Evolution's Rainbow. Joan Roughgarden

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СКАЧАТЬ male Belding’s ground squirrels don’t. Why? The power relationship is probably not as simple as it seems. Are the female Idaho squirrels being caged against their will? Maybe, maybe not. If asked, an Idaho male squirrel might say lovingly that he was “protecting” his female during their brief marriage from the unwanted advances of rival males. And she might agree, happy to have him there. The courtship that precedes mating suggests her acquiescence. And she is capable of physically repelling a male if she wishes: the day after her three-hour burst of sexuality, an Idaho female squirrel constructs a nest and excludes all other squirrels from a small territory around it. So a female Idaho squirrel may want to be guarded.

      Why does the last male sire most of the young in the Idaho squirrel, and the first male sire most of them in Belding’s squirrel? Could female squirrels control whose sperm fertilizes their eggs?⁷ Could a female manipulate a male to guard, or not guard, by controlling his sperm? Can a female select the first male’s sperm to do the fertilizing if she wants him to have no incentive for staying, and select the last male’s sperm if she wants him to hang around and guard? Presenting mate guarding as a tactic by which males protect their investment ignores the female perspective. Females are viewed as land in which males plant seeds and which they guard if necessary. Yet females are probably active players in whether they’re guarded or not.

      The Idaho and Belding’s squirrels may have evolved to experience pleasure differently. A female Idaho squirrel may like being squeezed into a burrow, and a male may enjoy having a female behind him as he stuffs the burrow’s entrance, like a guy taking a girl for a spin in his red convertible—fun for both. Yet for a female Belding’s ground squirrel, squeezing into burrows could be a total turnoff and the reason she doesn’t permit the guarding. Species differences in how power is eroticized make it difficult to discern whether animals have freedom of choice during mate selection and in their family lives.

      Among primates, the amount of sexual coercion varies greatly from species to species, as does the overall level of both between-sex and within-sex aggression.⁸ Outside the breeding period, male mountain gorillas aggress against females one to four times a day, olive baboons about one time per day, and red howler monkeys only 0.04 times per day. The context for the aggression varies from competing for food, coming to the aid of one female against another, and breaking up fights between females. These aggressions are not directly sexual coercion, but rather reflect a social atmosphere of violence.

      Within the breeding period, male rhesus monkeys attack females who consort with male rivals. Similarly, among chimpanzees, one of our closest primate relatives, males attack females who consort with lower-ranking male rivals, rather than attacking the low-ranking males themselves. Using “a fair amount of brutality,”⁹ chimpanzee males persuade females to accompany them away from the group where they were living. One-third of all conceptions result from matings between pairs separated from the group for several days to over a month. Chimpanzee males also intimidate females into submitting to their advances later on. Lack of resistance does not necessarily mean willing participation, but may reflect experience with the male’s previous aggression.

      The record-holders for male sexual coercion are the orangutans, in which most copulations by subadult males and nearly half of all copulations by adult males occur after a female’s fierce resistance has been violently overcome.¹⁰ Other primate species showing lots of male aggression against females include white-fronted and wedge-capped capuchins, black spider monkeys, and brown lemurs. Females’ counterstrategies include avoiding areas where males are found, joining a male’s territory or harem to gain his protection, and forming coalitions to fight off the males.

      Yet other primate species enjoy a peaceful life. Male aggression against females is rare in bonobos, a primate species as closely related to humans as the relatively violent chimpanzee is. Male sexual coercion is also rare in patas monkeys, red-backed squirrel monkeys, brown capuchins, woolly spider monkeys, and black-and-white ruffed lemurs. As with the bonobo/chimpanzee contrast, closely related species differ in the prevalence of male sexual aggression.

      No explanation exists for why some societies develop coercive power relations between the sexes, whereas others form equitable power relations.¹¹ Although some species resemble the tough-talking television show “NYPD Blue,” others resemble the peaceful Mr. Rogers. How power relates to sex is not a biological universal. We may choose to live like some species and not others.

      

      MONOGAMY AND DIVORCE

      As we turn to long-lasting two-gender families—beyond the three-hour marriages of ground squirrels—the plot thickens. Ninety percent of bird species are monogamous in the sense that a family consists of one male and one female who cohabit a nest and raise young together in that nest during at least one breeding season. In contrast, 90 percent of mammal species are polygynous, with one male for many females.¹² Among mammals, females typically occur in groupings of two or more who are serviced by one male.

      But what do these statistics mean? Biologists have been slow to distinguish between economic monogamy and reproductive monogamy. Birds are identified as “monogamous” simply if they are living in the same nest and feeding nestlings together—an economic criterion. Biologists have unquestioningly assumed that the nestlings are the offspring of the couple in the nest. When distributed parenthood is discovered, biologists feel something is amiss—that one or the other member of the pair “cheated,” straying outside the marriage bond. However, birds have decoupled reproductive and economic monogamy; in some species these go together, and in others not.

      Black-capped chickadees (Parus atricapillus) of eastern Ontario are monogamous.¹³ During the summer, pairs settle in territories to raise their families—a suburbanite’s dream. During the winter, the chickadees cease living as couples and live in flocks of about ten birds. Because a chickadee’s average life spans several years, the birds are aware of each other through both the over-wintering period and the breeding period. During the winter, males and females sort themselves into separate dominance hierarchies.

      New couples form during the winter social period and settle as pairs for the summer to raise a family. As a couple, they forage together, excavate their nest cavity together, mate with one another, and defend their territory together. The male feeds the female although they both forage, and the male prevents other males from approaching the female. When forming couples, the highest-ranking female pairs with the highest-ranking male. Most couples remain together for more than one breeding season. Females may “divorce” their mate and/or mate with males other than their nest mate. A mating outside the pair-bond is called an extra-pair copulation, or EPC in biology jargon. Female chickadees divorce to obtain a male who is higher in the male dominance hierarchy than their present mate.

      In one study,¹⁴ seven females paired to seven high-ranking males were removed, temporarily creating seven very desirable widowers. Over the next two days, some of the remaining females deserted their partners and took up with the higher-ranking widowers. When the females who had been removed were restored to the site, they quickly chased the social climbers back to the males they had deserted. These hen-pecked husbands took their wayward spouses back. To complete the experiment, six females were also temporarily removed from six low-ranking males, but none of the remaining females left their mates to join these sad sacks. These males were saved from eternal loneliness only when their mates were returned to the site to rejoin them. Not only are female chickadees willing to desert their mate to acquire a higher-ranking male when one is available, but they are willing to mate with higher-ranking males in EPCs, resulting in many broods with mixed parentage.

      A close relative of the chickadees, a member of the same genus, is the European great tit (Parus major). Boxes for these birds to nest in have been constructed on the island of Gotland in southeast Sweden. In a 1985–89 experiment, eggs were removed from one pair’s nest and transferred to another pair in order to observe the effect on divorce. Pairs whose eggs were removed СКАЧАТЬ