The Human Mating System Note: this is the stereotypical EP view (e.g., Buss) Types of Mating Systems Random Monogamy Polygyny: variance in male reproductive success > variance in female reproductive success Polyandry:variance in female reproductive success > variance in male reproductive success What Factors Influence Mating System Evolution? Mode of fertilization (internal vs. external: if internal, can the female store sperm?) Offspring ecology (how much parental care is required) Degree of asymmetry in parental investment in offspring Mammal Reproductive Characteristics Internal fertilization, with an intromittent organ, the penis No sperm storage Offspring have huge energy demands Asymmetry in parental investment is extreme: females provide essentially all of the energy (gestation and then lactation) to raise a zygote to independence 1
Primate Mating Systems Monogamy rare occurs in some newworld primates and in gibbons Polygyny common occurs in most monkeys and in orangutans, gorillas, chimpanzees, bonobos Why are Most Mammals Polygynous? Precise male control of sperm transfer Huge energy cost of offspring Extreme asymmetry in parental investment in offspring females provide everything, males provide a sperm cell Especially in mammals, females are a reproductive resource over which males compete Evolutionary Results of Male-Male Competition for Mates Sexual size dimorphism Male weapons Fighting Male delayed maturation Increased male mortality rates Sperm competition (sometimes) Forced copulation (sometimes) Infant killing by males (sometimes) Behavioral emphasis on copulation In mammals, the degree of sexual dimorphism is generally correlated with the degree of polygyny Variance in male RS/variance in female RS Male Mass/Female Mass 2
Weapons Triangular relationships: 2 men compete over one woman Male baboon Female baboon Human Sex-Specific Survivorship Sperm Competition 100000 females males Number Surviving 80000 60000 40000 USA 1994 20000 0 0 20 40 60 80 100 120 Age 3
Bellis, M. & Baker, R. 1995. Human sperm Study Couples used condoms each time they mated Immediately after mating, contents of the condom were placed in fixative, then delivered to the lab by the next day In lab, measure ejaculate volume and number of sperm ejaculated With each sample, the couple reported: Time since previous mating Percent time apart during that interval Bellis, M. & Baker, R. 1995. Human sperm Predictions The number of sperm ejaculated should vary with likelihood of sperm competition The number of sperm ejaculated should not vary with time since the previous mating Bellis, M. & Baker, R. 1995. Human sperm Bellis, M. & Baker, R. 1995. Human sperm Number of sperm ejaculated was independent of ejaculate volume Time since the last ejaculation was a weak predictor of number of sperm ejaculated Number of sperm ejaculated varied positively with partner body mass Number of sperm ejaculated varied inversely with percent time together since the last ejaculation 4
Infant Killing Infant Killing Daly & Wilson, 1988: Homicide Infant killing in the adaptive pattern that occurs in primates living in single-male groups (and sometimes in others, e.g., chimps) If it occurs in humans, it should happen when a mother and her young child live with a male who is not the child s sire. Daly & Wilson (1985): Child abuse and other risks of not living with both parents Evolutionary hypothesis: Parental feeling should vary with prospective fitness value of child to parent when step-parent is called to fill parental role toward unrelated child, we may anticipate an elevated risk of lapses of parental solicitude (inc. child abuse) Previous studies had various problems. (1) confound with SES (2) is abuse a specific effect or just one aspect of a bad rearing environment ( broken home syndrome)? test: compare parent+step-parent households with single parent households. Daly & Wilson (1985): Child abuse and other risks of not living with both parents Hamilton, Ontario. Called homes in survey to get base-rates on family composition (2 natural parents; 1 natural + 1 step; 1 natural alone). Used postal code to tag with SES. Child abuse sample: 99 kids (info: children s aid societies) Police sample: 542 kids (99 runaways, 449 criminal offenses) Had household composition data for these cases. Two data sets combined to give rates. 5
Behavioral Emphasis on Copulation Buss, D. 1994. The Evolution of Desire. New York: Basic Books Anonymous questionnaires at elite American universities Sought to discover whether men and women have different attitudes about copulation Question How strongly are you seeking a spouse? How strongly are you seeking a one-night stand? How many sex partners do you want in: The next month The next 2 years Your lifetime Would you consider having sex with a desirable partner you have known for: 5 years 2 years 6 months 1 week Less than 1 week 0.8 1 4-5 Buss, 1994 Women Fairly strongly Not strongly Probably yes Probably yes Neutral Definitely not Definitely not Men Fairly strongly Pretty strongly 2 8 18 6
Clark, R. & Hatfield, E. 1989. J. Psychology and Human Sexuality 2: 39-55 Clark, R. & Hatfield, E. 1989. J. Psychology and Human Sexuality 2: 39-55 Hired attractive men and women to approach strangers of the opposite sex on college campuses and to pose the question: I have been noticing you around campus. I find you very attractive. A. Would you go out with me tonight? B. Would you come over to my apartment tonight? C. Would you go to bed with me tonight? Question Date? Come to my apartment? Go to bed with me? Women 0.5 0.06 0.0 Men 0.5 0.69 0.75 Conclusions Compared to women, men are Larger, more muscular More violent More likely to die at any age Much more likely to kill a same-sex rival More directly focused on copulation, sex partner variety All evidence points to an evolutionary history of polygyny in humans Human males show evidence of adaptation to sperm competition Human males show infanticide in the adaptive pattern (debateable) Human males show strong behavioral interest in copulation alone The Double Standard Imagine a married couple where each of the partners (separately) has an adulterous affair (disregard the extra-pair partners). Despite the apparent symmetry of this situation, most societies consider this an asymmetrical situation, and one of the married pair is considered to be a victim and to have a serious grievance. In many societies, the victim has legal recourse and may even be permitted violent revenge or receive only mild punishment for revenge. (a) Which one is considered the victim, and why? (b) What is the victim called (the term dates to Shakespeare s time or before)? cuckold (c) That term is derived from animal behavior from what do you think? cuckoo a brood parasite 7