required to ensure complete fertilization -but if larger ejaculates contribute to victory in sperm competition, males at risk of competition should release more sperm -if sperm released is unimportant to outcome, male should release same amount regardless of risk of competition -1 group raised alone, mated privately -1 group raised with another male, mated in that males’ presence -males mated with a potential rival released 2.5x as many sperm -therefore large ejaculates contribute to victory in sperm competition

Male-male competition sperm competition - other examples large ejaculates mate guarding prolonged copulation copulatory plug repellant pheromones applied to female -other adaptation -large ejaculates, mate guarding, prolonged copulation, copulatory plug, repellant pheromones applied to female, damselflies scrub out previous sperm -sperm competition – sperm of 2 males have the opportunity to fert a female egg -male enhances his chances of fert by producing more sperm, eg. chimps vs gorillas

Male-male competition sperm competition - other examples large ejaculates

Male-male competition infanticide - lions infanticide -competition between males continues past conception -young male lions travel as a group and take over a pride by evicting old lions -coalition holds a pride about 2 yrs total -male keen to quickly father cubs -females cannot breed until pups are weaned -males kill cubs so females become available to mate -kill pups, females come to breeding condition about 8 months earlier on average infanticide responsible for ~25% of cub deaths annually, -infanticide helps males, detrimental to females -females can defend young, risk death -spontaneous abort any pregnancies when a new coalition of males takes over, no wasted reproductive effort

Male-male competition infanticide - lions infanticide -competition between males continues past conception -young male lions travel as a group and take over a pride by evicting old lions -coalition holds a pride about 2 yrs total -male keen to quickly father cubs -females cannot breed until pups are weaned -males kill cubs so females become available to mate -kill pups, females come to breeding condition about 8 months earlier on average infanticide responsible for ~25% of cub deaths annually, -infanticide helps males, detrimental to females -females can defend young, risk death -spontaneous abort any pregnancies when a new coalition of males takes over, no wasted reproductive effort

Inter-sexual selection Female choice better genes resource acquisition preexisting sensory bias ‘sexy sons’ female choice -male advertise, female choose, leads to elaborate displays -female choice responsible for great # of advertisement displays -choosing good genes for her offspring and acquisition of resources offered by the males

Female choice barn swallows -barn swallows -males larger, brighter, tail feathers 15% longer, sexual dimorphism -females prefer males with longer tails -shortened tails, sham, control, elongated -if no preference, all treatments mate equally -males with short tails take longer to attract a mate -females choosy, prefer longer tail feathers -females with short tail mates attempt to compensate by copulating out of pair with large tail males -displays by males are indicators of genetic quality -choosy females will secure better genes for their offspring -shorter pre-mating period allowed time for second clutch -second clutch allowed for greater # of offspring

better genes - gray tree frogs Female choice better genes - gray tree frogs -female gets better genes for her offspring = handicap hypothesis -better displays may indicate better genetic quality; indicator shows male condition -depends on honest, expensive signals, can lead to exaggerated traits -handicap principle – if they are handicapped but still do well, they must have good genes -grey tree frogs -males produce long or short calls, females prefer to mate with long calling males -fertilized females clutch with sperm from long and short calling males -raised tadpoles on generous & restricted diets -better performance by offspring of long callers – as measured by larval growth, time to metamorphosis, mass at metamorphosis, larval survival, post-metamorphic growth -offspring of long callers had significantly higher fitness -consistent with good genes hypothesis long-call males had higher fitness offspring

resource acquisition - hanging flies Female choice resource acquisition - hanging flies nuptial gifts -female acquisition of resources -hanging flies -male catches insect, releases pheromone, female comes to eat and mate -copulation while she eats -large prey, longer it takes her to eat, more sperm he can fill her with -if she finishes quickly (<20min) breaks off and flies away -females prefer males with large gifts -provides her more nutrients, can lay more eggs, and saves her need to hunt for herself -males have same kind of analysis, if they fill her up and she is still eating, male grabs gift back and flies off to find another mate

pre-existing sensory biases - tungara frogs Female choice pre-existing sensory biases - tungara frogs preexisting sensory bias -females preference evolved first, male display followed -tungara frogs -males make high pitched whines and low chucks -neuro-biology shows female ear biased toward low freq components of chuck -similar bias in closely related spp that does not display chuck -conclude female bias for low freq sounds and male call evolved to exploit it -chucks in tungara frogs attract females but also predatory bats -conflict between sexual and natural selection females prefer low frequency calls

pre-existing sensory biases - swordtails & grassfinches Female choice pre-existing sensory biases - swordtails & grassfinches -swordtails and platyfish -females prefer to mate with long tailed males even though their own spp does not have long tails -grassfinches, zebra finches – glued feather crests on head of males and females preferred those males -preferences for features may exist before the feature evolves females prefer males with artificial enhancements

Female choice ‘sexy sons’ stalk-eyed flies peacocks sexy son -once a particular male advertisement display is favoured by females, selection on females will automatically reinforce a preference for the fashionable trait -female choosing fashionable males will have more fashionable sons and therefore more grandchildren -benefits of access to mates are balanced by costs of producing the traits -sexy sons can lead to coevol of male traits and female prefs and can greatly exaggerate male traits -could lead to runaway selection -all above hypotheses are mutually compatible stalk-eyed flies peacocks

-females prefer some trait -males with that trait leave more offspring Runaway selection -females prefer some trait -males with that trait leave more offspring -male offspring will possess that trait and female offspring will prefer males that possess that trait -leads to a positive feedback loop, get exaggeration of the trait -if trait becomes a detriment to survival, sexual selection balanced by natural selection -runaway selection -females prefer some trait – longer tail -males with longer tails leave more offspring -male offspring will have longer tails and female offspring will prefer longer tailed males -leads to a positive feedback loop -exaggeration of tail length -tail length becomes a detriment to survival -sexual selection balanced by natural selection

Role reversal Species in which males invest more in each offspring, and are thus a limiting resource for females, are the exceptions that can prove the rules of sexual selection diversity in sex roles -sexual selection stronger for females than males -pipefish, male provides all parental care -female deposits eggs in male brood pouch -male provides oxygen and nutrients until they hatch -energetically expensive but more importantly takes time -female can produce eggs quicker than males can brood them so males are limiting sex -females compete for access to males, males are choosy -paired choice tests, males choose larger females with larger skin folds; females less choosy, no discrimination between males -also found in spotted moorhens, spotted sandpipers, giant waterbugs

Role reversal male pipefish brood their young, and access to males is a limiting resource for females. females fight among themselves and males are choosy

Role reversal