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From this module you will learn:


1.  Why Sex?

Question: parthenogenesis (asexual reproduction) has a twofold advantage in projecting the individual's genes into the next generation (see next exhibit)!  If so, why would sexual reproduction evolve at all, especially given all the complications sexual reproduction entails?

Answer: sexual reproduction enhances the genetic variability of the offspring, allowing adaptation to unforeseen environments, and resistance to co-evolving parasites.  (This issue is the object of a lot of research.  See Matt Ridley.  1993.  The Red Queen.)

2.  Why Two Sexes?

Given that sexual reproduction has evolved, a separate question is why there has been the evolution of just two sexes, male and female, rather than more than two sexes, or a "unisex" system in which any individual can  reproduce with any other.  According to the mathematical model of Parker, Baker, and Smith the generic difference between males and females (small sperm versus large ovum) evolved by a process of disruptive selection.

Q - Why is sexual reproduction paradoxical?

Q - What is the main advantage of sexual reproduction?

Q - What does parthenogenesis have to do with the Parthenon in Athens (if anything)?

Q - What is disruptive selection?


 Main results of A. J. Bateman's experiments with fruit flies:  Bateman (1948) concludes:
"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".
Greater variance of reproductive success (RS) of males is also typical of human populations. Effective polygyny = (Variance in RS of males)/(Variance in RS of females); it is a measure of reproductive competition among males.  For example, in Xavante, effective polygyny = 12.1/3.9 = 3.1 (the variance in reproductive success of males is more than three times that of females).

Robert Trivers (1972) has generalized Bateman's results as the theory of parental investment.  Parental investment is:

"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)
The sex investing the most in the offspring (usually, but not always, the female) constitutes a limiting resource for the other sex and therefore the object of competition.   As a result:
"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 more so that competition is greater among males.  As a consequence, in many species males are inclined to "riskier" behavior and have lower life expectancy.  The riskier lifestyle and greater vulnerability of males is a recurrent pattern in nature: In some species, competition among males is expressed in a social hierarchy in which higher ranking males have greater sexual access to females and greater RS as a result. The beauty of Trivers' theory of parental investment is that it is so general.  It works even in rare cases of "sex-roles reversal" in which males invest more parentally than females.  Examples of sex-roles reversals include seahorses, the giant water-bugs, and phalarope birds. Q - "Because of different reproductive strategies men generally have a higher mean number of sexual partners"  TRUE/FALSE?

Q - "Because of different reproductive strategies men generally have a higher variance in number of sexual partners"  TRUE/FALSE?


In most species females are the sex investing most in each offspring.  Therefore, a female can maximize her RS by choosing a male judiciously.  What criteria should she use?

1.  The "Domestic-Bliss" Strategy

In species in which males care for the offspring, the female may try to select a male who will stick around and care for the offspring.

To do this:

  •  look for signs of domesticity, such as patience during a long courtship (Q - Is a long period of courtship useful only for females, or does it have advantages for males also?)
  •  select a male who controls resources to provide for the offspring, such as a good quality territory
  • The domestic-bliss strategy underlies a system of polygynous mating in certain birds called resource defense polygyny.  In resource defense polygyny a male defends a territory and is joined by one or more females attracted by the resources it contains.  NOTE:  Research has shown that a similar model may apply to a human population, the Kipsigis of Kenya studied by Monique Borgerhoff-Mulder.

    2.  The "He-Man" Strategy

    In species in which males do not participate in raising the offspring, all a female can do is try at least to select a male with "good genes".  Dawkins calls this the he-man strategy. 

    To implement the he-man strategy:

    3.  The "Madame Bovary" Strategy

    Emma Bovary is the heroin of the 1856 novel Madame Bovary by Gustave Flaubert.  In the novel Emma finds it difficult to choose between her husband and her lover.  (It all ends disastrously.)

    The Madame Bovary strategy: If you can, keep both a husband to provide care for the offspring, and a lover to provide "good genes".

    Researchers have found a number of clues suggesting that female infidelity played an important role in the evolution of human sexuality:

  • Studies of (supposedly) monogamous birds using genetic testing have found high rate of "illegitimacy", indicating high incidence of extra-pair copulation.
  • Comparative studies of colonial birds (birds living in large colonies) have found that mating in private (as opposed to publicly) is associated with a higher prevalence of extra-pair copulation; by analogy the human tendency to mate in private suggests that extra-pair copulation may have been prevalent in human evolutionary history.
  • The absence of estrus (= distinct period of sexual receptivity, or "heat") in the human female reduces her mate's ability to monitor her sexual activity; an influential theory is that estrus has been lost in humans because its absence gives a female more control over the paternity of her offspring.
  • The relatively large testicles of human males (with size intermediate between very large testicles of promiscuous chimpanzees and small ones of harem-keeping gorilla) suggest a high degree of sperm competition during human evolution; sperm competition is associated with high levels of extra-pair copulation.
    Exhibit: Testis size and sperm count in Hominoidea (SEB Table 12-1 p. 318)
  • Some studies using genetic testing have found high rates of illegitimate paternity in contemporary human populations.
  • These clues suggest that the Madame Bovary strategy has evolved to be part of the behavioral repertoire of human females.  It is furthermore possible that a greater concern of males about the fidelity of their mate (i.e., jealousy) may have evolved as a "counterstrategy" to the Madame Bovary strategy (see later).

    Q - According to Dawkins, a female tendency to mate with older males is an instance of what reproductive strategy?

    Q - The assumption that it may be better for a female to be the second wife to a wealthy male than the first wife to a poor male is part of what theory?

    Q - Why have human females lost the estrus (period of "heat" accompanied by physiological changes marking ovulation and sexual receptivity in other mammals)?


    1.  Comparative Reproductive Strategies

    Frequency of various mating systems in birds and mammals
    Mating System Birds Mammals
    Monogamy 92% Sometimes (especially carnivores)
    Polygyny 2% Predominant
    Promiscuity 6% Predominant
    Polyandry <0.5% Almost never
    In different species, male strategies of reproduction range from heavy parental investment in the offspring (the Dad strategy) to no parental investment at all (the Cad strategy).  Whenever males invest parentally (i.e., engage in the Dad strategy) they are under selective pressure to prevent their mate from being inseminated by another male.  In such species males typically seem to exhibit concern about the sexual fidelity of their mate ("jealousy", in human terms) and engage in a variety of guarding behaviors to keep rival males away from their mate.   

    Q - Why do birds exhibit a greater propensity to monogamy (with high levels of paternal investment) compared to mammals?  A - The "cruel bind" argument (Trivers): internal fertilization in mammals gives males a prior opportunity to desert.  In birds, the egg is detached from the female earlier, so the incentive to provide care is greater for male.

    What is the "natural" mating system of the human species, on a continuum from strict monogamy (one man, one woman) to a high degree of polygyny (one man, many females)?  In comparing species, a greater degree of polygyny is associated with greater sexual dimorphism ( = difference in size and appearance between male and female):

    The typical human sexual dimorphism of 1.08 (measured as the ratio of average stature of male to average stature of female).  Interpolating the regression line of male harem size on sexual dimorphism for primates suggests that humans are "slightly" or "mildly" polygynous.

    2.  Generic Differences in Reproductive Strategies of Males & Females

    Summary of Daly & Wilson in SEB (p. 280):
     "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."
    A number of clues are consistent with parental investment theory in explaining human mating systems and behavioral differences between males and females:
  • 83.4% of human societies allow polygyny (as either a usual or occasional marital arrangement); however, the majority of human marriages are monogamous (can you see why?)
    Exhibit: Marriage practices in 849 human societies (SEB Figure 11-1 p. 282)
    NOTE: polyandry exists but is extremely rare, found only in two regions: in Nigeria (in the form of sequential cohabitation) and in the Himalayas (in the form of fraternal polyandry to prevent partition of family holdings).
  • In 67.4% of 860 societies in Ethnographic Atlas, the groom or his kin "pays" for the wife through bride-price, bride-service, or direct exchange of women; dowry exists in only 2.6% of societies.
    Exhibit: Exchange of considerations at marriage (SEB Figure 11-3 p. 289)
  • Because they invest parentally, human males in general exhibit a greater concern over the fidelity of their mate; this is reflected in the double standard in adultery laws and customs, where typically the husband of the adulteress is viewed as the victim, rather than the wife of the adulterer.
  • Male-female difference in the content of sexual jealousy, as revealed in a study by psychologist David Buss in which male subjects find sexual infidelity of mate (sexual intercourse with someone else) more upsetting than emotional infidelity (forming a deep emotional attachment to someone else, and female subjects find emotional infidelity more upsetting: "Women's jealousy [...] is triggered by cues to the possible diversion of their mate's investment to another woman, whereas men's jealousy is triggered primarily by cues to the possible diversion of their mate's sexual favors to another man" (David Buss.  1994.  The Evolution of Desire, p. 128)
  • The finding that physical injury of victim during a rape is associated with lesser (rather then greater) difficulties with the male partner (boyfriend or husband) of the victim after the rape.  (Why?)
  • Greater attention paid to the resemblance of a newborn baby with the father rather than the mother.
  • Widespread use of coercive constraints of women's sexuality (confinement, genital mutilations reducing the opportunity or desire for extramarital copulation, chador, chastity belts, etc.).
  • Greater male vulnerability (as measured by greater mortality at all ages).
    Exhibit: Sex differences in age-specific mortality (SEB Figure 11-4 p. 298)
  • Greater male risk-taking propensity (as illustrated by driver death risk).
    Exhibit: Driver death risk in U.S. 1970 (SEB Figure 11-5 p. 300)
  • Q - What is the main ultimate cause of differences between male and female reproductive strategies in mammals?

    Q - In what species would one predict that males are most concerned with the fidelity of their mates (i.e., jealous)?

    Q - Which marriage practice (monogamy, polygyny, polyandry) is most common among human societies?  Among human marriages?


    Genetically-based behavior differences between males and females are inherently paradoxical, since males and females share most of the same genes.  Therefore, a behavior increasing fitness in one sex should increase fitness in the other (for the same reasons that males have nipples)

    Sex differentiation depends on hormonal exposure during early development in the womb (the organizing role of hormones), and later production of hormones at puberty (the activating role of hormones).  NOTE: the role of hormones in sex differentiation was discovered by Alfred Jost.

    The "social interpretation" of sex differences as division of labor between the sexes in human societies: Are there behavioral/psychological differences between males and females underlying sex roles?

    A classic literature review by Eleanor Maccoby and Carol Jacklin (1974.  The Psychology of Sex Differences) identified only four "real" sex differences:

  • greater verbal ability of females
  • greater visual-spatial ability of males
  • greater mathematical ability of males
  • greater aggressiveness of males
  • BUT: Maccoby & Jacklin's method of using overly broad concepts (such as "sociability" or "dominance") tends to blur sex differences, making their list too conservative

    More recent research has found many more psychological differences between males and females (Doreen Kimura.  1992.  "Sex Differences in the Brain."  Scientific American, September:119-125)

    Summary of male-female differences discussed by Doreen Kimura (1992)
    Difference Size*
    Target-directed motor skills M>F 0.75
    Spatial tasks (e.g., imagine rotating objects in space) M>F 0.70
    Mathematical reasoning  M>F N/A
    Navigating through a route** M>F N/A

    Arithmetic calculations F>M N/A
    Recalling landmarks from route  F>M N/A
    Precision manual tasks F>M N/A
    Vocabulary F>M (?) 0.2
    Non-verbal reasoning F>M (?) 0.3
    Verbal reasoning F>M (?) 0.17
    Verbal fluency (e.g., list words beginning with given letter) F>M 0.22
    Perceptual speed (e.g., identify matching items) F>M 0.25
    Ideational fluency (e.g., list objects that are the same color) F>M 0.38

    * "effect size" is difference in mean scores divided by the standard deviation of scores
    ** Kimura also found that men and women use very different techniques to navigate through a route: women tend to rely on landmarks (such as a building or distinctive tree) while men rely preferentially on spatial cues such as distance and direction

    Are  M-F differences a relic of sexual division of labor in hunting-gathering lifestyle during human evolution?
  • M:  hunting large game, using weapons
  • F:  gathering plant food, tending home, caring for children

  • Exhibit: Doreen Kimura's conclusion (Kimura 1992, p. 125)


    The r-strategy versus K-strategy of reproduction: many offspring with high mortality rate (r-strategy) versus few offspring with low mortality rate (K-strategy).

    EX: the evolution of two types of dandelions in lawns that are frequently mowed (small, fast growing r-strategist dandelion) versus lawns that are infrequently mowed (larger, more slowly growing K-strategist dandelion).

    As a species, humans are K-strategists

     Last modified 14 Sep 2004