Fatherhood

Evolution and Human Paternal Behavior

Peter B. Gray, Kermyt G. Anderson

• Publisher: Harvard University Press
• Year: 2010
• ISBN:
  9780674246362
• Topic: Social Sciences
• Subtopic: Anthropology
• Index: Social Sciences

We've all heard that a father's involvement enriches the lives of children. But how much have we heard about how having a child affects a father's life? As Peter Gray and Kermyt Anderson reveal, fatherhood actually alters a man's sexuality, rewires his brain, and changes his hormonal profile. His very health may suffer—in the short run—and improve in the long. These are just a few aspects of the scientific side of fatherhood explored in this book, which deciphers the findings of myriad studies and makes them accessible to the interested general reader.Since the mid-1990s Anderson and Gray, themselves fathers of young children, have been studying paternal behavior in places as diverse as Boston, Albuquerque, Cape Town, Kenya, and Jamaica. Their work combines the insights of evolutionary and comparative biology, cross-cultural analysis, and neural physiology to deepen and expand our understanding of fatherhood—from the intense involvement in childcare seen in male hunter-gatherers, to the prodigality of a Genghis Khan leaving millions of descendants, to the anonymous sperm donor in a fertility clinic.Looking at every kind of fatherhood—being a father in and out of marriage, fathering from a distance, stepfathering, and parenting by gay males—this book presents a uniquely detailed picture of how being a parent fits with men's broader social and work lives, how fatherhood evolved, and how it differs across cultures and through time.

1

Our Founding Fathers

The geneticist Angus John Bateman conducted seminal breeding experiments in the 1940s with captive fruit flies, experiments that, we will see, have relevance for understanding the evolution of human male behavior (Bateman 1948). When he provided sufficient food resources to his subjects, he observed sex differences in the relationships between fruit fly mating opportunities and reproductive success. He found that the number of offspring a fruit fly male fathered was related linearly to the number of mates to which he was given access. If he had one mate, then he had x offspring; two mates, then 2x offspring; three mates, then 3x offspring. This relationship did not hold for females, however. Females tended to have approximately the same number of offspring, whether they had mated with one, two, or three males. The additional mating opportunities did not translate into additional offspring for females in the same way they did for males.

Bateman’s results probably seem intuitively obvious. The key, though, is that these captive fruit fly patterns appear to generalize quite broadly with respect to sex differences in reproductive constraints in nature. Male reproductive success tends to be ultimately limited by reproductive access to females. For females, resources such as food play a more important role: female reproductive success tends to be ultimately limited not by reproductive access to males but by sufficient resources that can be used for reproduction. A consequence is that males can be expected to compete heavily for limited reproductive opportunities, whereas females are expected to prioritize access to reproductively limiting resources.

This does not mean that Bateman’s experiments are the end of the story—far from it. Among animals in the wild, there are cases where it pays females to mate with multiple males. Idaho ground squirrel females bear more offspring if they mate with multiple males; interestingly, in the same litter they may have pups fathered by different males. In other cases, it may benefit females to mate with multiple males to garner favors. If a female can acquire additional food resources by mating with multiple males, her reproductive success may be enhanced. A leading hypothesis advanced to account for chimpanzee females mating with multiple males is that the practice “confuses paternity,” thereby allowing a number of males to think they might be an infant’s father and in turn be less prone to harming it. Back to fruit flies, though: Bateman suggested that the sexual asymmetries could be traced to differences in gamete size. Females, by definition, possess the larger gamete (the egg), whereas males have the smaller gametes (sperm). Initial sex differences in investment in gametes, Bateman felt, were magnified into sex differences in reproductive investment and hence constraints more broadly.

Decades later, the eminent evolutionary biologist Robert Trivers (1972) argued that the key to understanding sex differences was not in gamete size but in relative parental investment between the sexes. One advantage of Trivers’s insights is that they moved the locus of sex differences closer to home—from the very distant focus on gametes to the more readily observable parental care in the wild. In a chapter in the 1972 book Sexual Selection and the Descent of Man, he contended that the sex which invests more in parental care will become the reproductively limiting sex. In most species, especially mammals, the sex investing more in parental care is females. As a consequence, males are expected to seek mating opportunities with females. In the language of evolutionary biology, males tend to specialize in “mating effort”—investment in male-male competition, in courtship, and in mate guarding. Females, conversely, tend to specialize in “parenting effort”—investment in the survival and reproductive prospects of their offspring.

A plus of Trivers’s framework was that it could account for “sex-role reversals,” for species in which the typical sex differences break down. Consider jacana birds. Living in ponds and marshes in parts of Africa and the Americas, these birds possess enormous feet to help them balance upon lilies and the like. Interestingly, male jacanas provide the bulk of parental care. Females drop their eggs off with their males. Males take over, incubating them and fostering their offspring’s success. The outcomes of this social arrangement are that males are the reproductively limiting sex. Females compete more among themselves for access to males than males do among themselves for access to females. Females are larger and more colorful than males, thanks to their need to compete among themselves and to advertise to prospective male mates. Jacanas are thus exceptions to typical sex roles that help prove the rule.

Exceptions to Trivers’s framework resulted in a reformulation of the basis of sex differences in reproductive strategies. For example, in some species of mouth-brooding frog, males provide more parental care than do females. However, males tend to compete among themselves for access to females more than females compete among themselves for access to males. Such exceptions led to the concept of “potential reproductive rates” as the basis for sex differences in reproductive strategies. The sex with the slower potential reproductive rate is the one over which competition will occur (Clutton-Brock and Vincent 1991).

The sex differences in reproductive strategies that Bateman, Trivers, and others have described resonate with ideas proposed by Charles Darwin. He advanced the concept of sexual selection in the latter part of The Descent of Man and Selection in Relation to Sex (1871). As Darwin noted, selection favors traits that enhance survival and reproductive success. Among sexually reproducing species like ourselves, acquiring mates and raising offspring who themselves succeed in obtaining mates present tremendous challenges. These processes have been written into our morphology, physiology, and behavior. We are all descended from a long, unbroken line of males and females who successfully reproduced. As a result, we bear many marks resulting from sexual selection.

Darwin divided what he termed “sexual selection”—selection operating in the sexual and reproductive realm—into two main parts: male-male competition and female choice. He noticed that males commonly battle among themselves in the effort to mate. The antlers, large canines, and extra muscle mass that many male mammals bear compared with that of females serve these competitive ends. Females, conversely, often have the ability to select a mate. A female may look upon the male suitors in her environment and choose the one that best suits her.

The textbook example of female choice is the peacock’s train. Peacocks are famous for their elaborate, colorful, and long tails. These are completely lacking in the peahen. Yet Darwin felt that natural selection could not explain the presence of the peacock’s tail: it is energetically expensive to grow and maintain, raising the male’s nutritional requirements; its mass slows the bird down, making it more difficult to forage and evade predators; and its bright colors are the equivalent of a neon “all you can eat buffet” sign for predators. Darwin argued that the existence of the peacock’s train could be traced to female preferences for showy feathers.

Darwin did not delve into the varieties of female mating with multiple males, and he devoted less attention to cases in which male choice operates. Still, he addressed the ultimate bases of animal sex differences in reproductive strategies, bases that would later find great resonance with Bateman’s fruit fly experiments and more. These bases allow us to speak of peacocks and human fatherhood in the same sentence.

Patterns of Nonhuman Paternal Care

Approximately 90% of bird species are socially monogamous (a single male and female bonded together, with or without sexual fidelity) and have males who invest in offspring. The peacock is not one of them, but if you have seen pairs of doves or pigeons cooing together, then you get a better picture of the typical avian world. It appears that because birds lay eggs that can be cared for by either males or females, this favors greater male family involvement. A male can sit on eggs, incubating them; can perhaps provide food to a mate while she incubates the eggs; and can later aid in feeding the young and his mate. It should be no surprise, then, that among the vast majority of avian species males and females are about the same size as each other and tend to look fairly similar. If you are in it together over the long haul, there is less need to show off or battle with others in the attempt to seek additional mates.

Mammals, of course, have internal gestation (with the exception of the egg-laying mammals—the duck-billed platypus and the echidnas). Males can hardly sit on fertilized eggs growing in a kangaroo female’s pouch or a llama’s womb. Males are unleashed. Male investment in offspring can in principle (and in practice often does) end at the moment of ejaculation. Female mammals have the obligate investment through the gestation of their young and must also invest in lactation and care of the young for various lengths of time postpartum. The consequence is that, unlike birds, mammals tend to exhibit sex differences in parental investment and potential reproductive rate. As a result, in only around 3–5% of mammalian species do males and females form long-term bonds with each other (pair-bonds) and do males provide parental care (Clutton-Brock 1991).

The rarity of mammalian paternal care raises other questions: In which species of mammals does paternal care occur? What forms does paternal care take?

A number of social carnivores, such as wolves, dwarf mongooses, and coyotes, have paternally investing males (Clutton-Brock 1991). Males appear to play important roles providing food to pups and mates. In a risky world of hunting, where food is served in the form of valuable, unpredictable, and shareable meals, it helps when there is more than a single adult (the mother) seeking prey on a daily basis. By pooling the risk with an adult male and female, and maybe even other adults and juveniles who do not breed in a social group, selection favors paternal involvement.

Another clustering of paternal mammals occurs among New World monkeys (H. J. Smith 2005). Here, you can find some of the poster fathers of the natural world. Take the titi monkey, of which there are a handful of species inhabiting forests in South America. Not only do male and female titi monkeys sometimes intertwine their tails while perched together on a tree branch, but fathers tend to play important roles carrying their young. A father may lug his little ones through the forest, handing them to their mother for nursing, then putting them back on his shoulders. In experiments with captive titi monkeys, off-spring given the choice of heading to their mother or their father will favor their fathers

Among marmosets and tamarins, small-bodied monkeys in South America, we also find numerous species of paternally investing males (Digby, Ferrari, and Saltzman 2007). Here, males play crucial roles in family life. Females commonly give birth to twins. The combined weight of twins may comprise 20% of a mother’s body weight at birth. That is akin to a 135-pound woman giving birth to a 27-pound baby. Males help shoulder some of the burden with their young. Commonly, fathers carry the young through the forests they inhabit. As the young are weaned, sometimes the fathers also help provide their young with food, one of the few primate cases in which this occurs. The importance of paternal care in these species has been illustrated through research with captive animals. If cotton-top tamarin fathers are removed from the family scene, mothers often abandon the young. This behavior provides compelling evidence of the importance of paternal care in the form of carrying and provisioning young.

The Descent of Paternal Care: Old World Monkeys and Apes

Paternal care is largely absent among our more closely related primate cousins, Old World monkeys and apes. Take the abundant and pesky rhesus macaque or vervet monkey as examples. Very few males of these species provide any meaningful amount of paternal care. That is the standard protocol for Old World monkeys. Or take baboons living in Botswana. In a wonderful book describing their social lives, males are shown to develop “friendships” with particular females who have off-spring (Cheney and Seyfarth 2007). These adult males are typically not the fathers of the young. But by developing a strong relationship, in part by being nice to a mother’s baby, a male increases the likelihood of mating with her when she next seeks a mate. Interestingly, in this multimale, multifemale baboon society, males appear to recognize more often than they would by chance alone which offspring they have fathered and come to their offspring’s defense when duty calls (Buchan et al. 2003).

What about the apes, to whom we are more closely related than Old World monkeys? Genetic evidence reveals that we last shared a common ancestor with the lesser apes—the twelve or so species of gibbons and siamangs—approximately 20 million years ago (T. Bartlett 2007). The lesser apes live in peninsular and island southeast Asia. These lesser apes tend to form social systems including one adult male, one adult female, and their offspring; adults defend their territories from intruders of their own species who might seek to mate with their partners. Sometimes they do engage in extrapair mating. Yet generally adult males and females of these species tend to be socially and relatively monogamous sexually. Consistent with this, males and females are about the same size; if observing males and females in a shadowy forest, you might have difficulty discerning males from females. Interestingly, however, males are not that involved in paternal care. Of the lesser apes, siamang males appear to provide the greatest amount of parental care. Siamang males may harbor their young on their backs for part of the day. Occasionally, males may provide a morsel of food for their off-spring. The remaining species of apes are referred to as the great apes, in part because they are larger than their lesser ape cousins (Campbell et al. 2007; Strier 2007). Of the great apes, we last shared a common ancestor with orangutans approximately 15 million years ago. Orangutans live on two islands—Borneo and Sumatra—in southeast Asia. Important to our story, orangutan males do not provide paternal care. Instead, adult males tend to pursue one of two reproductive strategies. As young adults, when they might be subordinated by fully adult males, males may have a smaller appearance, not so different in size from females. These smaller males may attempt to force copulations upon adult females. They are not preferred mates and thus appear to opt for the only reproductive avenue available, sexual coercion. By being about the same size as females, young males are also agile in their forested environment, allowing them to pursue females wherever they go. Prime adult males, bulked up on testosterone, are larger in size, weighing about twice as much as adult females (Knott and Kahlenberg 2007). Prime adult males have large fleshy cheek pads that facilitate their “long calls” through the rainforest (which make them heard if not seen). They can manhandle young adult males if competing directly but have difficulties pursuing the smaller adult males in trees. Females prefer to mate with these prime orangutan males. Even then, however, these trysts tend to be short lived: males do not stick around to observe their pregnant partners or to welcome their offspring into the world. Orangutan males are the definition of a typical, uninvolved mammalian male.

The other great apes—gorillas, common chimpanzees, and bonobos—live in Africa. We last shared a common ancestor with gorillas around 12 million years ago, and with common chimpanzees and bonobos approximately 6 million years ago. Gorilla males do provide paternal care. Yet the care they provide differs in a number of ways from how we tend to think of human paternal care.

Gorillas tend to form polygynous groups (or harems) based on one or sometimes two adult males, multiple adult females, and their off-spring (Robbins 2007; H. J. Smith 2005). Gorilla males exhibit some gentle fatherly ways even if males mark their entry into a new social group in a violent flurry, commonly entailing infanticide. In a polygynous species, the unmated males may wait their opportunity to mate with females, forming bachelor herds. When an unmated gorilla male attempts to break into a group already led by an adult male, the intruder encounters resistance. The mated gorilla, aided by his bulk (and potentially supportive females), attempts to keep the unmated gorilla at bay. Such forms of contest competition between males may account for the huge sexual dimorphism present in gorillas: males weigh about twice as much as females, apparently to utilize this extra bulk during male-male challenges.

If a mated gorilla loses the contest, he loses his keys to reproductive success—the adult females in the group of which he had previously been a part. This may not be good news for the females. It is the nursing young of such females who may now be in jeopardy. A significant fraction of the young present during the arrival of a new male may be killed by the new arrival. Why would he do such a thing? That hardly seems like a good way to entice the mother as a mate. Functional hypotheses for infanticide in gorillas suggest that males may kill young offspring of lactating females in order to terminate females’ investment in young to which the new male is not related; after all, he did not father those offspring. Once the young are killed, mothers no longer experience the physiological effects of lactation that can delay a return to ovulation, so they begin cycling sooner. And when they begin cycling, they seek a brawny male that may prevent the attack upon their young from happening again. The new male has already demonstrated his abilities by overcoming resistances of the previous male; the new male may be a good defender in the future. So here a female may mate with the very male who killed her previous offspring.

In established gorilla groups, males play several important paternal roles. Their role of protectors can be appreciated by the dynamics of infanticide described above. Males may also be indulgent with their own offspring. Their little young may play on their backs, finding a quiet calm of encouragement. Gorilla fathers do not help feed their offspring, however; they are not providing resources such as food to their mates or offspring. Gorilla males do not appear to play active roles in the socialization of their young. They are not offering the gorilla equivalent of social guidance (son—be tough—if you don’t stand your ground, nobody else will do it for you). The patterns of gorilla paternal care make sense: gorilla males can protect and be indulgent with their young without sacrificing an ability to keep male competitors away. The multiple mothers of their multiple offspring can provide the childcare and guidance their young need. So gorilla adult males may be paternal, but in very limited ways.

Of the living primates, including apes, we are most closely related to common chimpanzees and bonobos. The ancestors of common chimpanzees (hereafter, referred to just as “chimpanzees”) and bonobos themselves diverged from each other only about 2 million years ago. So we might imagine ...

Table of contents

• Cover
• Title
• Copyright
• Dedication
• Contents
• Preface
• Introduction
• 1. Our Founding Fathers
• 2. A World of Diversity: Cross-Cultural Variation in Paternal Care
• 3. Men and Marriage
• 4. Fathers and Fertility
• 5. Who’s the Dad?
• 6. Father Involvement, Father Absence, and Children’s Outcomes
• 7. The Makings of a Stepfather
• 8. Having It All? Fatherhood, Male Social Relationships, and Work
• 9. The Descent of Dad’s Sexuality
• 10. Babies on His Brain
• 11. Health and the Human Father
• 12. Rewriting the Manual
• Appendix
• References
• Acknowledgments
• Index