All discussion of mating strategies and sex differences begins with Darwin s theory of Sexual Selection

Transcription

1 All discussion of mating strategies and sex differences begins with Darwin s theory of Sexual Selection Natural Selection Survival Mating Success Parental Care Intrasexual Selection: Weapons that are used in male-male competition for access to females. Intrasexual Competition Mate Choice (Epigamic) Epigamic Selection: Ornaments or displays that are attractive to females. Female choice key. selection equally on both conventional sex role reversed Pronghorn: an interesting case. Female gets only sperm (genes) from male. She spends lots of time choosing among males. Kids of popular males are healthier and develop faster. But in pronghorn, female choice is based not on ornaments or displays, but on the behavior of the male, in particular his ability to keep her in the harem! ~ no selection on either 1

2 sexual dimorphism 1. Not always obvious whether trait due to epigamic or intrasexual selection 2. Not all dimorphism due to sexual selection. 3. Multiple selection pressures on these traits Red-winged Blackbird strong epigamic selection on both sexes? Anisogamy Trivers 1972: Parental investment and sexual selection. Anisogamy = Unequal gametes Eggs: few, large, energetically expensive Sperm: many, small, cheap 2

3 PARENTAL INVESTMENT (PI) 1. initial PI: females produce large egg cells, males produce small sperm 2. specialized adaptations directly related to reproduction (e.g., gestation, nursing): usually the female 3. behavioral adaptations (and costs): egg-guarding, protecting and feeding young: females or males (more commonly the female) In most animals, female PI is much greater than male PI SEXUAL SELECTION Trivers: Key to sexual selection is parental investment (PI). Trivers: Sexual selection = 1. Sex investing less will compete for sex investing more (intrasexual selection) 2. Sex investing more will be discriminating in choosing mates (epigamic selection) Typically: Female PI > Male PI so usually its males competing among themselves for females and females choosing among males. Let s consider some consequences The major sex differences (Trivers 1985) (1) Relative parental investment Typically: female PI > male PI (2) Degree of intrasexual selection Typically greater in males (3) Discrimination in mate choice Typically females are discriminating, males are not. General Sex Differences in Behavior Male: ardent, indiscriminant Female: reluctant, discriminating FEMALES Don t you Dare. MALES Laissez Faire. 3

4 Trivers: Differential Parental Investment drives Sexual Selection Definitely not meant to be taken too seriously! It s a caricature, but shows where you could get if you take the sex differences argument to its extreme! Sexual Selection - Female PI Sex role reversal: females compete more intensely than males for mates (and males are more choosy) a test of Trivers hypothesis that differential PI drives sexual selection Male PI > Female PI Female RV > Male RV female competition and male choice Seahorses male gestation & lactation Mormon crickets spermatophores Polyandrous species (e.g., Jacana) mating systems terminology derived from anthropology 4

5 polygyny: male RV > female RV monogamy: male RV female RV polyandry: female RV > male RV polygamy: polygyny or polyandry promiscuity: no bonds, males and females mate multiple times Most common mating systems: mammals: polygyny, -only PC birds: monogamy, shared PC fish: polygyny, -only PC most other animals: polygyny, -only PC Important point: The degree of asymmetry in RV is an indicator of the intensity of sexual selection. Trivers: PI drives choice in the sex investing more and competition in the sex investing less. Thus as PI in one gender, RV in the other. 5