Whiptail Lizard Sexual Selection Charles Darwin and Alfred Russell Wallace Noticed that males of many species have highly elaborated traits that seem maladaptive secondary sexual characters Antlers Irish Elk extinct Horns Bighorn Sheep Elephant Tusks Hercules Beetle Male Female African Lion Mane is used for protection during male clashes male male competition Pheasant Elaborate bright red wattle on cheek Pheasant Elaborate spur used in male male fighting and by females to choose Notice the fake spur on the right Some males try to trick females but do not fight Two Types of Sexual Selection Male male competition intra sexual selection Mate female choice inter sexual selection Male male competition Within group dominance Female defense polygyny Territorial polygyny Lekking competition for a position in the lek Within group dominance Grey Wolves Female defense polygyny Northern Elephant Seals Territorial defense polygyny Impala Waterbucks head butting Sperm competition Grouse a lekking species Mate Choice Two broad categories Resource based Non resource based In most cases females are the choosy sex but not always Why Why are females choosy Investment cost eggs are expensive relative to sperm internal fertilization and carrying young to term is costly When are males choosy sex role reversal When their contributions exceed the cost of making eggs The chances of mating with multiple females is small biased sex ratio many females and very few males Example Sea Horse males care for the young Mate choice Why choose in the first place mate with correct species better fertilization ability or higher fecundity provides more food better parental ability better breeding territory or defended resource lower risks or hazards e g predation partner offers higher heritable viability or other heritable qualities that are important Mate choice Direct benefits proximate benefits Example Bush crickets nuptial gifts 1 Males provide spermatophores which females feed on Often consist of up to 30 40 of male body weight i e very costly 2 More spermatophores allow female to lay more eggs Mate Choice Indirect benefits ultimate benefits Good genes Others e g Fisherian runaway sexual selection which will not be discussed Good Genes Females or males choose a mate which offers high quality genes influencing survival Mate quality is indicated by a secondary trait Secondary trait must be heritable Must be heritable variation in mate quality No low or high cost to males bearing the trait e g bearing the cost is an indicator of good genes handicap model Good Genes Example Gray Tree Frog Hyla versicolor Short vs Long Calls Fitness Measure High Food Low Food Larval growth LC LC Time to meta LC Mass at meta Larval survival Postmeta growth LC Final Topics Lekking and the evolution of leks Sensory bias exploitation What are leks An aggregation of males which females visit only for the purpose of mating No male parental care only contribution is sperm An area that males aggregate and mate with females that is not associated with feeding etc Display sites of males contain no significant resources to females e g nesting site Females have the opportunity to choose or shop among available males Evolution of leks lower predation risk for males and females passive attraction more males is better hotspot model males congregate in areas that increase the probability of encountering females black hole model females are not choosy but wish to avoid dangers associated with mating hotshot model females choose the best male poor males congregate near good males in the hopes of increasing their chances Bowerbirds An example of sexual selection in a lekking species Males build and decorate courtship arenas called bowers on leks for the purpose of courting females Females prefer males with lots of blue and yellow objects at the bower They also prefer neat bowers While the female grey sights and watches the male produces an elaborate courship display including dances jigs wingflips and vocalization Males hoping to woo females will even clasp a yellow flower or feather in the beak Males with the best of all these traits get the most matings Many species of bowerbirds build elaborate bowers decorate them with colorful objects and have elaborate courship displays Fembots Sensory Bias Exploitation Latent preferences in females are used by males to gain greater reproductive success Physaleumus Frogs P coloradum P postulosum CHUCKS evolve in males CHUCK preference evolves in females P coloradum Males attract mates by calling using WHINES P postulosum Males attract mates by calling using WHINES and some use CHUCKS Experiment Take tape playback unit with recording of P coloradum WHINES with P postulosum CHUCKS to pond Play and observed relative to just WHINES how many females are attracted Female Pref WHINE NO WHINE CHUCK YES Swordtails Swordtails Platyfish Platyfish No swords When males are modified to have swords the female Platyfish prefer them Swordtails Swords evolve in males Sword preference evolves in females Swordtails The males of some species have swords Females prefer males with long swords Helpful behaviors Alarm calls e g Belding ground squirrel Sentinel behavior e g meerkats Nest helping Eusocial behavior Actor performs some action that benefits another the recipient How do you explain the evolution of helpful behavior Mutualism actor benefits Reciprocal altruism actor eventually benefits Kin selection indirect selection Interaction Social Donor Social Recipient Mutualism Reciprocity delayed Altruism Selfishness Spitefulness Mutualism Groups of lions can bring down larger prey and better defend the prey from other lions and hyenas Bluegill Male Males form nest sites of 50 to 100 males Mutualistic because predation is lower if your nest is surrounded by others Male lions will often cooperate in ousting resident males from a pride or in defending a pride from other outside groups of males Often the males in a pride are closely related Reciprocal Altruism Dispense an altruistic beneficial act which is later returned as an equally beneficial act Most likely when Repeated interactions between individuals Many opportunities for altruism Good memories Potential altruists interact in symmetrical situations Prisoner s dilemma Player B C cooperate Player A D defect C cooperate D defect R both receive light sentence S suckers payoff T reduced sentence for defector P both receive intermediate sentences Prisoners dilemma T R P S R S T 2 Best
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