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 Intrasexual Natural Selection Sexual Selection Survival Mating Success Parental Care Intrasexual Competition Mate Mate Choice (Epigamic) Sexual Selection Intersexual or Epigamic Intrasexual Selection Epigamic Selection Elephant Seal Wilson s Bird of Paradise 1

2 Epigamic Selection Superb bird of paradise 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! selection equally on both conventional sex role reversed 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 ~ no selection on either Red-winged Blackbird 2

3 Sexual selection can occur even in monogamous species with biparental care its just less intense Northern Cardinal strong epigamic selection on both sexes? Anisogamy Anisogamy = Unequal gametes Eggs: few, large, energetically expensive Sex role reversal: Wattled Jacana Sperm: many, small, cheap 3

4 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 General Sex Differences in Behavior Females as the discriminating sex Male: ardent, indiscriminant Female: reluctant, discriminating FEMALES Don t you Dare. MALES Laissez Faire. Males as the less discriminating sex 4

5 Kinds of Mate Choice 1) Good looks 2) Good genes 3) Good parent 4) Good resources Mate Choice? Rational choice how signal? how detect? indirect benefits boil down to PI direct benefits Mate Choice and Sexual Selection Direct benefits Female choice leads to increased fecundity (more offspring). Good genes Female choice leads to improved genetic quality of offspring re survival. * Runaway selection Female preference increases because it is linked to sexy son advantage. Handicap selection Female prefers signals that are handicaps. * Sensory exploitation Male evolves display trait that exploits pre-existing sensory bias in female. * non-rational choice hypotheses The field is now wide open and many opinions are possible. Indeed, it remains to be seen whether female choice is an arbitrary and amusing evolutionary sideshow or a powerful force driving male evolution for female benefit. Trivers 1985 Direct Benefits Polygyny threshold model Female chooses mated male (polygyny) rather than unmated male (monogamy) if she gets more direct benefits from that choice. 5

6 Good Genes Good Genes controls -20 eyespots Uakari monkey Malaria parasites Petrie & Halliday 1994 Cut out eyespots on some males and did mate choice tests. Honest signals are uncheatable, and provide accurate information about health, vigor, and general condition Female peacocks definitely prefer males with more eyespots... but are they better quality males? Good Genes Honest signaling: Handicap model % of chicks surviving after 2 years Alcock Fig Petrie 1992 Fitness cost or benefit benefit optimum cost low quality cost high quality Mean area father s eyespots (mm 2 ) 8 males X 4 females each 96 offspring released Apparently #eyespots does signal better quality males (pass on better genes) Signal intensity An epigamic signal will be an honest indicator of the condition (quality) of the male if it is less costly (a smaller handicap) for good condition (high quality) males than for low condition (low quality) males. 6

7 Sensory Exploitation Sensory biases in zebra finches: Females prefer males with certain color bands Females prefer males with artificial feather hats over normal males! Human Mating Systems & Mate Choice Monogamy most common but sequential, also extra-pair mating Polygyny (mild) also fairly common Polyandry (milder) also found (rare) Bi-parental care, division of labor Buss (1994): The Strategies of Human Mating 1. Human mating is inherently strategic. Strategies solved specific problems in human evolutionary history. The manifestation of these strategies need not be through conscious psychological mechanisms; in fact they generally will not be. Sex Differences in Mate Choice Human mean mate preference scores in 9,474 people from 37 different cultures. Rating scores vary from 0.00 (irrelevant or unimportant) to 3.00 (indispensable). 2. Mating strategies are contextdependent in particular, we need to distinguish short-term vs. longterm strategies. 3. Men and woman have faced different problems, therefore have evolved different strategies. 7

8 Hypothesis 1: Short-term mating is more important for men than women Hypothesis 2: Men seeking a short-term mate will solve the problem of identifying women who are sexual accessible Hypothesis 3: Men seeking a short-term mate will minimize commitment and investment Hypothesis 4 and 5: Men seeking a shortterm mate will solve the problem of identifying fertile women, whereas men seeking a long-term mate will solve the problem of identifying reproductively valuable women Hypothesis 6: Men seeking a long-term mate will solve the problem of paternity confidence Hypothesis 7: Women seeking a short-term mate will prefer men willing to impart immediate resources Hypothesis 8: Women will more selective then men in choosing a short-term mate Hypothesis 9: Women seeking a long-term mate will prefer men who can provide resources for their offspring Hypothesis 4 and 5: Men seeking a shortterm mate will solve the problem of identifying fertile women, whereas men seeking a long-term mate will solve the problem of identifying reproductively valuable women Fertility: probability woman is currently able to conceive (ex: 14 yo < 24 yo) Reproductive value: individual s expected future reproduction (ex: 14 yo > 24 yo) Importance of physical attractiveness 3 important, 0 unimportant Men Women Short-term Long-term Hypothesis 6: Fidelity more important to men Hypothesis 8: Women will more selective then men in choosing a short-term mate Study: imagine two scenarios: 1) your partner having sex with someone else, or 2) your partner falling in love and forming a deep emotional attachment to someone else Men: 1 is worse than 2 Women: 2 is worse than 1 3 definitely yes -3 definitely no 8