Evolution's Rainbow. Joan Roughgarden
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‘Because it’s not safe for girls.’ “5 Well, how safe is it outside the cage? Why does a cage exist at all? If we look at squirrels, we can see what biologists call “mate guarding,” a male caging a female.
The Idaho ground squirrel (Spermophilus brunneus) lives in populations of one hundred to three hundred individuals in short-grass meadows.6 The squirrels hibernate in burrows most of the year, only becoming active from late March to early August. The males wake up from hibernation about two weeks before the females. Females become sexually active for about three hours in the afternoon on the first day after hibernation. Three hours on this one day is all the family life a squirrel has.
A successful date for a male squirrel means walking behind a female while sniffing and licking her genitals, going with her into a burrow, mating there for five minutes, and then reemerging. As a sign of successful mating, the female acquires a “sperm plug” that can be seen by a human observer and probably by other squirrels as well. The male then stays within one meter of her and keeps her in a small area by “herding.” About every forty minutes, he follows her back into the burrow, where they mate again. She acquires a fresh sperm plug, and he turns around to block the entrance with his body. The male rebuffs an average of four other males who try to mate with the female. If a male is displaced, he is likely to take about an hour and twenty minutes to locate another female, and she is usually guarded by some other male.
A litter typically consists of five pups. If the female has been guarded by only one male for these three hours, then the whole litter is sired by him. If more than one male has guarded the female, paternity analysis shows that her litter is sired mostly by the male who was the last to guard her or by the male who guarded her the longest.
Thus the family life of the Idaho ground squirrel consists of three hours per year. The Idaho ground squirrel family is solely a unit for reproduction. The male doesn’t hang around and help raise the young. The male is said to guard the female to protect his “investment” of sperm by making sure that his female doesn’t mess around. He doesn’t mess around himself because if he stopped guarding he would lose his investment, and in any case finding another female is nearly impossible.
A close relative, Belding’s ground squirrel (Spermophilus beldingi) of the midwestern United States, does not mate-guard. In this species, twenty-five minutes are all a male needs to locate and initiate courtship with another female. Female Belding’s squirrels typically mate with three to five males. The female’s first mate sires most of the litter, followed by the second mate, then the third mate, and so on, in contrast to the Idaho squirrel, where the last mate gets most of the sires. A Belding’s male doesn’t waste time guarding a female because his investment of sperm is secure—as the first mate, he’s already guaranteed siring most of the young. Instead, these males hurry to find more females to mate with, and with so many nearby, why not?
A cage, then, is not biologically universal. These two closely related species of squirrels have completely different power relationships between males and females. Male Idaho ground squirrels guard the females, while 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