Evolution's Rainbow. Joan Roughgarden
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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 divorced more often, presumably because they were not able to raise as many young as the population baseline. Conversely, pairs given extra eggs divorced less often, presumably because they did raise more young. Success at raising a family thus seems to be a factor in whether birds decide to divorce.15
One survey of marriage fidelity among birds shows annual divorce rates as low as 2.4 percent in the barnacle goose (Branta leucopsis) of northern Europe, 2.5 percent in the silvereye (Zosterops lateralis), a berry-eating forest bird from Australia, New Zealand, and Fiji, and 2.7 percent in Cory’s shearwater (Calonectris diomedea) of Long Island to Nova Scotia.16 The highs were 36 percent in the European shag (Phalacrocorax aristotelis), which is similar to a cormorant but more marine, and 30.6 percent in Parus major, the European woodland songbird mentioned above.
Divorce rate correlates with mortality. In birds where the annual survival rate is only 40 to 80 percent, the divorce rate is high, and in birds where the annual survival rate is 90 percent or more, the divorce rate is low.17 Lots of eligible widows and widowers make for a hot singles scene. And when the singles scene is hot, the action doesn’t stay confined to singles. When divorce rates are high, lots of mating also takes place outside of the nest. The data for birds show a positive statistical relation between rates of divorce and EPCs.
Thus monogamy among birds seems to be an economically beneficial social institution, with divorce and some out-of-wedlock matings a regular part of the picture too. Bird females seem to have lots to say about their own lives, choosing partners and initiating divorce when advantageous. When we turn to mammals, though, we have to face the fact that monogamy seems rare. Why?
One explanation for why birds are more often monogamous than mammals is that flight endows female birds with more opportunity to choose their mates than female mammals have.18 Female birds can check out prospective husbands by flying from party to party around town, whereas a female mammal is stuck walking to the nearest block party. With so much choice around her, a female bird can demand a husband who is faithful and helps with the dishes, while a female mammal can’t. However, this theory assumes that a male generally doesn’t want to stick around and help with the young, can’t stand doing dishes, and must be manipulated to do so by a female’s threat of turning to someone else if he doesn’t. I don’t accept this logic. I feel the male’s perspective should be stated differently. He has two directions in which he can invest social effort. Within-sex effort involves competing with other males and/or building coalitions with them to access females. Between-sex effort involves “coalition-building” with a female to raise offspring together. Whether a male winds up with more offspring overall from within-sex or between-sex coalition-building depends on circumstances. This is the animal equivalent of balancing career and family.
Monogamy then emerges when (a) building relationships with a female is more advantageous to a male’s reproductive success than building relationships with other males, and (b) building a relationship with a male is more advantageous to a female’s reproductive success than raising young by herself or in conjunction with other females. In general, different mating systems emerge from different optimal allocations of social effort to between-sex and within-sex relationships.
Although not as commonly as in birds, monogamy among mammals does happen. It occurs in 15 percent of primate species and is common among wild canines, among others. In most monogamous species, the husband contributes to parental care by building a den, burrow, or lodge, defending the family’s feeding territory, feeding his wife when she’s nursing, and carrying the young around (driving the kids to soccer after school). In the monogamous prairie vole, Microtus ochrogaster, when a female produces a bigger-than-normal litter, a second nest is built, the young are divided up, and the male cares for one nest and the female for the other. Thus monogamy occurs among mammals, although not as commonly as in birds.
But why is monogamy rare in mammals? Mammalian females have internalized embryonic development in a uterus or pouch, whereas avian females leave the developing embryos as eggs in the external environment. This difference affects who can control the offspring. A mammalian male who wants to control offspring must somehow control the female herself, whereas an avian male can directly control the eggs in the nest. A mammalian female knows the embryo developing in her body is hers alone, not an egg deposited there by some other female. In birds, a female may derive from a monogamous marriage both male provisioning of the young and male protection of the nest, not only from predators but from “dumpers”—other females who deposit foster eggs in the nest.19 The male gains the female’s initial investment in eggs, plus her additional provisioning. Neither male nor female mammals benefit from marriage as much as birds do.
EXTENDED FAMILIES
Let’s now take a look at two-gender families larger than two individuals: extended families. The groove-billed anis (Crotophaga sulcirostris) is an insectivorous black bird with a large, deeply grooved bill. It lives in marshes and open pastures in Central America and is related to the cuckoo. Family organizations of the anis may consist of twosomes with one female and one male, foursomes with two females and two males, sixsomes, and even eightsomes.20 The foursomes are not sixties-style communes of free love. An anis foursome is two couples cohabiting a one-bedroom flat with one crib. Nests are built in thorny trees or vines. Each male guards one of the females. A female lays an egg every one to two days. The time from egg-laying to fledging is three weeks.
The two females in a foursome