Chapter 4:
Male and Female:
The Fundamental Differences
Copyright © 2002 Michael E. Mills
Opening Comments.
As we learned in Chapter 2, human females and males have somewhat dimorphic brains. Following this proximate difference, we will now ask an ultimate, or "why," question: Why did the brains of males and females evolve to be different? Both sexes must have faced similar problems of survival in the ancestral environment, given that both evolved in the same ecological niche. However, we shall see that the set of reproductive problems males and females faced were often quite different. It is these two different sets of problems that caused the sexes to evolve dimorphic bodies and brains. However, before we examine the set of reproductive problems unique to each sex, here we will first identify the most fundamental, enduring differences between males and females. We should note that the focus in this chapter will be on mammalian sex differences. As we shall see, mammalian gestation and lactation place particular constraints on female reproductive output.
As was noted in the previous chapter, the defining difference between males and females is anisogamy (AN-i-SOG-e-me): males have small gametes, females have large ones. A spermatozoon is one of the smallest cell in the human body. It is 32,000 times smaller than the ovum. An ovum is one of the largest cells in the human body -- so large it can be seen with the naked eye. Males produce about 12 million sperm per hour; fertile females generally produce one "ripened" ovum per month. A single ejaculate contains about 175 thousand times more gametes than the number of ova a woman produces in her entire lifetime.
Although these sex differences are impressive, they only hint at the evolutionary import of anisogamy. Evolutionary theory is, in essence, a mathematical theory. Evolutionary hypotheses are mathematically derived from the basic assumption that organisms are "designed" by natural selection to maximize their reproductive success. We can develop mathematical models to predict (or postdict) how anisogamy results in sex differences. Such models suggest that anisogamy ultimately leads to all of the robust differences between the sexes (Trivers, 1972).
The Two Fundamental Sex Differences.
In mammals, anisogamy has resulted in two very fundamental reproductive sex differences, one favorable to males, the other favorable to females. These are, respectively, the sex differences in 1) the maximum number of potential offspring an individual can produce in his or her lifetime, (sometimes called "maximum reproductive rate"), and 2) assurance of genetic parentage (the probability that a particular offspring is genetically related to oneself) (e.g., Daly & Wilson, 1983; Symons, 1979). We will examine these two fundamental mammalian sex differences, and
their consequences, below.
Sex Differences in Maximum Reproduction.
As a thought experiment, imagine a room populated by 30 males and 30 females. Consider how many gametes are currently available for potential fertilization in that room. It is likely that only a few females are within three or four days of their ovulation -- so perhaps a similar number of ova are potentially available for fertilization. Of course, sperm is far more plentiful. Each male ejaculate contains about 200 - 300 million sperm. If we multiply the number of males in the room by the number of available sperm per man, we can estimate the number of sperm in the room to be about nine billion. For each ripe ovum in the room there are over two billion sperm potentially available to fertilize it. The reproductive output of mammalian females is limited, not particularly by her limited number of large gametes, but more so by time and resource constraints imposed by internal gestation, birthing, weaning, and female parental care. Males, in contrast, are not subject to the same reproductive limitations.
To illustrate this point, consider another thought experiment. What do you think is the maximum number of children that any one woman could possibly produce in her lifetime? Let's say that a typical woman is potentially fertile, on average, from about age 13 to age 45 -- a reproductive window of about 32 years. If she had one child per year, she could bear a total of 32 children.
The upper reproductive limit for males is very much higher. Males produce viable sperm starting at about age 13, and they continue to produce viable sperm virtually until they die. If a man lives for 73 years, that gives him a reproductive window of about 60 years. Because fertization, gestation, birth, and lactation is all accomplished external to a man's body, the only theoretical factor limiting a man's maximum reproductive output is the number of fertile females that he can inseminate. Suppose a male volunteered (all in the name of science, of course!) to copulate, on average, with two different ovulating woman each day for his entire reproductive life. If half of these copulations resulted in a pregnancy, he could produce 365 children in a year. Over his 60 reproductive years, he could potentially father about 21,900 children. In reality, of course, the actual number of offspring sired by one man is much less!
What do you think the actual world records are? For women: 15, 20, or 30 children? For men: 50, 100, 200? The recorded maximums are slightly different from our theoretical upper limits of 32 for women and 21,900 for men. It is slightly more for women because we forgot to consider multiple births (twins, triplets, etc.) -- and much less for men (due to real-world constraints). The Guiness Book of World Records (Guiness Media, 1998) reports the largest number of children born to one woman is 69 -- by a Russian woman (mostly in sets of 2, 3 or 4). The current documented human male record in this regard is held by King Moulay Ismail of Morocco (also known as "Ismail the Bloodthirsty") who, depending on various sources, was reported to have sired either 888 offspring (McWhirter and McWhirter, 1975) or as many as 1,056 (Barash, 1982).
Sex Differences in Assurance of Genetic Parentage.
The second fundamental difference between the mammalian sexes involves the degree to which a male or female is assured that any particular offspring is genetically his or her own. Because offspring literally emerge from the female body, mammalian females are certain that they are genetically related to her offspring. This is termed "maternity security." In contrast, males are never entirely assured they are genetically related to any particular child. This is termed male "paternity insecurity." Of course, males may rarely consciously question the paternity of their putative offspring. But historically, over evolutionary time, males who were more emotionally and behaviorally predisposed to take actions that, in effect, increased their odds of paternity assurance, were likely to be more reproductively successful.
Since men today are the progeny of such fathers, we would predict them to be predisposed to take actions to defend their paternity assurance. Worry about paternity is not an unjustified or exaggerated concern of men. Indeed, blood tests show that perhaps more than 10% of husbands have been cuckolded -- one or more of their children that they think are their own have been fathered by another man. Diamond (1985) offers compelling evidence documenting male paternity uncertainty:
People have reason to lie when asked whether they've committed adultery, which makes it difficult for serious scholars to get accurate information about this important subject. One of the few available sets or hard facts emerged as a totally unexpected by-product of a medical study performed nearly half a century ago for a different reason. I recently learned of the findings, which have never been revealed until now, from the distinguished medical scientist who ran the study. (Since he doesn't want to be identified, I'll call him Dr. X.) In the 1940s Dr. X was studying the genetics of human blood groups, which are molecules acquired only by inheritance. Each of us has dozens of blood group substances on red blood cells, and each substance must come from either our mother or father. Dr. X's research plan was straightforward: go to an obstetrics ward of a respectable
As the adage suggests, "Mommy's babies are Daddy's maybes." The sex difference in assurance of genetic parentage, along with the sex difference in maximum reproduction, leads to both morphological and behavioral consequences, as summarized in Figure 4.1. We will identify each of these consequences below, and, of course, the rest of this book will be devoted to studying each of them in some detail.
Morphological Consequences.
One morphological consequence of these two fundamental differences is that males are generally larger (which is advantageous in physical contests with other males to gain access to and/or sequester females--an evolutionary consequence of intra-sexual sexual selection). This is the case in humans--males are physically about 10% - 15% larger and heavier than females. In other species, males may have more physical ornamentation to attract females (such as the peacock's tail).
There is a second interesting morphological difference between females and males. In humans, both females and males have nipples, yet only females lactate (produce milk for their offspring). Why? Would it not be in the reproductive interests of a man to also have the ability to lactate -- in case his partner has died, or has been seriously injured -- leaving him with an infant? Obviously, a male who found himself in such situation might find it in his reproductive interest to lactate for his own offspring. But there is the rub. To what extent is the male assured that infant is in fact genetically his own? If he does not have very high assurance of paternity, it would not be in his reproductive interests to invest the time and biological energy to nurse another man's offspring.
Since the ability to lactate evolved in females, it would be relatively easy for a genetic mutation, or "cross-over," to occur such that males too could also lactate. (Interestingly, the female capacity to orgasm is maybe the result of just such a genetic cross over -- from male ejaculation). In fact it is likely a few males in human evolutionary history did in fact have the capability to lactate. But apparently, over evolutionary time, the marginal benefit accruing to males who could lactate was offset by the costs associated with misidentification of offspring -- lactating for another man's infant would be a serious misdirection of parent investment. Moreover, the benefits of lactating were probably limited by the relatively rarity with which a man with an infant was widowed.
In contrast, it is always in the reproductive interests of a female to lactate since she enjoys absolute assurance of maternity--that is, she is absolutely assured that she shares 50% of her genes with the infant suckling at her breast.
This appears to be a satisfactory evolutionary analysis to explain why males do not lactate, but then why do males need nipples at all? Perhaps this is a consequence of the general trend of nature to produce a female. Fetal defeminization does not destroy this nascent female structure (however, de-feminization does destroy the female internal reproductive tract, the Mullerian ducts).
Male nipples are thus a vestige of the fact that all males were sexually bipotential before the 6th week of gestation. Apparently there was insufficient evolutionary advantage to destroying these female structures during defeminization, and little or no cost of not doing so. In other words, there is apparently no evolutionary pressure to get rid of nipples in a developing male fetus.
Behavioral Consequences for Females
The rest of this book will be primarily devoted to an exploration of the behavioral, rather than the morphological, differences between females and males. Several behavioral sex differences are predicted by evolutionary theory. Below is a brief summary of behaviors predicted for females.
Sexual choosiness and caution.
Females have little to gain by copulating with many different males (and, as we will explore later, possibly much to lose). Also contributing to female sexual choosiness and caution are factors related to optimal timing of pregnancy and subsequent child rearing. Conditions that make the present undesirable for such events (e.g., stress, famine, unavailability of a committed male partner, etc.) militate against activities likely to cause pregnancy.
Desire for male status, resource control, and commitment.
Females are predicted to be attracted to signs of male status, wealth and commitment. The number of offspring a female can have is limited by her ability to adequately provision and protect them. The degree to which a resource rich male is also willing to commit to a female, and help her provision for and protect her offspring, will proportionately increase her reproductive success.
Assurance of maternity and female parental investment.
In that female has assurance of maternity, it is clearly in her interests to invest heavily in her own offspring.
Behavioral consequences for males
Several behavioral consequences of the two fundamental sex differences for males include:
Indiscriminant sexuality.
Males, in general, tend not to be discriminating in choice of sexual partner. This is because their minimum possible investment to produce a viable offspring is extremely small: literally a few minutes of their time and a small amount of replaceable sperm. But males are not totally indiscriminant, they do discriminate based on age. As expected from an adaptionist approach, males worldwide demonstrate a marked preference for nubile sexual partners. Beyond that, and given a low cost, low risk opportunity, males don't discriminate much.
Imagine that a person of the opposite sex approached you on campus said: "I've been noticing you around campus, and I find you to be very attractive." Imagine they this stranger then asks you one of the following three questions: would you (1) go out with me; (2) come over to my apartment; or (3) go to bed with me? How do you think men and women would respond to each of the questions?
Clark and Hatfield (1982) performed such a study. They found that both sexes were equally likely to accept the date. Seventy-five percent of the men accepted the invitation to "go to bed;" none of the women did. The research confederates who asked the above questions reported that the men they approached were usually responded positively. Women reacted negatively, especially to second and third questions.Below are the results of the study:
Results of the Clark and Hatfield (1982) study:
Percentage of males and females who agreed to each type of invitation.
INVITATION MALES FEMALES
Go out on date | 50% | 56% |
Go to apartment | 69% | 6% |
Go to bed | 75% | 0% |
The Coolidge Effect: Desire for multiple sexual partners.
Since the factor that limits male reproductive output is the number of reproductive age females to which a male can gain sexual access, males tend to desire sexual variety for its own sake more than females, who generally have little to gain from sexual partner variety.
Strategies to assure paternity.
To deal with paternity insecurity -- to increase the odds that a particular offspring is genetically their own -- males have evolved a number of emotional and behavioral characteristics, including jealousy guard females from other males,
and seek fidelity in a female with whom they are investing long term.
Shakespeare, In Love's Labour Lost, writes:
The cuckoo then in every tree;
Mocks married men; for thus sings he,
"Cuckoo; cuckoo"; O word of fear
Unpleasing to a married ear!
A man's reproductive investment is indeed lost if he is cuckolded.
Less investment in offspring.
Males generally invest in offspring less than do females, given their lack of paternity assurance. When they can, they often redirect their energies to obtain additional matings with other females, rather than invest in offspring. However, the more monogamous the species, and the higher the probability of paternity, the more likely males will invest in offspring. Males will also invest when it is clearly impossible for a female to successfully raise offspring without his assistance.
Most species of birds are monogamous for this reason.
Exceptions to the rule: Sex role reversals.
The above robust sex differences are predicted by evolutionary theory to apply, not only to humans, but far more generally--to females and males across many different species. However, there are species to which these generalizations do not apply, and some in which the robust "sex roles" are actually reversed! Such species present a puzzle. As we discussed in Chapter 1, such unexpected anomalies can present a serious challenge to a theory. The challenge to evolutionary theory is the degree to which it can explain both the cross-species general trends, as well as the exceptions, in a manner consistent with its theoretical postulates.
Species with fully or partially reversed robust "sex roles" include Moromon crickets and moorhens (Williams, 1966), as well as water bugs pipefish seahorses, certain species of frogs, iacanas, and three-spined stickleback fish (Daly & Wilson, 1983). The fact that there are no mammals with reversed "sex roles" may provide us with a clue to unravel this apparent mystery. One of the most impressive sex role reversals occurs in in the pipefish seahorse (Syngathadae). Unlike mammals, in this species, internal gestation is provided by the male! A female deposits her eggs (using a penis-like appendage) in a pouch near his stomach called a "brood pouch." There he fertilizes them with his sperm. The male provides a blood stream connection via a placenta and nourishes and protects his offspring. He "gives birth" by expelling the fry with a series of muscular contractions (somewhat analogous to human labor).
In this species who has more assurance of genetic parentage -- the male or the female? In pipefish seahorses, "daddy's babies are mommy's maybe's" -- the female suffers from maternity insecurity while the male has complete assurance of paternity. She is not certain that any baby seahorses that emerge from his pouch are genetically her own -- the eggs could have come from another female seahorse. As as consequence, she does not invest in her offspring, while the male invests heavily. Here we have not only have a reversal of assurance of genetic parentage, but also a reversal in which sex can potentially produce more offspring. Reproduction in male seahorses is constrained by the limited space in their brood pouch, as well as the time investment required for gestation. Female seahorses are limited only by the number of different males willing to accept their eggs. In this species it is the females who are sexually indiscriminant, prefer multiple mating partners, and are aggressive in courtship. Males are cautious and discriminating about choice of sexual partner.
We can expect that when the two fundamental differences between the sexes are reversed, we will see a full reversal in robust sex roles. The reason that there are no mammalian species with reversed sex roles is because in none of them do females have greater potential reproductive output than males, nor are there any mammalian species in which males have more assurance of genetic parentage than females.
Other basic sex differences.
Minimum possible parental investment.
To understand sex differences in sexual psychology, Symons (1979) suggested that it is not the typical parental investment that is most relevant, but the minimum possible investment. As suggested above, the male minimum possible investment in successful reproduction is enormously different from the female minimum. What is the female minimum? For humans: a nine month gestation, three to four years of nursing (for ancestral women), and perhaps a decade or more of food provision, protection, and socialization. In contrast, the male minimum reproductive effort is a few minutes for copulation and the investment a trivial amount of semen that is quickly replenished. The the combined effect of the sex differences in maximum potential reproductive output and minimum possible parental investment leads to several robust sex differences.
Reproductive variance.
Review the table below. Assume the sex ratio is 50% males and 50% females. There is something puzzling in the data -- can you find it?
Percentages of Never Married Individuals
(Source:
Age: 20 - 24 | Women | Men | % More Unmarried Men |
1970 | 36% | 55% | 19% |
1980 | 50% | 69% | 19% |
1990 | 63% | 79% | 16% |
Age: 25 - 29 | Women | Men | % More Unmarried Men |
1970 | 10% | 19% | 9% |
1980 | 21% | 33% | 12% |
1990 | 31% | 45% | 14% |
Age: 30 - 34 | Women | Men | % More Unmarried Men |
1970 | 6% | 9% | 3% |
1980 | 10% | 16% | 6% |
1990 | 16% | 27% | 11% |
In each census, for all age groups, there are always more never married men than women. Given that polygyny is illegal in the
This anomaly is solved when we realize that more men than woman have multiple marriages (Chamie & Nsuly, 1981). Men who engage in "serial marriages" are, in a sense, "monopolizing" women from other men who, consequently, have more difficulty attracting a wife. For males, multiple mates generally mean greater reproductive success. This is not the case for females, who are likely to have the same number of offspring regardless of the number of different mates she has (Bateman, 1948).
The variability in reproductive success between individuals of the same sex is termed "reproductive variance." Across virtually all species, males generally have higher reproductive variance than do females (Payne, 1979). Males who can successfully monopolize female reproductive capacity produce a great number of offspring (e.g., "Ismail the Bloodthirsty"). Less successful males may have no offspring at all, either because females are unwilling to copulate with them or because they are prevented from doing so by other males.
The more polygynous a species, the higher the male reproductive variance, and, generally, the greater the sexual dimorphism. An extreme example of this can be seen in northern elephant seals. Le Boeuf and Reiter (1988) studied seals born on
Reproductive Success of Male Northern Elephant Seals
(after Le Boeuf and Reiter, 1988)
Reproductive Success | Number of Male Elephant Seals |
Sired 93 offspring | 1 |
Sired 82 offspring | 1 |
Sired 41 offspring | 1 |
Sired 18 offspring | 1 |
Sired 16 offspring | 1 |
Sired 11 offspring | 1 |
Sired 4 offspring | 1 |
Sired 3 offspring | 1 |
Sired 0 offspring | 11 |
Did not survive to breeding age | 119 |
Out of the 19 males that survived to reproductive age, two of them monopolized the majority of the matings: they fathered 175 (65%) of the 268 pups. Fifty-eight percent of the males were excluded from reproduction entirely. In contrast, 100% of females that survived to sexual maturity gave birth to at least one pup.
Greater male reproductive variance has been documented in virtually every species for which it has been measured, including fruit flies (Bateman, 1948), lions (Bertram, 1976), yellow baboons (Hausfater, 1975), and humans (Daly & Wilson, 1983). In humans, for example, among the Brazilian Xavante Indians, Salzano, Neel, & Maybury-Lewis (1967) found that only 1 out of 195 women was still childless by the age of 20. In contrast, by the age of 40, 6% of the men had not become fathers. The maximum number of children produced was eight for a woman and 23 for a man. The average number of children was 3.6 for both sexes, but the reproductive variance of males was 12.1 and 3.9 for the females.
Generalized Graph of Human Reproductive Variance
The figure below shows a generalized graph of the reproductive variance curves for men and women. Note that there are two groups of males for which there is no female equivalent -- the males at the extreme high and low ends of the distribution. At the high end are the males that win a "reproductive jackpot" by monopolizing the reproductive capacities of many females. In humans, these are typically the wealthy, high status males -- the chiefs, the headmen, the leaders, the emperors. At the other, lower end of the curve are the men who are so poor, unattractive, unintelligent, socially awkward, and/or have such dismal future prospects that no woman would voluntarily choose such a male to father her child(ren). The men at the bottom of the distribution are thus reproductively disenfranchised.
Among healthy, fertile women, are some so unattractive that no male who would be willing to copulate with them? Virtually, no. The penalties of a "bad" mating that a woman must endure can be severe, however a “bad” mating costs a male, at minimum, a few moments of his time and a small amount of sperm. Apparently, there are virtually no reproductively disenfranchised women. There is probably a male somewhere who is willing to copulate with just about any female.
In a reproductive sense, males are playing a higher stakes game than females (Alexander, 1979). Males may win a reproductive "jackpot" or lose utterly. Players of high stakes games (with much to either win or to lose) are likely to behave differently than players of low stakes games -- the high stakes players are generally willing to take greater risks (ref). As we will explore later, men generally take more risks than do women (Wilson & Daly, 1985), and this generally the case across virtually all species (ref).
Reproductive value.
Reproductive value is defined as an individual's potential current and future reproductive output. We will use the term here to refer to the value of an individual as a reproductive partner to the opposite sex. For men, a woman's reproductive value is largely a function of her age, since it correlates highly with current and future fertility (see Figure 4.3).
Generalized Graph of Human Reproductive Value
(After Daly & Wilson, 1988; Figure 4.3, p. 74)
For women, a man's reproductive value is largely a function of his status and wealth. The figure above shows the generalized age-related curves of reproductive value for high and low status males. The great majority of men, of course, are between these two extremes of reproductive value.
Closing comments.
Before we examine human robust sex differences, the next three chapters will be devoted to study female and male behaviors in a variety of non-human species.