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 wind up with four eggs total. A female in a twosome can also produce four eggs by herself. Thus the number of eggs laid per female is lower in a foursome than in a twosome. In a foursome, the females start laying eggs at different times. The starter lays bigger eggs and has a longer time between successive eggs than the follower. Both females stop laying eggs at about the same time. Each female “tosses” out of the nest some of the eggs already laid by the other, with the follower winding up mothering on average 63 percent of the eggs and the starter only 37 percent of the eggs. After all the tossing is over, four eggs are left in the nest.
Even though the follower mothers more of the eggs, she doesn’t necessarily successfully raise the most offspring. The starter lays larger eggs, which hatch earlier, so these chicks have a better chance of surviving than chicks from the follower. All four birds in a foursome work together to provide for and protect the eggs. The males divide the day into unequal shifts. The oldest male incubates at night and most of the daylight hours. As a result, he also incurs the most hazard and the highest death rate. Yet he also fathers the most young.
But why should a female, who can lay four eggs in a twosome without worrying about a nest mate tossing her eggs out, bother living in a foursome? The answer is that the larger group provides protection against egg predators. Once loss of eggs to predators is taken into account, the starter in a foursome produces the most young, a female in a twosome produces an intermediate number of young, and the follower in a foursome the fewest. For the anis, the benefit from predator protection of living in extended families of two couples outweighs the disadvantages of a rancorous life at home.
The family life of tamarins offers a pleasant contrast to that of anis. The saddle-backed tamarin is a tiny monkey that lives in the tropical rainforest of southeastern Peru, including the Manu National Park.21 Among tamarin families, 22 percent consist of one female with one male in a monogamous relationship, 61 percent of one female with multiple males, 14 percent of multiple females with multiple males, and 3 percent of males only. In the families with one female and multiple males, the female mates with all the males. The matings take place in view of the other males without any sign of aggression. The males in this species not only help take care of the young, but they cooperate with one another when doing so. The females usually give birth to twins, and the males carry the babies with them through the treetops. The males and females give the babies fruits and large insects to eat.
The twin babies are 20 percent of their mother’s weight, and are 50 percent of her weight by the time they can walk and climb on their own. Just
