Evolution's Rainbow. Joan Roughgarden
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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 one female and one male aren’t enough to raise the twins; three adults seem to be the minimum to do the job. Even the families consisting of one adult male and one adult female are accompanied by older children who help out. This family organization is called cooperative polyandry.
Other mammals with cooperative polyandry include African hunting dogs (Lycaon pictus) and dwarf mongooses.22 Cooperative polyandry also occurs in birds, including the Australian white-browed scrubwren (Sericornis frontalis) that lives near Canberra, the Tasmanian native hen (Tribonyx mortierii), the Galápagos hawk (Buteo galapagoensis), the English dunnock (Prunella modularis), the New Zealand pukeko (Porphyrio porphyrio), and the Venezuelan striped-back wren.23 No biological reason prevents these guys from cooperating with each other and helping around the house.
Lions, in contrast, seem to take the idea of a war between the sexes very seriously. We may be misled by the common picture of lions as cooperative hunters. Although lions seem to work together, not only at hunting but also in rearing cubs and roaring in a unified chorus, the truth makes human political infighting seem benign.24 Lion family organization is multimale polygyny—gangs of males guarding groups of females, called prides. Males form lifelong alliances among one to eight lions. Most members of an alliance are brothers or cousins, but others are unrelated. Once mature, these coalitions take charge of a pride of females and father all the offspring born in the pride for a period of two to three years. After that, a rival coalition moves in and evicts them. The males work together more effectively when battling with a rival gang than in any other situation. How well a male does depends on how well his coalition does—gang warfare in the extreme. A male lion is nowhere if not in the right gang.
Victorious lions don’t make ideal boyfriends. A cub matures in two years, and a female isn’t interested in mating during that time. However, if a cub dies, a female mates again in as little as two days. In their hurry to become fathers, an invading gang of males may kill off more than one quarter of the cubs, which quickly brings their mothers into reproductive condition. The invading males don’t share fatherhood equally. One or two father almost the entire pride’s litter.
To counter the male danger to the cubs, the females band together to raise the young. The females enjoy a reproductive life span of about ten years, during which five gangs may come and go. The females give birth in secrecy and keep their litters hidden in a riverbed or rocky outcrop until the cubs can move on their own. Then they bring the cubs to a place where they are nursed together in what is called a “crèche,” a word meaning a public nursery for infants of working women.
The lionesses nurse their own cubs and those of others as well. This shared lactation is not entirely altruistic. The lioness gives milk primarily to her own cubs and rejects the advances of others. She needs her sleep, though, and while she is asleep, cubs who are not her children are able to nurse. Although a lioness prefers her own cubs, the strength of this preference depends on how closely related the other cubs are to her. If a pride consists mostly of close relatives, a lioness is more generous with cubs who are not her own than if she is in a pride of comparatively unrelated females.25 In sum, female lions raise their young in crèches to defend against infanticidal males rather than to provide nutritional benefits from shared nursing. Ironically, house mice have the same family dynamics as lions,26 as though a lion were no more than a mouse that roars.
THE BIG CITY
Some animal species live in what might be called cities. In these cases, family life shows many of the sophisticated intricacies of human urban living. Consider the vampire bat. Your first impulse may be to shudder at the terrifying vision of vampire bats, in the dead of night, swooping down to drink the blood of an unsuspecting victim. Yet vampires have a wonderful story to tell of social cooperation.27
The vampire bat (Desmodus rotundus) is a rather small bat, not much bigger than a plum. A vampire can hang by its feet from the hair of a horse’s mane and bite the horse’s neck. It doesn’t suck the blood; instead, it removes a small patch of flesh with its razor-sharp incisors and laps up the blood flowing from the wound. A vampire’s saliva has an anticoagulant to keep the blood from clotting. After one bat has drunk its fill, another continues at the same spot. Horses can dislodge feeding bats by tossing their heads, swishing their tails, or rubbing against trees.
Life as a vampire is hard. Bats are warm-blooded and, without feathers or fur, lose lots of heat. Their requirements for energy are huge. A vampire bat consumes 50 to 100 percent of its weight in each meal. Yet up to one-third of the bats may not obtain a meal on any given night. Going without a meal is dangerous. A vampire dies after sixty hours without food because by then its weight has dropped 25 percent, and it can no longer maintain its critical body temperature. To survive, vampire bats have developed an elaborate buddy system for sharing meals. The sharing takes place between mother and pup, as well as between adults.
One study of vampires on a ranch in Costa Rica focused on a population divided into three groups of a dozen females. The members of a group often stay together for a long time, twelve years in some cases, and get to know one another very well. The group of a dozen adult bats is a family unit from a vampire’s standpoint. Most of the group consists of females, each of whom usually cares for one pup. A female pup stays in the group as she matures, whereas a male pup leaves. The females in a group span several generations. Group membership is not entirely static, however. A new female joins the group every two years, so at any time the females in the group belong to several lineages, called matrilines.
The bats live in the hollows of trees. Imagine a hollow tree with an opening at its base and a long vertical chamber reaching up into the tree trunk. The females congregate at the top of the chamber. About three males hang out, so to speak, in the tree hollow. One male assumes a position near the top of the