EEOB 400: Lecture 9 Sexual selection Sexual dimorphism Sexual dimorphism Sexual dimorphism Photo: Bill Love “Considering that colors of chameleons often reflect their “mood”, one has to wonder why 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 Sexual dimorphism Weapons Sexual dimorphism Weapons female male male Sexual dimorphism Ornaments Sexual dimorphism Sexual dimorphism Sexual size dimorphism Bizarre sexual dimorphism The peacock’s tail Extravagant male ornaments The peacock’s tail greatly impairs his mobility…how could such a trait evolve? Sexual selection Darwin’s second “major” book: 1871 On the Descent of Man, and Selection in Relation to Sex Why a theory of sexual selection? Darwin needed a theory to explain the many extravagant traits that seem to reduce survival e.g. the peacock’s tail What is sexual selection? Sexual selection is distinguished from natural selection by the following criterion: Sexual selection arises through variance in mating success Sexual selection Is sexual selection different from natural selection? Darwin saw them as distinct - only sexual selection could produce traits that compromise survival The basic principles are identical – selection favors whatever gets more genes into the next generation In sexual selection, fitness is measured relative to members of the same sex Two kinds of sexual selection Intrasexual selection – mating success determined by within-sex interactions e.g., male-male combat Intersexual selection – mating success determined by between-sex interactions e.g., female choice of males (also referred to as epigamic selection) Mating systems Monogamy One male mates exclusively with one female Polygamy Individuals mate with more than one partner Polygyny Some males mate with more than one females Polyandry Some females mate with more than one males Promiscuity Males mate with more than 1 female and vice versa Monogamy Polygyny Promiscuity Mating systems Mating systems influence sexual selection Strength of sexual selection Variance in mating success All males have same mating success = 1 mate Some males = 0 mates Some males = 1 mate Some males = 2 mates Most males = 0 mates One male = 8 mates Monogamy Moderate Polygyny Strong Polygyny Harem polygyny Elephant seals (Mirounga) - breeding females cluster together on beaches - allows males to defend a harem of many females at once Male 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 engage in violent, bloody fights over females – large size confers an advantage in male combat Lekking polygyny Lekking - males aggregate in particular areas called leks, display for females Lek Combination of male competition… Males may fight for position in center of lek …and female choice: Females choose a mate…often dominant male or male in the center Lekking in Black Grouse, Fallow Deer & Stalk-Eyed Flies Territorial defense polygyny Territory An area that is defended for exclusive use of the defender against rivals Territories may be defended by males or females and for multiple purposes What 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 territory In many species, males set up territories around females (or vice versa) – in this case it becomes similar to a harem defense polygyny Male combat % females Why are these males fighting? % males Red Deer (Cervus elaphus) # offspring surviving to 1 yr. Reproductive success is more variable in males than females Many stags never reproduce, some may sire up to 24 offspring! Alternative reproductive tactics Remember…there may not be a “best” way to be a male (or female) Sex roles Macho males and choosy females? 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 resource over which the other sex competes ( = sexual selection) Anisogamy: difference in the size of male and female gametes Gametic investment: sperm are consequently cheaper than eggs This predisposes females to a greater level of parental investment Males can easily produce enough sperm to fertilize all of a female’s eggs, but the reverse is not true: mating opportunity limits male reproductive success fecundity limits female reproductive success Polyandry Jacanas Most jacana species exhibit harem polyandry and “sex role reversal” Males maintain small territories Males perform all parental care Females mate with multiple males and then leave eggs with males The number of males a female mates determines her reproductive success because she doesn’t care for eggs Sexual dimorphism Females larger than males by 60% in mass (unusual for birds) Females aggressively fight other females and also kill their chicks (infanticide) Polyandry When is polyandry favored? When males become limiting resource for reproduction This often occurs when low offspring survival requires male parental care, so that males have the greater Reproductive effort and lower reproductive rate What is the consequence? Sexual selection is stronger on females than males Saddleback Tamarins, Spotted Sandpipers, and Red-necked Phalaropes are examples of polyandrous species Intra- and inter-sexual selection Intrasexual selection All of the examples thus far have involved interactions within a sex - male-male combat - sperm competition - female competition in polyandrous species Darwin’s theory of intra-sexual selection was readily accepted, even in his time Intersexual selection Intersexual selection has always been controversial We know mate choice occurs because we can observe it directly However, it is debated if and how some sexually dimorphic traits evolve in response to mate choice The peacock’s tail Extravagant male ornaments The peacock’s tail greatly impairs his mobility…how could such a trait evolve? Male ornaments Long-tailed widowbird Experimental manipulation of tail length Males with unnaturally long tails attract females away from the nests of “normal” males or males with shortened tails Andersson (1982) Nature 299:818 Male ornaments Barn swallow Experimental manipulation of tail length Males with elongated tails: 1. Obtain mates more quickly 2. Have greater reproductive success 3. Experience a cost in tail size the following year From Moller (1994) Hypotheses for male ornaments Fisher’s “runaway” hypothesis Mate choice originally evolved to facilitate adaptive choice for traits conferring a survival advantage Once female preference evolved, any genes that conferred survival advantage but compromised attractiveness would not be passed on because surviving males would fail to mate Ronald Fisher Zahavi’s “handicap” hypotheses Extravagant male traits are costly to develop and maintain Choosing a mate with “good genes” requires an honest signal of genetic quality Only males in good condition (those with good genes) will be able to fully develop and maintain an ornament Amotz Zahavi Fisher’s runaway model Survival Selection Sexual Selection Total male fitness (survival + mating) Fitness Female choice adaptive for survival Fitness due to survival Tail length Zahavi’s handicap hypothesis Some candidate “handicaps” Note that the handicap itself need not be heritable…it need only provide a reliable index of fitness, and fitness must be heritable Traits that encumber the owner are physiologically costly (exertion in flight) as well as being more expensive to develop Bright color honestly signals immunocompetence and parasite/disease resistance Asymmetry is indicative of developmental instability and possibly “bad genes”. Symmetry is chosen in some species Sexual vs natural selection Guppies (Poecilia) - sexual selection can favor traits that reduce survival - laboratory selection studies by John Endler Natural selection (predation) favors spot patterns that match background Sexual selection (mate attraction) favors male patterns that contrast background http://evolution.berkeley.edu/evolibrary/article/artificial_01 Sexual vs natural selection Marine iguanas (Amblyrhynchus cristatus) - One of the few lekking reptiles - Large males most successful at lekking - Sexual selection favors large males - Males 2x more massive than females Cost of large size - During El Nino years, food is scarce - The largest iguanas are unable to meet their minimum energy demands and literally starve to death - Most of the largest animals are males, so sexual and natural selection have conflicting effects on male body size Good times, bad times Martin Wikelski’s website: http://www.princeton.edu/~wikelski/ Sexual dimorphism and sexual selection Sexual selection - Selection arising from variance in mating success within a sex - Not all sexual dimorphism arises from sexual selection - Natural selection can also differ between sexes Fecundity selection - Selection on female fecundity = number of offspring produced - Fecundity is different than mating success = number of mates - In many species where females are larger than males, dimorphism is though to reflect fecundity selection favoring large femlae size, since larger females can produce more eggs Sexual dimorphism and sexual selection Niche divergence - Males and females evolve to fill different ecological niches, adaptive if it reduces competition for limited resources (e.g., food) - Another example of natural selection causing sexual dimorphism