Extended Sexuality in Women: What Is It For?
In chapter 3, we, following others, argued that extended female sexuality typically functions to obtain male-delivered material assistance. As we also noted, no mammalian female known to biology matches the amplified form of extended sexuality exhibited by women. Though women can possibly conceive on 5 or 6 days of their cycles in which ovulation occurs, with pronounced chances occurring just 2–3 days, women engage in and seek copulation throughout these cycles. Indeed, in aggregate data, human mating frequency varies very little across the cycle, aside from a drop at menstruation (see chapter 10). Furthermore, women of reproductive age often have nonovulatory cycles and mate frequently within these cycles. And human females may be sexually active during years of adolescence before establishing reliable ovulatory cycles. Indeed, human female adolescents appear to be more sexually motivated than adolescents in other primates in which adolescent females exhibit sexuality (see chapter 6). Finally, women are proceptive and receptive when pregnant. Naturally, if the male-assistance hypothesis of extended female sexuality applies specifically to women, women benefit through male-delivered material assistance by mating during infertile times of their lives. But precisely how?
In this chapter, we explore answers to this question. The precise nongenetic material benefits that females obtain from males through extended sexuality vary across species. As we saw in the last chapter, many Old World female primates obtain the benefit of reducing male aggression toward offspring by mating promiscuously with multiple males, particularly during periods of extended sexuality. In pair-bonding birds, by contrast, the benefits females obtain through extended sexuality are typically very different: In many species, females enhance the flow of investments their primary pair-bond mates provide to them and their offspring by mating exclusively (or almost exclusively) with their pair-bond mates during periods of extended sexuality. In yet other species, females may directly exchange sex for food. The same hypothesis—the male-assistance hypothesis—applies to all of these instances of extended female sexuality. But the contexts in which females obtain male assistance vary greatly as a function of the mating systems of species and the social and economic relations between males and females these systems entail.
Key questions that women’s extended sexuality raises, then, are the following: What is the nature of the mating and parenting system (or systems) in which human mating adaptations have evolved? And, in light of that mating system, what male-delivered material benefits have shaped the nature and form of women’s extended sexuality?
Patterns of Marriage: A Cross-Cultural Perspective
Marriage is nearly universal across human societies. (Purportedly, the Na, an ethnic minority living in the Himalayan foothills in China, lack any such institution. Rather, brothers and sisters live together for life. Siblings help women care for offspring. Fathers do not. See Hua, 2001.) Traditionally, many anthropologists and other scientists have inferred human mating or parenting arrangements from marital arrangements (see, for example, Low, 1990, and references therein). These inferences are potentially risky, as institutional arrangements need not directly map onto patterns of mating or parental care (see Leach, 1988; also Hawkes, 2004; Symons, 1979). Moreover, institutional arrangements may lend little insight into key issues pertaining to mating, such as whether men have been selected to invest in offspring due to benefits deriving from paternal care (e.g., Hawkes, 2004). Nonetheless, they are a convenient point from which exploration into human mating patterns can embark.
The Standard Cross-Cultural Sample (SCCS) is a collection of 186 modern and historical human societies selected by Murdock and White (1969) for their distinctiveness. Purportedly, they are weakly redundant representations of human culture, as they do not closely derive from common cultures or possess similarities due to horizontal cultural diffusion. Accordingly, they are thought to be appropriate for study of cross-cultural universals and associations between cultural and ecological variables. Within the SCCS, more than 80% of societies permit polygyny. Less than 20% are completely monogamous, and 1% are characterized by a nonzero level of polyandry (multiple husbands taken by one female simultaneously). In about 60% of the SCCS societies that permit polygyny, however, more than 80% of wives are in monogamous unions (Murdock & White, 1969).
As agriculture, herding, and other relatively recent means of production may alter mating arrangements, Frank Marlowe (2003b) sought to examine the mating arrangements of the 36 foraging societies within the SCCS. (Data on levels of polygyny were available on 30 of them. Foragers, for this study, were defined as groups who attain less than 10% of their diet from cultivated foods or domesticated animals.) All women were monogamously married in just three societies. The median percentage of polygynously married women, however, was just 10%; women’s rate of polygyny was 12% or less in almost two-thirds of foraging societies. In about one-quarter of them, by contrast, 35% or more of the women were married polygynously.
Are humans, then, a socially polygynous or a monogamous species? Clearly, both kinds of marital arrangements exist. Polygyny has commonly been permitted in human societies. By far, however, most marital arrangements across human societies, as represented by the SCCS, have been monogamous unions. Patterns of marital arrangements suggest, then, that humans facultatively mate: In most instances, males and females form monogamous marital bonds. But in many societies, minorities of women enter into polygynous relationships, wherein a woman shares one husband with multiple wives. In very rare instances, a woman may have multiple husbands simultaneously.
The Human Adaptive Complex, Biparental Care, and Pair Bonding
The Role of Male Hunting
A traditional view in anthropology is that human pair bonding derives from the importance of male provisioning for offspring—a form of biparental care (e.g., Lancaster & Lancaster, 1983; Lovejoy, 1981; Westermarck, 1929). In most primate species (including our closest relatives), individuals of both sexes are largely responsible for their own subsistence after at most a few years of care following birth. Though males provide a variety of material services to females, including sometimes sharing food in exchange for sex (e.g., Dunbar, 1987), in these species mothers harvest the overwhelming majority of calories consumed by offspring during pregnancy and lactation. In contrast, in the majority of human foraging populations studied to date, the average adult male generates more calories than he consumes. These food resources yield benefits for reproductive women and juveniles by making calories and macronutrients such as protein available to them to consume. Marlowe (2001) estimated that, on average, men produce 64% of the calories in the 95 foraging societies on which sufficient information is available. In Hillard Kaplan and colleagues’ (Kaplan, Hill, Lancaster, & Hurtado, 2000) analysis of studies that carefully measured produced foods in nine hunter-gatherer societies, men generated on average about 66% of all calories consumed.
The primary activity through which men generate surplus calories that subsidize women and children’s diets is hunting (which, for purposes here, is broadly defined to include any activity aimed to harvest animal meat, including fishing). Though women forage and extract roots (and, in a meaningful minority of societies, produce more calories than men produce), only rarely do they hunt to a substantial degree (for an exception, see Hart, Pilling, & Goodale, 1987, on the Tiwi of Australia). Human foragers, according to this view, are adapted to a diet consisting of high-quality, calorically rich foods. Compared to close phylogenetic relatives, humans consume large amounts of high-quality but difficult-to-extract resources such as animal protein, roots, and nuts. Whereas chimpanzees obtain about 95% of their calories from collected foods requiring no extraction (e.g., fruits, leaves), for instance, only about 8% of calories consumed by modern hunter-gatherers are from foods requiring no extraction. Vertebrate meat in particular accounts for, on average, 30–80% of human hunter-gatherer caloric intake but just 2% of chimpanzee diets (Kaplan et al., 2000). Male subsidization of female and juvenile diets is largely achieved through men’s hunting of animal meats (see figure 4.1).
Figure 4.1 The energy production and consumption of females (top) and males (bottom) in relation to age averaged across three hunter-gatherer societies (Ache, Hiwi, and Hadza). Females do not show a net positive production until they reach 45 years of age, which is sustained until about age 70 years. Males achieve net productivity in the age range of about 17–60 years. Based on figure 3 of Kaplan et al. (2000); reprinted with permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.
Building on earlier views (e.g., Lancaster & Lancaster, 1983; Westermarck, 1929), Kaplan et al. (2000) explain male hunting as the outcome of selection for male parental effort. According to these authors, male hunting functions to harvest nutrients not only for self but also to foster the viability and health of reproductive partners and offspring. Kaplan et al. (2000), however, expand on earlier views and situate human biparental care within a larger set of human coevolved features, which they refer to as the human adaptive complex (see also Kaplan, Lancaster, & Robson, 2003).
First, humans are characterized by a very long period of juvenile dependence. Whereas chimpanzees are responsible for their own foraging by age 5, human children in foraging societies are dependent on subsidies to their diet until they are almost 20. Children’s need for subsidies (caloric consumption minus self-production) peaks when they are about 11 (Kaplan et al., 2000).
Second, people also live relatively long lives. If a human forager lives into adulthood, he or she has a very good chance of living into his or her 60s (see, e.g., Hill & Hurtado, 1996, the best documented demography on a foraging people, the Ache of Paraguay). Longevity in humans is fostered by substantial investment in somatic repair and immune systems, as well as risk-averse strategies to avoid predation (see Robson & Kaplan, 2003).
Third, human productivity continues well into adulthood. Indeed, Kaplan et al. (2000) estimate that male Ache foragers reach peak productivity (net rate of energy capture) when they are in their 40s. And, in a study of hunting within the Tsimane Indians of Bolivia, Gurven, Kaplan, and Gutierrez (2006) found that direct encounter with important prey items and successful capture of prey involve skills not fully developed for 10–20 years following onset of adulthood, despite strength peaking in the early 20s. Fourth, Kaplan et al. (2000) argue that the expansion of the brain during human evolution, as well as an extended, subsidized period of learning, have made possible the high rates of productivity characteristic of humans. Uniquely human capacities to engage in innovative thinking and flexible problem solving, in their view, make it possible for humans to subsist in virtually every terrestrial environment found on earth. Humans also have evolved capacities to engage in highly cooperative forms of social alliances that support high rates of productivity. (Full consideration of the debates about what forms of selection led to brain expansion during the course of human evolution is beyond the scope of this book. For a recent overview of ideas and controversies, see contributions in Gangestad & Simpson, 2007.)
These features purportedly constitute an “adaptive complex” in that each contributes to the effectiveness (or possible adaptiveness) of others. If, for instance, humans did not live long lives and produce surplus calories well into adulthood, they simply could not afford to have their long, sustained period of juvenile dependence. The math is simple: On average, individuals must produce as much during their life spans as they consume. To overcome the very substantial net caloric deficit they build up during their first 20 years, then, humans must be adapted to live long, productive lives. (Put otherwise, no species could afford to simultaneously possess the extended period of childhood of humans and the adult life history profile of chimpanzees, particularly male chimps, whose production never meaningfully exceeds their consumption throughout their entire lives.) At the same time, the extended period of juvenile dependence may very well permit the acquisition of skills, knowledge, and ability to learn (“embodied capital”; e.g., Robson & Kaplan, 2003) that fosters high rates of productivity well into adulthood.
These features render a human life course very different from the life histories of our closest living relatives, the two chimp species, as well as, presumably, our common ancestors with those relatives, who lived 5–8 million years ago. Biparental care purportedly is yet another key feature in the coevolved human adaptive complex.
The Evolution of Biparental Care: Complementarity
Biparental care is relatively uncommon in the biological world. In theory, selection could favor any mixture of parental care by mothers and fathers. Empirically, however, the typical evolved solution to parenting that characterizes species is one in which members of one sex—usually females—are fully responsible for parental effort. The other sex—typically males—incur by far the greatest costs of mating effort, costs of seeking and competing for mates. This parenting solution characterizes most mammals. A recent conceptual analysis suggests that differences in the strength of sexual selection across the sexes themselves lead this solution to be, by far, the modal one (Kokko & Jennions, 2002). When females particularly prefer a small subset of males (often for their intrinsic good genes; see chapter 7), the cost of caring for young is great for those males; they give up precious mating opportunities if they exert effort to invest in offspring. The large cost of parental effort that the most desired males pay leads them not to care for offspring in most cases, which means that females alone are typically responsible for parental care. (For further discussion of these issues, see chapter 5.)
Exceptions to the typical solution do exist, of course. Biparental care characterizes the majority of bird species, some rodents, and some primates, among other species. Recent modeling suggests that complementarity of each sex’s parental investment may be an important factor leading biparental care to be favored over the typical division of reproductive efforts (Kokko & Johnstone, 2002). Complementarity of the sexes’ parental efforts exists when the total beneficial effect of the sexes’ efforts exceeds what would be the sum of the individual beneficial effects of males and females were they investing in offspring separately. (Put otherwise, complementarity implies nonadditive, multiplicative effects of each parent’s investments.) It may be critical to the evolution of biparental care because, with complementarity, a father’s investment not only has its own fitness benefits but also ratchets up the fitness benefits of the mother’s investment. Within the aerial niche occupied by most bird species, complementarity may partly exist because, while one parent flies away to gather food for offspring, the other guards the nest. If one parent alone were to leave the chicks in order to forage, the chicks could be easy prey for predators.
What gave rise to biparental care in humans? The traditional argument is that humans, too, evolved to occupy a niche in which complementarity of parental efforts exists. Dependent juveniles demand caretaking. They simultaneously require substantial nutritional subsidies. Women cannot effectively engage in many forms of foraging while simultaneously caring for and protecting offspring (particularly infants). Male foragers, then, perform most of the hunting, which functions to subsidize the diets of dependent children and caretaking mothers. Women with small children forage in ways that are compatible with child care. Though women and their kin (e.g., postreproductive women) may subsidize offspring to some extent, the bulk of the subsidies to children in modal foraging societies are calories that men harvest (e.g., Marlowe, 2001, 2003a). Male subsidization, then, directly increases female reproduction by increasing offspring quality (lowering the childhood mortality rate) and/or increasing the rate of reproduction (decreasing the interbirth interval) that females can possibly achieve. (For a comparative analysis of biparental care as a function of trade-offs between maternal care and foraging, see Ember & Ember, 1979.)
Obviously, women could not have evolved to become dependent on subsidies achieved through men’s hunting (and children could not have become dependent on these subsidies) without men first providing some measure of subsidy. The argument, then, is that male efforts that led to subsidy and the remaining elements of the human adaptive complex (e.g., the long period of juvenile dependence) coevolved over time, in increments, as did human entry into and deepening commitment to an ecological niche requiring capture of high-quality food items. On average, men and women who entered into codependent relationships in which men subsidized the diets of their partners and children, according to this perspective, outreproduced those who did not.
An analysis of close to 100 foraging societies by Marlowe (2001) is consistent with the view that women can and do turn the surplus of calories generated by men into production of offspring and thereby reproductively benefit from this surplus generated through male hunting. Though, on average, men generate about two-thirds of the calories in foraging societies, the degree of male contribution to the diet varies considerable across foraging societies (from about 40% to close to 100%). If women and offspring directly benefit from male subsidies, women’s fecundity should be relatively great in societies in which male contribution to subsistence is relatively large. Marlowe (2001) found precisely this association. The effect of men’s contribution of subsistence on female fecundity appears to be at least partly mediated through the interbirth interval: In societies in which men contribute more calories to the diet, the delay between the birth of one offspring and the same woman’s next offspring appears to be shorter. (For an overview of the energetics of human pregnancy and lactation, see Dufour & Sauther, 2002; see also Ellison, 2001.)
According to this male-hunting-as-parental-effort theory, the nuclear family is a key economic unit in the evolution of human mating relations. For subsidies generated by male hunting to function as parental effort, nutrients that men generate must flow from them to mates (and then to offspring) or directly to offspring. Male-female pair bonding importantly cements the psychological foundations of these resource flows. As we discuss later, doubts that male-generated subsidies flow within the nuclear family in this manner has led some scholars to question the plausibility of the male-hunting-as-parental-effort theory.
When biparental care and mutual mate choice exists, the sexes may choose each other on similar grounds and, as a result, sexual selection may favor the same traits in both sexes. Alternatively, the sexes may choose on different bases, in which case mutual mate choice and sexual selection on each sex may lead to sex-specific exaggerated traits. Within primates, there are well-established associations between sexual dimorphism of body and canine size and mating system (e.g., Plavkan & van Schaik, 1999). In species that pair bond, the sexes are more similar in body size and possess canines of more similar size, relative to species whose mating is characterized by promiscuity or high levels of polygyny. Again, however, mutual mate choice need not imply that the sexes are sexually selected to be similar in all ways.
Though many studies have investigated mate preferences of college students and community samples (e.g., Buss, 1989a,b), few have investigated mate preferences in foraging societies. A rare exception is a study by Marlowe (2005) on mate preferences in the Hadza of Tanzania, a foraging society in which men generate about 40% of the calories and 12% of women are mated polygynously. Consistent with the view that men and women engage in a division of labor within a family, Hadza women highly value men’s foraging abilities and intelligence, whereas Hadza men value age-related fertility in a mate more than women do. Marlowe did not detect sex differences in preferences for a partner’s personal character and physical attractiveness.
Why Polygyny?
A standard explanation for polygyny in human societies, where it occurs, is Orians’s (1969) polygyny threshold model. In this model, males differ in their resource holdings. Polygyny is favored if a female would do better for herself by becoming the second mate of the male with the greatest resource holdings than by becoming the first mate of the best unmated male available to her. This model has been successfully applied to an understanding of polygyny within some human societies (e.g., the Kipsigis of Kenya; Borgerhoff Mulder, 1990).
A more complex model is one that derives from the idea that mates provide more than resources. They provide genes as well. If males vary considerably in their genetic quality and, hence, in the extent to which they can provide good genes to a female’s offspring, a female could be better off by becoming the second mate of a male of high genetic quality (despite having to share male-provided resources with his first mate) than by becoming the first mate of the best available unmated male (in terms of a composite of material resources and genes; Weatherhead & Robertson, 1979).
In chapter 7, we treat the sources and nature of variation in genetic quality in more detail. There, we discuss the idea that the presence of parasitic organisms can lead to greater expression of genes that influence fitness and thereby to greater genetic variance in fitness (i.e., greater phenotypic variance in condition as a function of genetic variation). In turn, greater genetic variation in fitness among males should increase the chances that some portion of females will find it worthwhile to enter polygynous unions. Low (1990) coded the prevalence of seven different parasites in regions occupied by the societies of the SCCS. She predicted and found that, indeed, in cultures in which people are exposed to relatively great levels of parasite stress, polygyny is relatively common. Marlowe (2003b) found the same association in foraging societies.
In societies in which women are responsible for a relatively large share of their own subsistence and their offspring’s diets, one might similarly expect women to be more likely to enter into polygynous unions. In such instances, the marginal gains a monogamously mated woman enjoys by receiving all of a mate’s provisioning to offspring, as opposed to receiving half of his total provisioning to two wives, are less than if men bring in a larger percentage of calories for offspring. Consistent with this prediction, Marlowe (2003b) found that, across forager societies, women’s contribution to subsistence is positively correlated with the level of polygyny.
As Marlowe (2003b) also noted, parasite prevalence and men’s contribution to subsistence account for less than 30% of the variance in levels of polygyny in foraging societies. A variety of other factors may be important (e.g., threats of sexual coercion, which demand effective male protection). As well, the interests of parents and grandparents may be in conflict with those of their daughters and granddaughters (discussed later); in some cultures, polygynous marriages may be arranged in the interests of these family members, not in those of the daughters who marry (see Marlowe, 2003b).
Have Men Been Selected Directly to Invest in Offspring?
Critique of the Male-Hunting-as-Parental-Effort Theory
The male-hunting-as-parental-effort theory has been seriously challenged over the past two decades. The fundamental difficulty that this theory faces, according to critics, is that nuclear families are, in fact, not the potent economic units in foraging societies that this theory implies (Hawkes, 1991, 2004; Hawkes, O’ Connell, & Blurton Jones, 1991, 2001). In the Hadza of Tanzania and the Ache of Paraguay, for instance, hunters have little control over the distribution of meat generated through their efforts, as meat is shared widely across community members. This pattern particularly characterizes the sharing of meat from large game. A Hadza hunter’s own family receives no more meat from a large game animal he kills than the meat they receive from the same-sized animal killed by a neighbor. Yet men allocate a substantial portion of their time to hunting large game. Large-game hunting cannot be explained as parental effort if, in fact, a man’s hunting does not differentially advantage his own offspring over the offspring of other men. (Indeed, as Hawkes et al., 2001, also claim, Hadza men actually generate fewer calories per unit time hunting large game than smaller game, an added reason that it does not effectively or efficiently generate calories for a nuclear family.)
According to Kristen Hawkes (2004), men’s hunting functions as mating effort—effort to gain access to mates—rather than as parental effort. Men garner prestige through successful hunting exploits, particularly big-game hunting. Ultimately, prestige translates into mating opportunities (including mating with other men’s wives; see also Kaplan & Hill, 1985). As Marlowe (2003a) notes, compared to poor hunters, good hunters among the Hadza are more likely to obtain second or new wives once their first wife has reached the age of menopause. Put otherwise, hunting is a form of “showing off” (e.g., Hawkes, 1991).
Of course, the diets of women and their offspring are subsidized by male hunting, as the male-hunting-as-parental-effort emphasizes. But these subsidies, in the male-hunting-as-mating-effort view, are not generated directly by women’s own mates or by children’s own fathers. Rather, they are generated by the efforts of the community of men to gain mates. In economists’ terms, the substantial surplus calories generated by male hunting that benefit women and offspring are “positive externalities” of men’s showing off—windfalls they enjoy, not benefits men’s efforts were designed to achieve. In adaptationist terms, the surplus calories that men’s hunting generates for their community are fortuitous by-products. Men do not gain fitness directly as a result of generating food surpluses that enhance the fitness of mates or offspring. They gain fitness from hunting, especially large-game hunting, because success at it leads to mating opportunities.
Hawkes et al. (2001) do argue that the diets of women and their children are subsidized through the efforts of family members, but husbands do not play the primary role in this regard. Rather, maternal kin—most important, mothers of mothers (i.e., children’s grandmothers)—work to directly subsidize the diets of women of reproductive age and their offspring. Hawkes et al. (2001) explain the long life span after the end of women’s reproductive years as an extended period of productivity affecting women’s own fitness by enhancing the fitness of offspring and grand-offspring (see also Hawkes, 2003).
According to this framework, then, the nuclear family is not an economic unit that is key to understanding the evolution of human male-female relations. Men and women do have offspring together and form marriages. But the extended maternal family (a grandmother, her offspring, and her grand-offspring) is of greater economic importance, according to this view, than the nuclear family. Because a man invests time and effort to hunt to gain access to mates in general and not one mating partner in which he invests exclusively, sexual monogamy is not highly important to either men’s or women’s fitness outcomes from this perspective. According to the male-hunting-as-parental-effort theory, by contrast, the fitness of both sexes can be harmed through a spouse’s infidelity. A cuckolded man invests substantially in another man’s offspring. And a woman whose husband has offspring with another woman may suffer because a portion of her husband’s parental effort is diverted away from her own children and toward other children.
Hadza women do prefer to marry good hunters. (Indeed, Hadza women mention this attribute more often than any other when asked what characteristics they desire in a husband; Marlowe, 2005.) And good hunters have more surviving offspring than poor hunters do. But, Hawkes and others have argued, women do not prefer good hunters for their foraging returns. Nor do good hunters have more surviving offspring because they bring more food to their families. Rather, good hunters possess social prestige, which may benefit their wives indirectly. As well, effective hunting may be an honest signal of a man’s genetic quality, which may benefit women’s offspring (see Hawkes & Bliege Bird, 2002; Smith, 2002; see also chapter 7). Finally, good hunters obtain better wives, who work harder and more proficiently than the wives of other men. In one analysis, offspring nutritional status covaried positively with women’s foraging returns but not with men’s (see Hawkes, 2004).
A Blended View
The male-hunting-as-parental-effort and the male-hunting-as-mating-effort theories can be presented in extreme forms (and, indeed, they sometimes are—if not by their proponents, then by their critics). But a blended position or mixed model is also possible. Men’s hunting may function as parental effort as well as showing off; historically, men may have benefited reproductively from hunting in currencies of enhanced viability of offspring as well as mating opportunities. Accordingly, men’s hunting may arise from psychological adaptations with two different functions (indeed, at least partly served by distinct adaptations)—parental effort and mating effort.
In such a mixed-model view, different hunting endeavors may differentially benefit men through parental investment and mating effort. Hawkes et al. (2001) emphasize that men’s large-game hunting is not an effective or efficient means of provisioning offspring. But large-game hunting (in at least some foraging societies) may benefit men substantially in the form of mating effort. By contrast, men in foraging societies have much more control of the distribution of captured small game and may preferentially direct it toward primary partners and offspring, such that hunting of small game functions as an effective means of paternal investment. Hawkes et al. (2001) observe that Hadza men spend much more time hunting large game than returns to their families warrant, which implies that male Hadza foragers do not allocate their time to foraging in ways that maximize gains through parental investment So long as opportunities for mating with women other than current primary partners are available to men (in the form of extra-pair mating or additional wives) and can be achieved through foraging, however, men should not be expected to allocate foraging time in ways that maximize the fitness benefits of parental effort alone (see, for instance, Wedell, Kvarnemo, Lessells, & Tregenza, 2006). Of course, that need not imply that male foraging often does function as parental effort.
Indeed, Marlowe (2003a) offers data on Hadza foraging rates and activities strongly supporting a blended view. Overall, married Hadza women produce as many calories as do married Hadza men. Women with small offspring, however, do not. Compared with all other married women, women whose youngest children are 3 years of age or younger harvest about one-third fewer calories. And women with infants 1 year of age or younger harvest only about half as much. Women’s child care, it seems, does interfere with effective foraging. When women have young children, however, their husbands make up for the shortfall. Hence, whereas in couples without children 8 years of age or younger, wives produce more calories than husbands do (approximately 3,300 vs. 2,900), in couples with infants less than 1 year of age men produce almost 70% of the calories (approximately 1,700 by wives vs. 3,800 by husbands). Hadza men, then, appear to facultatively adjust their work efforts (and perhaps the prey items they target) in response to the direct food production of wives, as it varies with the presence or absence of young children. The view that men’s work functions solely as mating effort has a difficult time explaining this pattern (see Marlowe, 2003a, for a discussion of possible alternative explanations). As Marlowe (2003a) also emphasizes, men’s production in this study was not achieved exclusively through hunting. Collection of honey accounts for about 30% of the calories that Hadza men produce. Men typically have substantial control over the distribution of honey they collect and, according to informal reports, they try to direct what they do not eat themselves to their families. Men who are good hunters also tend to harvest more honey and other foods than other men do (possibly due to good hunters’ overall vigor). Hence women married to good hunters truly do benefit directly from their husbands’ foraging skills.
Finally, separation of men into fathers and stepfathers provides additional evidence that men’s production functions partly as parental effort. Approximately 30% of Hadza children have stepfathers. In contrast to genetic fathers, stepfathers do not enhance food production in response to the presence of young stepchildren in the household (see also Marlowe, 1999).
The Hadza are at the low end of the cross-cultural distribution of men’s contributions to subsistence (with men producing only about 40% of the calories compared with, on average, about 64% of men across foraging societies; Marlowe, 2001). In foraging societies in which wives produce lesser amounts of food (e.g., in colder climates), men might be expected to engage in even more parental efforts through food production than Hadza men perform. At the same time, there is little reason to doubt that even men with children do allocate time to activities that function purely or largely as mating efforts, including forms of hunting.
Consider as well recent work by Robert and Marsha Quinlan. Across societies of the standard cross-cultural sample, pair-bond stability (a low divorce rate) positively predicts older ages of infants at weaning (Quinlan & Quinlan, 2008). And in a rural village on the Caribbean island of Dominica, women with coresident mates weaned children at later ages than did women without coresident mates (Quinlan, Quinlan, & Flinn, 2003). As lactation interferes with women’s ability to produce food, male subsidy purportedly permits women to invest in young offspring through nursing. (At the same time, we note, male contributions to subsistence are actually predictive of shorter interbirth intervals in foraging societies; Marlowe, 2001. Taken together, these results imply that male subsidies increase the total amount of time a woman allocates to reproductive effort through both gestation and lactation.)
Trade-Offs in Allocation of Effort to Parenting and Mating
In species in which males and females form pair bonds and share parenting duties but in which both sexes also engage in extra-pair copulation (EPC), males face a trade-off between parental effort and mating effort. Parental effort increases male fitness by increasing offspring quality or increasing the rate of offspring production by a mate. Mating effort can increase male fitness should he succeed in obtaining extra-pair matings. How much males allocate to parental effort and mating effort, respectively, should hence depend partly on whether males can successfully obtain extra-pair matings through mating effort. Through comparative analysis of pair-bonding birds, Møller and Thornhill (1998a) demonstrated how this trade-off may work. The prevalence of extra-pair paternity (percentage of offspring sired by males other than females’ social partners) varies considerably across species (or across populations even within single species). In species in which the extra-pair paternity rate is relatively high, male attractiveness (which affects males’ ability to obtain EPCs) negatively covaries with male parental feeding. In these species, males can potentially obtain EPCs, and those males whose efforts to obtain extra-pair matings are purportedly most profitable (attractive males) do less parental care, arguably to allocate greater effort toward pursuit of extra-pair matings. By contrast, in species in which the extra-pair paternity rate is low, attractive males do no less (and, in some instances, do more) feeding of young than unattractive males. In those populations, neither attractive nor unattractive males have much chance to obtain extra-pair matings. Hence, both attractive and unattractive males allocate much time to parental care. (See also Møller & Jennions, 2001. The negative relationship between male quality and male assistance to females is also observed in some insects; for a review, see Bussière, 2002.)
Several lines of evidence suggest that men face this same trade-off. Among the Hadza, fathers with relatively many mating opportunities engage in less parental care than do fathers with few mating opportunities (Marlowe, 1999). And among the Tsimané Indians of Bolivia, men without dependent children have more EPCs than when they do have dependent children (Winking, Kaplan, Gurven, & Rucas, 2007). These studies yield evidence consistent with men facultatively adjusting the amount of effort they put toward mating and parenting, respectively, as a function of the (historical) payoffs to each, where payoffs vary with men’s personal characteristics (their attractiveness) or life circumstances (having a dependent child). (See also chapter 7. For related discussion of and additional evidence for trade-offs men face, see Gangestad & Thornhill, 1997a; Gangestad & Simpson, 2000.)
Do Men Possess Design That Functions to Allocate Effort to Parenting?
We argued in chapter 2 that compelling evidence that particular selection pressures have effectively shaped an organism’s phenotype historically is to be found in the nature of the organism those selection pressures shaped. That is, effective selection on an organism leaves its signature in the design of the organism. The most compelling evidence that men were historically shaped to allocate effort to parenting, then, should be found in evidence that men possess design that functions to allocate effort to parenting. Studies showing that men respond to circumstances that, in theory, affect the payoffs to parenting and mating effort do, in fact, suggest that men possess design to engage in parental effort. Related work may shed light on the nature and design of physiological mechanisms that underpin adaptive modulation of effort: investigations of factors that affect men’s testosterone (T) levels.
Across a wide range of taxa, T appears to facilitate male mating effort by channeling energetic resources to features particularly useful in male-male competition (e.g., muscles) and, due to necessary trade-offs, away from other targets of allocation (e.g., immune function; see Ellison, 2001, 2003; see also chapter 7). In some species in which males invest in offspring (e.g., marmosets, some birds), male T levels drop after the birth/hatching of the mates’ offspring (e.g., Nunes, Fite, & French, 2000; Nunes, Fite, Patera, & French, 2001). Men’s T levels too appear to drop when they become mated or have offspring (e.g., Berg & Wynne-Edwards, 2001; Booth & Dabbs, 1993; Burnham et al., 2003; Gray et al., 2004; Gray, Kahlenberg, Barrett, Lipson, & Ellison, 2002; Gray, Yang, & Pope, 2006; Mazur & Michalek, 1998; Storey, Walsh, Quinton, & Wynne-Edwards, 2000). In birds, marmosets, and men, drops in T may facilitate paternal investment. In fact, men who have lower T levels respond more prosocially to infant cries than do men with higher levels of T (Fleming, Corter, Stallings, & Steiner, 2002).
One set of studies further illustrates the facultative nature of men’s allocation of effort to mating. As mentioned, in Western samples, men who are mated in serious dating or marital relationships typically have lower T than single men do. In two studies, McIntyre et al. (2006) found this same difference in men’s T as a function of mating status. The effect of mating status, however, was moderated by men’s interest in pursuing extra-pair relationships with women other than primary partners. Men who claimed to have little interest in and history of extra-pair relationships revealed the typical drop in T when mated, as compared with being single. Men who claimed interest in and had a history of extra-pair relationships, by contrast, showed no difference: Such men had T levels just as high when they were in relationships as when they were single. (Perhaps of related interest, across species of birds, male T covaries with the total extra-pair paternity rate, but not well with overall levels of polygyny; see Garanszegi, Eens, Hurtrez-Boussès, & Møller, 2005.)
Though these data offer strong hints that men have adaptation shaped for the function of parental investment (trading off against mating effort), more research is needed. One study found that, when polygynously mated, Kenyan Swahili men’s T levels remain high (perhaps because maintaining multiple mates requires sustained mating effort; Gray, 2003). And alternatives must be ruled out. For instance, perhaps men simply have lower opportunity to engage in male-male competition when they have offspring as a result of uniquely modern social practices, leading to lower T levels. Or females may manipulate men’s T levels in their own interests and against male interests, such that changes do not reflect male adaptation (e.g., Gray et al., 2002).
Additional Features That May Speak to the Nature and Function of Human Pair Bonding and Paternal Care
Mutual Mate Choice Studies of mate preferences strongly point to mutual mate choice in modern societies. In Buss’s (1989b) classic study of mate preferences in 37 cultures, both men and women, on average, rated “kindness and understanding” as their number one preference. And a study by Buston and Emlen (2003) found that people tended to prefer valued traits in others that they perceived themselves to possess. The sexes differ with respect to particular mate preferences as well. For instance, men particularly prefer physical attractiveness in women, and this sex difference appears to be cross-culturally widespread (Buss, 1989b; but see Marlowe, 2005). The fact that both sexes possess strong preferences for long-term romantic partners, however—even if they differ in some respects—is additional evidence that humans exhibit mutual mate choice.
It is not merely that men and women prefer many of the same traits in mates. Men and women are similarly choosy when it comes to long-term partner choice. Kenrick, Sadalla, Groth, and Trost (1990) describe this phenomenon in terms of their “qualified parental investment model.” (See also Trivers, 1972, who described humans’ sexual selection system similarly.) In studies on college students, men and women claim to be nearly equally choosy when it comes to evaluating people as long-term mates (in that they identify, on average, a near equal minimum acceptable level of desired traits in a long-term mate). By contrast, when they evaluate people as potential sex partners—mateships lacking either party’s commitment to an enduring relationship—men are considerably less picky than women (Kenrick et al., 1990; Kenrick, Groth, Trost, & Sadalla, 1993). This pattern is consistent with men and women having evolved to engage in mutual mate choice in contexts in which partners cooperate to provide biparental care (while also being open to opportunistic mating outside of stable pair-bonds).
Female Mate Preferences in Long-Term and Short-Term Mating Contexts Evidence on what characteristics people prefer to see in mates and the mates of their children is consistent with an ancestry of paternal care. As we discuss at greater length in chapter 7, men’s degree of facial masculinity is purported to signal (or, ancestrally, to have signaled) men’s vigor and genetic quality. People tend to think that men with masculinized faces, however, are less likely to be good investors in romantic partners and offspring (see review in Penton-Voak & Perrett, 2001). Kruger (2006) presented women with two male faces—one masculinized through computer technology, the other feminized—and asked them to pick which they would prefer as an affair partner and as a marriage partner. On average, women preferred the man with the masculinized face for an affair partner (chosen 66% of the time) and the one with the feminized face for a marriage partner (chosen 63% of the time). This pattern is consistent with the idea that, because men invest in partners and offspring, women are willing to trade off some degree of genetic quality for greater willingness to care for offspring when choosing a long-term partner (see also Buss & Schmitt, 1993; Penton-Voak et al., 2003).
Preferences Parents Have About Their Daughters’ Mates Kruger (2006) also asked men and women which of the two men they would prefer to have as a son-in-law. As expected, both sexes preferred the feminized man as a son-in-law. Interestingly, these preferences were even stronger than women’s own preferences for a feminized man as a marriage partner (75% vs. 63%). This difference suggests a conflict of interest between daughters and parents over daughters’ ideal mates. Such a conflict would arise if fathers invest in offspring, but to variable degrees, and if grandparents invest an amount that depends on the level of paternal investment. That is, grandparents invest more in grandchildren if paternal care is minimal or absent than if paternal care is substantial (Buunk, Park, & Dubbs, 2008). In such a case, a daughter should not be willing to trade off as much genetic quality for paternal investment qualities in a mate as her parents should want to see her trade off. (This conflict of interest is, naturally, a particular instance of parent-offspring conflict; see Trivers, 1974). The fact that parents and offspring have conflicts of interest over the qualities of an optimal mate purportedly contributes to the common practice of parentally arranged marriages (though, we recognize, is not the sole reason for arranged marriages; see Buunk et al., 2008).
Romantic Love Both sexes have the capacity for romantic love, a capacity that, to our knowledge, can be found across cultures (e.g., Jankowiak & Fischer, 1992; see also Fisher, Aron, & Brown, 2005). The function of romantic love is not clear. One possibility is that love functions as a signal of intent to another person of commitment to a longterm interest in a relationship with the person (see Gangestad & Thornhill, 2007; for related and other views, see also H. Fisher, 2004; Frank, 1988).
Proprietariness and Threats of Infidelity As we noted earlier, if men invest directly in offspring, infidelity by both men and women threatens the other partner. In species in which males do not invest in offspring or provide substantial material benefits to females, multiple mating by males need not impose costs on females. After examining the cross-cultural record, Jankowiak, Nell, and Buckmaster (2002) concluded that, across cultures, sexual propriety within marriages and love relationships is a presumed right of both sexes.
Sensitivity of Investment Decisions to Levels of Paternal Uncertainty That men’s calibration of paternity certainty affects their investment in the offspring available to them for investment is strong evidence for psychological adaptation that functions as paternal care. In a large Western sample, Anderson, Kaplan, and Lancaster (2007) found that men directed more assistance toward offspring that they report are likely to be their own genetic offspring than toward offspring they suspect may be the product of their mate’s infidelity. Indeed, as we discuss in chapter 12, men’s perception of resemblance to offspring positively affects their willingness to assist them. Finally, abundant evidence indicates that men invest less strongly in their stepchildren than in their putative genetic children (see partial review in Anderson et al., 2007; additional evidence that men’s paternity confidence positively affects their investment is also reviewed by Anderson et al., 2007).
Cross-cultural patterning of “avuncular nepotism” also speaks to paternal investment patterns sensitive to paternity certainty (Alexander, 1979). In a minority of human cultures, men pass heritable wealth to their sisters’ sons rather than to their own putative children (their wives’ offspring). In some societies, men may invest time and effort into helping train or assist in other ways their sisters’ sons as well. In cultures with matrilineal inheritance patterns, the extra-pair paternity rate (typically estimated from ethnographic investigation of patterns of sexual behavior) tends to be relatively high (e.g., Flinn, 1981; Gaulin & Schlegel, 1980; Morgan, 1877). In turn, high extra-pair paternity (and hence matrilineal inheritance) tends to co-occur with particular means of subsistence. Cultures in which members depend on coastal fishing (e.g., cultures located in the insular Pacific) or horticulture are overrepresented among matrilineal societies. Coastal fishing may render it more difficult for men to guard mates. Women may be more efficient producers through horticulture and, hence, women in horticultural societies may be less dependent on men as providers. Matriliny is rare among pastoralists and agropastoralists.
As a number of scholars have observed, the extra-pair paternity rate would have to be exceptionally high (73%) for men to maximize inclusive fitness by investing in sisters’ offspring rather than their own offspring (e.g., Greene, 1979; Hartung, 1985; Holden, Sear, & Mace, 2003). (As the extra-pair paternity rate increases, a man’s mean relatedness to offspring obviously diminishes. But so too does his mean relatedness to his sister’s offspring.) Decisions about inheritance patterns, however, are not made solely by individuals whose resources are passed down. Men’s parents and grandparents may also exert influence over these decisions (Hartung, 1985). And, for it to pay parents and grandparents to prefer to invest in daughter lineages over son lineages, the extra-pair paternity rate need not be exceptionally high, particularly if the marginal benefit of wealth to sons is relatively modest (Holden et al., 2003). Whereas parents and grandparents can exert influence over decisions concerning transfer of heritable wealth, they may have a difficult time exerting influence over decisions about how a son allocates time. Perhaps for that reason, men’s decisions to allocate time to caring for their own offspring do not appear to meaningfully covary with estimated extra-pair paternity rates (Gaulin & Schlegel, 1980). (See also Chrastil, Getz, Euler, & Starks, 2006, for a review of literature on effects of paternal uncertainty on grandparental investment patterns.)
In sum, examination of the psychological makeup of modern humans reveals suggestive evidence that humans have been selected to pair bond, express mutual mate choice, and biparentally care for offspring. In these respects, we appear to have diverged substantially from our closest phylogenetic relatives (see also Geary, 2000).
The Male-Assistance Hypothesis of Extended Sexuality in Women
Male Assistance That Led to Selection for Women’s Extended Sexuality
We now turn to the male-assistance hypothesis of extended sexuality in women. The form of this hypothesis that applies to humans is, purportedly, at least very similar to the form that applies to other pair-bonding species. Men deliver material benefits and services (e.g., food, protection, shelter) to primary partners (and vice versa, though benefits women deliver to their partners are not key to understanding extended sexuality). Women’s copulability outside of the fertile phase of the cycle coevolved with male delivery of benefits to facilitate their flow, yielding a variety of forms of extended sexuality: sexual motivation during infertile phases of ovulatory cycles, during anovulatory cycles, during adolescence, and during pregnancy and lactation. That is, following the logic of Rodríguez-Gironés and Enquist’s (2001) model, female ancestors of modern women who possessed extended sexuality outreproduced other females specifically because they outperformed them in the realm of garnering male-provided material benefits. Men who paired with females with whom they could copulate regularly delivered greater flow of benefits (partly to achieve continued sexual access) than did men paired with females with whom they could copulate only during the fertile phase. In addition, men paired with women with extended sexuality may have been less likely to seek extra-pair partners or to pair-bond with women other than current mates (Petrie, 1992).
The argument, of course, is not that men benefit from copulation per se, simply for the sake of the “pleasure” of copulation. No doubt, men typically find sex pleasurable, but the pleasure that men (or women, for that matter) find in sex was selected, in theory, because sex has, on average, reproductive benefits. Men should not have evolved to find a class of sex pleasurable if that class of sex had no benefits or has net negative reproductive consequences. (Hence, men, as well as women, typically find the thought of incestuous sex disgusting; see Lieberman, Tooby, & Cosmides, 2003, 2007.) In Rodríguez-Gironés and Enquist’s (2001) model, males must possess imperfect knowledge of their mates’ fertility status, as it changes across the cycle. They need not be completely ignorant of females’ cycle-related fertility (and, as we discuss in chapter 11, men are not completely ignorant of the times that women are fertile in their cycles). Males simply need be unable to completely rule out possibility of conception. When males cannot completely rule out that a female has some risk of conception, they generally will be sexually selected to be motivated to copulate with a female (under appropriate circumstances). Men’s interest in copulating across the cycle, even in the absence of female interest, coupled with the female’s copulability across the entire cycle and at other infertile times, satisfies the assumption in the game-theoretic model that males do not have unambiguous direct cues of peak fertility. Presumably, it is the lack of unambiguous fertility cues that has selected for men’s sexual interests in women throughout their cycles and in adolescent women. (Later, we treat in greater detail how female adolescent sexuality functions to gain male material benefits in the context of the human mating system of pair bonding, including long-term pair bonding; see chapter 6.)
That women’s extended sexuality is extreme, in comparative perspective, is consistent with the view that pair bonding and male delivery of associated material services have been highly important to women’s fitness in human evolution. Table 4.1 lists a number of these purported benefits. As we and others have emphasized, they include male paternal care (e.g., Alexander, 1979, 1990; Alexander & Noonan, 1979; Geary, 2000; Geary & Flinn, 2001).
Table 4.1 The Contexts Proposed for the Direct Selection of Female Extended Sexuality in Human Evolutionary History
| I | Female–female competition for paternal care of offspring (protection, teaching, etc.) fueled by extremely altricial offspring, requiring intensive and extended parental care. |
| II | Female–female competition for male-provided calories for offspring and the female herself, given the relatively low caloric provision by females because they are pregnant or lactating or otherwise engaged in child-care activities that restrict their caloric access. |
| III | Female–female competition for males capable of and willing to protect the females and the females’ daughters from capture by raiding males and sexual coercion. |
| IV | Offsetting child maltreatment by adult males. |
Another service that male partners purportedly provided to women ancestrally is protection of mates and their female relatives from capture during raids and warfare and from sexual coercion by other men in the same group. (See Smuts, 1992; Smuts & Smuts, 1993; Thornhill & Palmer, 2000, for treatment of the importance cross-culturally to women of protection from sexually coercive males. See also Mesnick, 1997, and Wilson & Mesnick, 1997, on the bodyguard hypothesis for the evolution of human pair bonding.)
The benefits that women ancestrally garnered from men and that led to their extended sexuality need not have been delivered solely by primary partners. Male mating effort, leading them to deliver resources to women in hopes of gaining sexual access, may also have selected for women to possess extended sexuality (see, e.g., Hill, 1982; Symons, 1979). Though we do not dismiss the potential importance of these benefits, we suspect that women’s reliance on a continued flow of material benefits delivered by primary partners typically meant that it was not worth the risks of losing those benefits by being unfaithful for exchange of a single meal. Hence it would not commonly benefit women to be unfaithful to an investing primary partner (at least one she wished to retain) during extended sexuality (particular if she had small children; see Marlowe, 2003a), unless the material benefits gained through infidelity were considerable. In some bird species, females rarely or never engage in extra-pair copulation during extended sexuality. (Some of these same female birds, by contrast, do so during the fertile phase of their cycles, but not to garner material benefits; rather, they often appear to do so to obtain genetic benefits for offspring. See chapters 9–12.)
Do Women Possess Extended Sexuality to Confuse Paternity Through Multiple Mating?
As we discussed in chapter 3, Hrdy’s paternity confusion hypothesis is one important variant of the male-assistance hypothesis for female extended sexuality. Reduction of maltreatment of offspring by resident males through promiscuous mating has probably been a very important benefit leading to the evolution of female extended sexuality in a number of nonhuman primate species (Dixson, 1998; Hawkes, 2004; Heistermann et al., 2001; Hrdy, 1981, 1997, 2000; Hrdy & Whitten, 1987; Palombit, 1999; Pazol, 2003; van Schaik, 2000; van Schaik & Dunbar, 1990; van Schaik et al., 2004; Wrangham, 1993).
Might human females have been shaped to confuse paternity through mating with multiple males during a period of extended sexuality as well? Possibly—though there is reason for doubt. In pair-bonding species in which females’ fitness is importantly affected by a flow of material benefits delivered by primary partners, female tactics to increase paternity confidence in these partners may be more successful than tactics to confuse paternity. Hence, once again, in a variety of bird species, females rarely or never engage in extra-pair copulation during extended sexuality, though they copulate with a social partner frequently. Similarly, women’s extended sexuality may be directed primarily, even if not exclusively, toward primary partners (see chapter 10).
Nonetheless, perhaps a strategy of dispersing paternity confidence widely by mating with multiple men during extended sexuality has been adaptive in women conditionally, in restricted sets of circumstances (see Hrdy, 2000; see also Beckerman et al., 1998).
Women’s Extended Sexuality Versus Concealed Ovulation
A number of writers have conflated extended sexuality and concealed ovulation (e.g., Strassmann, 1981; Symons, 1979; Turke, 1984). Specifically, some scholars have claimed that extended sexuality has functioned to garner material benefits by concealing ovulation (e.g., Alexander, 1979). The argument is that extended sexuality renders women’s sexual interests continuous—effectively unchanging—across the cycle. If males discern females’ fertile phase largely through females’ sexual proceptivity and receptivity, continuous sexuality leaves males (as well as females themselves) ignorant of when cycling females are fertile. Lack of knowledge of female fertility status, in turn, may alter males’ optimal strategy of allocating time and effort in a way that fosters paternal care. If males know which females are fertile at any point in time, they may be best off pursuing mating opportunities with those females. By contrast, if males do not know whether their mate or any other female is fertile at any point in time, many males may be better off investing in one female (with whom he mates and guards) and her offspring (see chapter 11).
Lack of perfect knowledge of female cycle-related fertility status is a necessary condition for the coevolution of female extended sexuality and male-delivered benefits during extended sexuality (Rodríguez-Gironés & Enquist, 2001). For female extended sexuality to function to obtain nongenetic material benefits or services from males, males cannot be able to rule out that a female in extended sexuality is fertile; hence males must lack perfect discrimination of female fertile phases from nonfertile phases for extended sexuality to evolve. Nonetheless, it is a mistake to conflate extended sexuality and concealed fertility within the cycle. They are two distinguishable and, at least potentially, functionally distinct phenotypic traits (or suites of traits). As we discuss in detail in chapter 5, sexual selection strongly favors male capabilities to discern and act on valid cues of female fertility status. Female receptivity may or may not be a valid cue. Typically, males attend to by-products of alterations in female chemistry across the cycle indicative of fertility status, though these cues may not be completely unambiguous cues of cycle-related fertility. Concealment of fertility status implies adaptation to suppress the cues to which males have evolved to attend and by which they discern female fertility status. Extended sexuality can evolve with or without concealed fertility. If the cues of fertility status that males attend to are sufficiently ambiguous, such that it pays them to copulate with (and deliver material benefits to) nonfertile females (whose fertility status, of course, males cannot perfectly gauge), female extended sexuality can evolve (in response to fitness gained through male delivery of material benefits) without any female adaptation for concealed ovulation whatsoever. For an expanded discussion of this theme, see chapter 11.
Women’s extended sexuality should also not be conflated with continuous sexuality. As we noted in chapter 3, females of many species are sexually receptive outside their fertile period and hence possess extended sexuality. As we have emphasized, from a comparative perspective women’s extended sexuality is extreme; women are sexually receptive throughout their cycles and during a variety of other nonfertile times. By no means do these claims imply, however, that females of species who exhibit extended sexuality, including women, express sexual preferences and motives that are unchanging (or continuous) throughout their sexually receptive periods. Indeed, we argue that, across all species with extended sexuality, instances in which female sexual interests do not change across their reproductive cycles are relative rarities. Instead, females of these species almost always express dual sexuality—sexual preferences and motives during extended sexuality that differ from those exhibited during fertile cycle phases (see chapter 8).
Summary
Women’s sexuality is extended to a degree not matched by other female mammals. This amplification, similar to female extended sexuality in other species with this adaptation, was favored by direct selection because those females with extended sexuality received material benefits and services from males by exchanging infertile sex for them. Women’s extended sexuality evolved in the context of the human mating system, which reveals important adaptations that served to affect amplification of extended sexuality. These adaptations account for, at least partly, facultative marriage and associated pair-bond arrangements, most commonly social monogamy with small to moderate levels of polygyny and rarely polyandry, and biparental investment in offspring. Comprehensive studies of foraging societies reveal that men subsidize the reproduction of their partners and their offspring, at least during critical times. Subsidies of calories and other nutrients are primarily achieved through men’s hunting, which increases female reproductive success by reducing interbirth interval and/or offspring mortality.
Two primary explanations for the evolution of men’s hunting activity have been offered and defended: that it functions as parental effort and that it functions as mating effort. Both receive some empirical support. They are not exclusive alternatives, and a blended view is most likely correct. Men hunt both as parental effort and mating effort; possibly, some adaptations for hunting serve one function and not the other.
Paternal care coevolved with the evolution of highly extended juvenile life, demanding intensive parental investment from conception through adolescence, and high levels of productivity that subsidize offspring well into the life span. Humans are very different from other mammals in the degree of these coevolved life history characteristics.
Evidence suggests that pair-bonded men possess psychological adaptation that conditionally alters the amount of effort allocated toward mating and parenting based on the ancestrally adaptive value of each. This evidence, as well as other forms of evidence suggestive of psychological design, is consistent with the reasonable view that men in pair bonds express design for both parental and mating effort.
Although paternal care was an important material benefit selecting for extended female sexuality in human evolutionary history, other material benefits from pair-bonded males in exchange for mating, such as protection from sexual coercion, were likely important as well. Ancestral women may have gained material benefits that affected selection for extended sexuality through extra-pair mating, but large benefits would have been required to offset possible lost investment from main partners, particularly for women with dependent children. Hence, though in many species of nonhuman primates mating with multiple males to reduce their maltreatment of offspring has importantly selected for extended female sexuality, women’s extended sexuality probably does not significantly reflect this function.
Women’s extended sexuality is distinct from concealed ovulation. Extended sexuality can evolve with or without concealed ovulation. Furthermore, women’s sexuality is not continuous in the sense of unchanging sexual motivation across the menstrual cycle. Women possess dual sexuality across their ovulatory cycles: estrous sexuality when fertile and extended sexuality when not.