4 Sexual dimorphismPhoto: Bill Love“Considering that colors of chameleons often reflect their “mood”, one has to wonderwhy mating elicits such a different color response in males and females.”- Lizards: Windows to the Evolution of Diversity (2003) L.J. Vitt & E.R. Pianka
11 The peacock’s tail Extravagant male ornaments The peacock’s tail greatly impairs his mobility…how could such a trait evolve?
12 Sexual selection Darwin’s second “major” book: 1871 On the Descent of Man, andSelection in Relation to SexWhy a theory of sexual selection?Darwin needed a theory to explain the manyextravagant traits that seem to reduce survivale.g. the peacock’s tailWhat is sexual selection?Sexual selection is distinguished fromnatural selection by the following criterion:Sexual selection arises through variance inmating success
13 Sexual selection Is sexual selection different from natural selection? Darwin saw them as distinct - only sexual selectioncould produce traits that compromise survivalThe basic principles are identical – selection favorswhatever gets more genes into the next generationIn sexual selection, fitness is measured relative tomembers of the same sexTwo kinds of sexual selectionIntrasexual selection – mating success determined by within-sex interactionse.g., male-male combatIntersexual selection – mating success determined by between-sex interactionse.g., female choice of males(also referred to as epigamic selection)
14 Mating systems Monogamy One male mates exclusively with one female Polygamy Individuals mate with more than one partnerPolygyny Some males mate with more than one femalesPolyandry Some females mate with more than one malesPromiscuity Males mate with more than 1 female and vice versaMonogamyPolygynyPromiscuity
15 Mating systems Mating systems influence sexual selection Strength of sexual selectionVariance in mating successAll males havesame matingsuccess = 1 mateSome males = 0 matesSome males = 1 mateSome males = 2 matesMost males = 0 matesOne male = 8 matesMonogamyModerate PolygynyStrong Polygyny
16 Harem polygynyElephant seals (Mirounga) - breeding females cluster together on beaches- allows males to defend a harem of many females at onceMale elephant seals weigh up to 3x more than females!Male reproductive success is highly variable:8 individual males inseminated 348 females in one study!Bull male elephant seals engagein violent, bloody fights overfemales – large size confers anadvantage in male combat
17 Lekking polygynyLekking - males aggregate in particular areas called leks, display for femalesLekCombination of male competition…Males may fight for position in centerof lek…and female choice:Females choose a mate…often dominant male or male in the centerLekking in Black Grouse, Fallow Deer & Stalk-Eyed Flies
18 Territorial defense polygyny TerritoryAn area that is defended for exclusiveuse of the defender against rivalsTerritories may be defended by malesor females and for multiple purposesWhat is being defended?Sometimes territories are defended simply for resources:food, basking sites, dens or hiding places, etc.This may still be important for sexual selection, e.g. if females “choose” male territories based on the resources within the territoryIn many species, males set up territories around females (or vice versa) – in this case it becomes similar to a harem defense polygyny
19 Male combat Red Deer (Cervus elaphus) Why are these males fighting? Reproductive success is morevariable in males than femalesMany stags never reproduce, somemay sire up to 24 offspring!% females % males# offspring surviving to 1 yr.
20 Alternative reproductive tactics Remember…there may not be a “best” way to be a male (or female)
21 Sex roles Macho males and choosy females? Bateman’s principle: Why do males and females fall into these “typical” sex roles?In part, it reflects a bias in the species typically studied……but is there also some inherent biological reason?Bateman’s principle:The sex which invests the most in offspring will become a limiting resourceover which the other sex competes ( = sexual selection)Anisogamy: difference in the size of male and female gametesGametic investment: sperm are consequently cheaper than eggsThis predisposes females to a greater level of parental investmentMales can easily produce enough sperm to fertilize all of a female’s eggs,but the reverse is not true:mating opportunity limits male reproductive successfecundity limits female reproductive success
22 Polyandry Jacanas Sexual dimorphism Most jacana species exhibit harem polyandryand “sex role reversal”Males maintain small territoriesMales perform all parental careFemales mate with multiple malesand then leave eggs with malesThe number of males a female matesdetermines her reproductive successbecause she doesn’t care for eggsSexual dimorphismFemales larger than males by 60% in mass (unusual for birds)Females aggressively fight other females and also kill their chicks (infanticide)
23 Polyandry When is polyandry favored? What is the consequence? When males become limiting resource for reproductionThis often occurs when low offspring survival requiresmale parental care, so that males have the greaterReproductive effort and lower reproductive rateWhat is the consequence?Sexual selection is stronger on females than malesSaddleback Tamarins,Spotted Sandpipers,and Red-neckedPhalaropes areexamples of polyandrousspecies
24 Intra- and inter-sexual selection Intrasexual selectionAll of the examples thus far have involved interactions within a sex- male-male combat- sperm competition- female competition in polyandrous speciesDarwin’s theory of intra-sexual selection was readily accepted, even in his timeIntersexual selectionIntersexual selection has always been controversialWe know mate choice occurs because we can observeit directlyHowever, it is debated if and how some sexuallydimorphic traits evolve in response to mate choice
25 The peacock’s tail Extravagant male ornaments The peacock’s tail greatly impairs his mobility…how could such a trait evolve?
26 Male ornaments Long-tailed widowbird Experimental manipulation of tail lengthMales with unnaturally long tails attractfemales away from the nests of “normal”males or males with shortened tailsAndersson (1982) Nature 299:818
27 Male ornaments Barn swallow Experimental manipulation of tail length Males with elongated tails:Obtain mates morequicklyHave greater reproductivesuccessExperience a cost intail size the following yearFrom Moller (1994)
28 Hypotheses for male ornaments Fisher’s “runaway” hypothesisMate choice originally evolved to facilitate adaptivechoice for traits conferring a survival advantageOnce female preference evolved, any genes thatconferred survival advantage but compromisedattractiveness would not be passed on becausesurviving males would fail to mateRonald FisherZahavi’s “handicap” hypothesesExtravagant male traits are costly to develop and maintainChoosing a mate with “good genes” requires an honestsignal of genetic qualityOnly males in good condition (those with good genes)will be able to fully develop and maintain an ornamentAmotz Zahavi
29 Fisher’s runaway model FitnessTail lengthTotal male fitness(survival + mating)Fitness due to survivalSurvival SelectionSexual SelectionFemale choiceadaptive for survival
30 Zahavi’s handicap hypothesis Some candidate “handicaps”Note that the handicap itself need not be heritable…it need only provide areliable index of fitness, and fitness must be heritableTraits that encumber theowner are physiologicallycostly (exertion in flight)as well as being moreexpensive to developAsymmetry is indicativeof developmentalinstability and possibly“bad genes”. Symmetryis chosen in some speciesBright color honestly signalsimmunocompetence andparasite/disease resistance
31 Sexual vs natural selection Guppies (Poecilia) - sexual selection can favor traits that reduce survival- laboratory selection studies by John EndlerNatural selection (predation) favorsspot patterns that match backgroundSexual selection (mate attraction) favorsmale patterns that contrast background
32 Sexual vs natural selection Marine iguanas (Amblyrhynchus cristatus)One of the few lekking reptilesLarge males most successful at lekkingSexual selection favors large malesMales 2x more massive than femalesCost of large sizeDuring El Nino years, food is scarceThe largest iguanas are unable to meettheir minimum energy demands andliterally starve to deathMost of the largest animals are males,so sexual and natural selection haveconflicting effects on male body sizeGood times, bad timesMartin Wikelski’s website:
33 Sexual dimorphism and sexual selection Selection arising from variance in mating success within a sexNot all sexual dimorphism arises from sexual selectionNatural selection can also differ between sexesFecundity selectionSelection on female fecundity =number of offspring producedFecundity is different than matingsuccess = number of matesIn many species where females arelarger than males, dimorphism is thoughto reflect fecundity selection favoringlarge femlae size, since largerfemales can produce more eggs
34 Sexual dimorphism and sexual selection Niche divergenceMales and females evolve to fill different ecological niches, adaptive if itreduces competition for limited resources (e.g., food)- Another example of natural selection causing sexual dimorphism
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