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Soci111 – Human Societies Module 6 – Strategies of Reproduction – Evolutionary Foundations of Sex Roles François Nielsen University of North Carolina Chapel Hill September 18, 2016 Outline Main Themes Paradoxes of Sexual Reproduction Parental Investment Theory Female Strategies Male Strategies Human Consequences Biological Basis of “Gender Roles”? Main Themes É why sexual (versus asexual) reproduction is paradoxical É why male-female distinction evolved Bateman’s experiments with drosophilae & Trivers’ theory of parental investment: É É É É É whichever sex invests most in offspring is object of competition whichever sex invests least competes more not necessarily the way we think! generic female strategies É É É domestic bliss strategy he-man strategy Madame Bovary strategy É generic male strategies: dad or cad É human implications of parental investment theory É evolutionary bases of men-women differences? Paradoxes of Sexual Reproduction Why Sex? É parthenogenesis (asexual reproduction) has a 2-fold advantage over sexual reproduction É how can sexual reproduction evolve? ⇐ sexual reproduction: É É É É increases genetic variability of offspring ⇒ adaptation to unforeseen environments ⇒ resistance to co-evolving parasites Paradoxes of Sexual Reproduction Why Just Two Sexes? É É why not 3, 4,. . . , or none? theory of Parker, Baker, & Smith: É É É É É male-female distinction = small gamete vs. large gamete evolved by disruptive selection = intermediate sizes selected against but: É É Joan Roughgarden in Evolution’s Rainbow shows intermediate strategies widespread in nature Paradoxes of Sexual Reproduction Review Questions É Q – Why is sexual reproduction paradoxical? É Q – What is the main advantage of sexual reproduction? É Q – What does parthenogenesis have to do with the Parthenon in Athens (if anything)? É Q – What is disruptive selection? Parental Investment Theory Bateman’s Experiments É É A. J. Bateman’s (1948) experiments with fruit flies compare reproductive success of males vs. females É É É É females gain nothing by mating with more than one male males gain by mating with many females males vary more in reproductive success (number of mates & number of offspring) than females Bateman (1948) concludes É “Greater dependence of males for their fertility on frequency of insemination [is] an almost universal attribute of sexual reproduction” leading to the evolution of “an undiscriminatory eagerness in the male and a discriminatory passivity in the female” Parental Investment Theory Male vs. Female Reproductive Success (RS) in Humans É Xavante Indians in Matto Grosso jungle of Brazil É show pattern of greater variance in male RS effective polygyny É É É É = ratio of male to female variance in RS measure of reproductive competition among males 12.1/3.9 = 3.1 in Xavante Parental Investment Theory Trivers’s Parental Investment Theory É É Robert Trivers (1972) (again!) generalizes Bateman’s results as parental investment theory É É É É É parental investment = “any investment by the parent in an individual offspring that increases the offspring’s chance of surviving (and hence reproductive success) at the cost of the parent’s ability to invest in other offspring” (p. 139) sex investing the most constitutes limiting resource for other sex “whichever is the sex with greater parental investment will be the sex that is courted, that competes less, and that survives better” (SEB p. 159) in most species females invest most ⇒ males É É É compete more inclined to riskier behavior have lower life expectancy Parental Investment Theory Male Riskier Lifestyle & Greater Vulnerability É a Female Soay sheep survive longer than malesa SEB Figure 7-7 p. 94 Parental Investment Theory Male Riskier Lifestyle & Greater Vulnerability É a Females have longer life expectancy in peoplea SEB Figure 5-9 p. 96 Parental Investment Theory Male RS Related to Rank in Dominance Hierarchy É a Correlation of male RS (copulatory frequency) with rank in dominance hierarchy in yellow baboonsa SEB Figure 5-4 p. 86 Parental Investment Theory Sex-roles Reversal in Seahorses É But theory says whichever sex invests most! É when male invest most, “traditional” sex roles reversed É e.g. seahorses, phalarope birds, giant waterbugsa a SEB Figure 7-5 p. 157 Female Strategies Domestic Bliss Strategy É female usually invest most in each offspring, so É É É three female strategies É É É É 1 will stick around is willing and able to invest in offspring to do this look for male who É É (1) domestic bliss, (2) he-man, (3) Madame Bovary strategies1 in species where male cares for offspring use domestic bliss strategy = select male who É É can maximize RS by choosing a male judiciously but by which criteria? is patient in long courtship controls resources, e.g. good quality territory Q – BTW is a long courtship useful only to females? Terms from Richard Dawkins in Selfish Gene Female Strategies Domestic Bliss Strategy & Orians-Verner-Willson Model É domestic-bliss strategy basis of O-V-W model of resource defense polygyny in migratory birdsa É Monique Borgerhoff-Mulder finds similar patterns of polygyny in Kipsigi people of Kenya a SEB Figure 6-2 p. 119 Female Strategies He-Man Strategy É in species in which males do not participate in raising the offspring É all a female can do is try and select male with “good genes” É this is the he-man strategy to do this É É É É É É good bet is select older male (age proof of survival ability) choose male with most conspicuous display, e.g. peacock with most wonderful tail originally may be sign of vigor & health can lead to runaway sexual selection = evolution of dysfunctional ornaments in males explained in Richard Dawkins, 1989, Selfish Gene, pp. 158–9 Female Strategies He-Man Strategy & Runaway Sexual Selection É peacock tail epitomizes how he-man strategy É É can lead to runaway sexual selection producing dysfunctional trait in males Female Strategies Madame Bovary Strategy É Emma Bovary is bored & starts adulterous affair É story ends badly Madame Bovary strategy = keep both É É Isabelle Huppert in Claude Chabrol’s Madame Bovary (1991) after 1856 novel by Gustave Flaubert É a husband to invest parentally in offspring a lover to provide “good genes” Female Strategies Madame Bovary Strategy É clues suggesting that female infidelity played important role in evolution of human sexuality É É high rates of “illegitimacy” among (supposedly) monogamous birds, indicating high incidence of extra-pair copulation in colonial birds (birds living in large colonies) mating in private is associated with higher prevalence of extra-pair copulation; by analogy É É É absence of estrus (= distinct period of sexual receptivity, or “heat”) in human female É É É humans mate privately means that extra-pair copulation prevalent in human evolutionary history? reduces mate’s ability to monitor her sexual activity evolved because gives female more control over paternity of her offspring? some studies find high rates of illegitimate paternity in contemporary human populations Female Strategies Madame Bovary Strategy É É a big clue about evolution of human sexuality compared to males in other primates É É SEB Table 12-1 p. 318 É É humans have large testicles large ejaculates suggests high degree of sperm competition in human evolution importance of Madame Bovary strategy Female Strategies Review Questions É Q – What might be a male counterstrategy to the Madame Bovary strategy? É Q – A female tendency to mate with older males found in some species where males do not invest parentally is an instance of what reproductive strategy? É Q – The assumption that a female may be better off as second wife to a wealthy male than as first wife to a poor male is part of what theory? É Q – What is the predominant theory why human females have lost the estrus (period of “heat” accompanied by physiological changes marking ovulation and sexual receptivity) found in other primates? Male Strategies: Dad or Cad Comparative Reproductive Strategies Table: Types of mating systems in birds and mammals Mating System Monogamy Polygyny Promiscuity Polyandry Birds 92% 2% 6% <0.5% Mammals Sometimes (carnivores) Predominant Predominant Almost never É Q – Why do birds exhibit a greater propensity to monogamy (with high levels of paternal investment) compared to mammals? É A – Internal fertilization in mammals gives male prior opportunity to desert. In birds, the egg is detached from the female earlier, so incentive for male to provide care is greater Male Strategies: Dad or Cad Comparative Reproductive Strategies É across species male strategies range from É É É É dad strategy = heavy parental investment in offspring, to cad strategy = no parental investment Q – A sociobiologist has said “In mammals, the male bring home the bacon, or nothing at all”. What does he mean? when males invest parentally É É É males are under selective pressure to prevent insemination of mate by other males ⇒ greater concern about fidelity of mate (“jealousy”) ⇒ guarding behaviors to keep rival males away from mate Male Strategies: Dad or Cad Comparative Reproductive Strategies É across species greater polygyny (measured as harem size) associated with greater sexual dimorphism ( = difference in size and appearance between male and female)a É Q – Is there a “natural” human harem size? É A – In humans length sexual dimorphism is 1.08; interpolating relationship harem size × sexual dimorphism suggests “slight” or “mild” polygyny! a SEB Figure 7-3 p. 153 Human Consequences Summary of Generic Male-Female Differences In most animal species, the female’s greater investment in each offspring means that her maximal reproductive potential is lower than the male’s. Males therefore compete among themselves for fertilization opportunities. Investing little in each offspring, males are selected to sow their seed wherever opportunity arises. Investing considerably in each offspring, females are selected to exhibit greater selectivity in their choice of mates. One feature on which females may exercise selectivity is the male’s willingness or ability to make an effective parental contribution. But wherever males do in fact invest parentally, they are under selective pressure to protect themselves against cuckoldry, and therefore males have a greater concern than females over the fidelity of their mates. Martin Daly & Margo Wilson, Sex, Evolution & Behavior, p. 280 Human Consequences Features of Human Mating Systems & male-Female Differences É 83% of human societies allow polygynya É however majority of human marriages are monogamous (can you see why?) polyandry rare É É É a Nigeria (sequential cohabitation) Himalayas (fraternal polyandry) SEB Figure 11-1 p. 282 Human Consequences Features of Human Mating Systems & Male-Female Differences É in 67% of human societies groom or his kin “pays” for bride througha É É É bride-price bride-service direct exchange of women É only 3% of societies have dowry (bride or her kin pays) É Q – What kind of society would have dowry? a SEB Figure 11-3 p. 289 Human Consequences Features of Human Mating Systems & Male-Female Differences É double standard in adultery laws & customs, reflecting É É É sex difference in content of sexual jealousy2 É É É male parental investment greater concern of investing male over fidelity of mate males: more upset by sexual infidelity of partner (intercourse with someone else) females: more upset by emotional infidelity (deep attachment to someone else) study of difficulty with male partner in aftermath of rape É É physical injury of victim during rape → less difficulty no physical injury → more difficulty (why?) É greater attention paid to resemblance of newborn baby with father rather than mother É coercive constraints of women’s sexuality (confinement, genital mutilations, chador, chastity belts, . . . 2 David Buss. 1994. The Evolution of Desire, p. 128 Human Consequences Features of Human Mating Systems & Male-Female Differences É greater male vulnerabilitya É É a males have greater mortality than females at all ages difference especially large for young persons SEB Figure 11-4 p. 298 Human Consequences Features of Human Mating Systems & Male-Female Differences É greater male risk-taking propensitya É a e.g., greater driver death risk SEB Figure 11-5 p. 300 Human Consequences Review Questions É Q – What is the main ultimate cause of differences between male and female reproductive strategies in mammals? É Q – In what species would one predict that males are most concerned with the fidelity of their mates (i.e., “jealous”)? É Q - Which marriage practice (monogamy, polygyny, polyandry) is most common among human societies? Among human marriages? Biological Basis of “Gender Roles”? Prenatal Sexual Differentiation É genetic basis of sex behavior differences paradoxical É É É Organizing role of hormones in the womb discovered by Alfred Jost sex differentiation is hormonala É É a males & females share most genes advantageous traits should be shared (e.g., nipples) in the womb (organizing role) at puberty (activating role) SEB Figure 10-3 p. 252 Biological Basis of “Gender Roles”? Hormonal Changes At Puberty É a activating role of hormones at pubertya SEB Figure 10-5 p. 255 Biological Basis of “Gender Roles”? Sexual Division of Labor in Human Societies É “social interpretation” of sex differences É a division of labor between sexes in human societiesa SEB Figure 10-1 p. 262 Biological Basis of “Gender Roles”? Sex Differences in the Brain É É Are there behavioral/psychological differences between males and females underlying gender roles? in classic literature review The Psychology of Sex Differences (1974) Eleanor Maccoby and Carol Jacklin had identified only four “real” sex differences É É É É É greater verbal ability of females greater visual-spatial ability of males greater mathematical ability of males greater aggressiveness of males Maccoby & Jacklin’s method criticized because É É É use overly broad concepts (e.g. “sociability” or “dominance”) which tends to blur sex differences making their list of “real” differences too conservative Biological Basis of “Gender Roles”? Sex Differences in the Brain É ← Doreen Kimuraa É more recent research finds many more psychological differences between males and females É e.g. Doreen Kimura. 1992. “Sex Differences in the Brain.” Scientific American September: 119–125 a 1933–2013. Emerita from Western Ontario U.; Visiting Professo Fraser U., British Columbia Biological Basis of “Gender Roles”? Sex Differences in the Brain É É ← Tasks favoring women Also big difference in techniques to navigate a route É É women rely on landmarks (e.g. building or distinctive tree) men rely on spatial cues (e.g. distance and direction) Biological Basis of “Gender Roles”? Sex Differences in the Brain É É ← Tasks favoring men Are M-F differences relic of sexual division of labor during human evolution in hunting-gathering lifestyle? É É M: hunting large game, using weapons F: gathering plant food, tending home, caring for children Biological Basis of “Gender Roles”? Male-Female Differences Discussed by Kimura (1992) Task Difference Target-directed motor skills Spatial tasks Mathematical reasoning Navigating through a route Arithmetic calculations Recalling landmarks from route Precision manual tasks Vocabulary Non-verbal reasoning Verbal reasoning Verbal fluency Perceptual speed Ideational fluency M>F M>F M>F M>F F>M F>M F>M F>M (?) F>M (?) F>M (?) F>M F>M F>M 3 Size3 0.75 0.70 N/A N/A N/A N/A N/A 0.20 0.30 0.17 0.22 0.25 0.38 “effect size” is difference in mean scores divided by standard deviation of scores Biological Basis of “Gender Roles”? Origins of M-F Differences in Primordial Division of Labor É Doreen Kimura’s conclusion to “Sex Differences in the Brain”