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Sexual Selection Spot the Male Sexual Selection 1 Spot the male !! Spot the male vs. female 2 1! Sexual dimorphism Very common in animals !! What is it? !! Differences in characteristics between males and females of a species Sexual dimorphism Very common in animals !! What is it? !! Differences in characteristics between males and females of a species 2! Sexual dimorphism Very common in animals !! What is it? !! Differences in characteristics between males and females of a species Sexual dimorphism Very common in animals !! Darwin noted, when males and females differ, males have !! Brighter feathers or plumage !! Larger weapons (e.g. antlers) !! A greater tendency to vocalize !! Larger muscles !! Greater aggressive tendencies !! A higher intensity in seeking mating opportunities !! Two questions ! 3! Sexual dimorphism Very common in animals !! How does sexual dimorphism arise? !! Ornaments, behaviors, plumage, etc. require energy and reduce survival !! Why is the larger, or more colorful, or more ornamented sex usually the male? The Fundamental Asymmetry of Sex !! Different levels of investment for males and females – Generalizations !! Males !! Sperm is cheap to produce !! Male investment in each mating is small !! Male reproductive success determined by # of matings (i.e. access to females) !! There is great variability in male mating success 4! The Fundamental Asymmetry of Sex !! Different levels of investment for males and females - Generalizations !! Males The Fundamental Asymmetry of Sex !! Different levels of investment for males and females - Generalizations !! Males 5! The Fundamental Asymmetry of Sex !! Different levels of investment for males and females - Generalizations !! Females !! Eggs relatively expensive to produce !! Investment in each mating is large !! Especially where female parental care is important and asymmetrical (e.g. most mammals) !! Female reproductive success determined by # of eggs she can produce or nurture, not access to males !! Relatively little variation in female mating success The Fundamental Asymmetry of Sex !! Different levels of investment for males and females – Generalizations !! Females 6! The Fundamental Asymmetry of Sex !! Different levels of investment for males and females – Generalizations !! Females Consequences !! Males will be competitive for opportunities to mate, females will be choosy !! The sexes will differ in their characteristics due to different selection pressures !! Sexual Dimorphism 7! Sexual Selection !! Differential success among individuals in acquiring mates !! Operates more strongly on males because relative reproductive success determined primarily by relative mating success !! Can operate against survival interests! !! I.e. there can be selection against survival if lower survival is compensated for by greater reproductive success Intrasexual selection - Male/Male !! Male-Male combat – Elephant seals !! Males monopolize access to females or survival of offspring (at least much of the time) 8! Intrasexual selection - Male/Male !! Male-Male combat – Iguanas !! Males monopolize access to females or survival of offspring (at least much of the time) Intrasexual selection - Male/Male !! Male-Male combat – Iguanas 9! Intrasexual selection - Male/Male !! Male-Male combat – Iguanas: Size matters Intrasexual selection - Male/Male !! Male-Male combat – Iguanas: Tradeoffs 10! Intrasexual selection - Male/Male !! Male-Male combat !! Males monopolize access to females or survival of offspring (at least much of the time) !! Access to mating opportunities determined by success in battle - very common !! E.g. iguanas, elk, salmon, bighorn sheep, elephant seals, etc. etc. !! Often leads to elaborate weapons, large size Intrasexual selection - Male/Male !! Male-Male combat 11! Intrasexual selection - Male/Male !! Sperm competition !! Females mate repeatedly, but male success is determined by whose sperm gets to the eggs !! Produce large amounts of sperm !! Remove the competitors sperm !! Flush out the females reproductive tract Intrasexual selection - Male/Male !! Sperm competition !! Females mate repeatedly, but male success is determined by whose sperm gets to the eggs !! Produce large amounts of sperm !! Also very common !! E.g. Chimps 12! Intrasexual selection - Male/Male !! Sperm competition !! Females mate repeatedly, but male success is determined by whose sperm gets to the eggs !! Produce large amounts of sperm !! Remove the competitors sperm !! Leave a copulatory plug !! Mate Guarding Intrasexual selection - Male/Male !! Infanticide !! Occurs when a single male or related males have access to one or more females, but only temporarily 13! Intrasexual selection - Male/Male !! Infanticide !! Occurs when a single male or related males have access to one or more females, but only temporarily !! When taking over access to females, new males may kill newborns or young fathered by other males !! Reduces investment in another males offspring !! Brings female into estrous sooner !! E.g. Gorillas, Lions Sexual Selection 28 14! Intrasexual selection - Male/Male !! Pretty good at explaining dimorphism in size and other factors directly related to dominance and/or sperm competition !! Costs of size, aggressiveness, sperm production, and weaponry are balanced by benefit to reproduction !! But what about conspicuous differences in color, ornamentation? Intersexual selection - Female Choice !! Males often bear conspicuous colorations or ornaments, or exhibit elaborate displays !! May involve major costs to survival, but little or no benefit in male-male combat !! E.g. Coloration in many birds and fish !! Peacocks, Widowbirds 15! Intersexual selection - Female Choice !! Often occurs where males are unable to monopolize access to females !! It pays to advertise! !! Female choice exerts powerful control over male reproductive success !! Females tend to be very discriminating !! Reflects investment in eggs, offspring Intersexual selection - Female Choice !! E.g. Costs and benefits in Widowbirds !! Males have very long tails !! Long tails are costly! 16! Intersexual selection - Female Choice !! E.g. Costs and benefits in Widowbirds !! Males have very long tails !! Long tails are costly! !! But very attractive! Intersexual selection - Female Choice !! E.g. Costs and benefits in Gray Tree Frogs 17! Intersexual selection - Female Choice !! What, exactly, are males advertising, and why should females care? Intersexual selection - Female Choice !! The Sexy Son Hypothesis !! Males are advertising nothing more than sexiness itself !! Females mating with sexy males will produce !! Sexy sons !! Daughters with a tendency to find these traits sexy !! Lots of grandkids !! Runaway sexual selection !! E.g. possibly in stalk-eyed flies 18! Intersexual selection - Female Choice !! Good Genes hypotheses !! Ability to produce and maintain elaborate ornamentation indicates good genes !! I.e. I can afford to take risks or waste energy !! There are several variations Intersexual selection - Female Choice !! Good Genes hypotheses !! Handicap hypothesis, or more generally a sign of vigor !! Ability to survive despite costly advertisements indicates good genes !! The bigger the handicap or more elaborate the display, the better the genes must be !! Females mating with males that are successful despite handicap are getting good genes in their offspring 19! Intersexual selection - Female Choice !! Calling in Gray Tree Frogs Intersexual selection - Female Choice !! Calling in Gray Tree Frogs 20! Intersexual selection - Female Choice !! Good Genes hypotheses !! Handicap hypothesis !! Parasite hypothesis !! Parasites reduce the ability to produce displays !! Males that can produce/maintain elaborate displays must be resistant to parasites !! Females mating with showy males will get parasite-resistant genes in their offspring Intersexual selection - Female Choice !! Good Genes hypotheses !! Handicap hypothesis !! Parasite hypothesis !! Developmental stability !! Developmental stress causes asymmetry !! Advertisements that are large and symmetrical imply resistance to developmental stress !! Females mating with symmetrical males get good genes for their offspring 21! Intersexual selection - Female Choice !! Direct Benefits – E.g. nuptial gifts Intersexual selection - Female Choice !! Preexisting biases !! Sword preference predates swords? 22! Intersexual selection - Female Choice !! Preexisting biases !! Sword preference predates swords? !! Maybe not, sensitive to phylogeny Sexual selection – More generally !! Freeman and Herron: !! Members of the sex subject to strong sexual selection will be competitive !! Members of the sex subject to weak sexual selection will be choosy !! Now ! consider pipefish and seahorses 23! Sexual selection – More generally !! Females create eggs, which are more costly than sperm, but ! !! Males rear the young in a brood pouch or patch !! Requires extensive parental care and, since rearing young takes longer than producing eggs, males are in limited supply Sexual selection – More generally !! Predictions? 24! Sexual selection – More generally !! Who will be choosier? !! Who will be ornamented? 25!.
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