Evolution's Rainbow. Joan Roughgarden
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encounters from other males. This dominant male fathers about half of the group’s young. Subordinate males take up stations near the base of the tree by the entrance. Other males are out of luck, roosting alone or in small male-only groups rarely visited by females.
The pattern of food-sharing is especially interesting. The food is transferred by one bat regurgitating into the mouth of another. (You wouldn’t want to be a bat, would you?) Most (70 percent) of the food transfers are from a mother to her pup. This food-sharing supplements the mother’s lactation. The other 30 percent involves adult females feeding young other than their own, adult females feeding other adult females, and on rare occasions, adult males feeding offspring.
Some adult females have a “special friendship” with females who are not their kin (males also have same-sex relations; see p. 141). This bond is brought about in part by social grooming. The bats spend 5 percent of each day grooming and licking one another. Some of this grooming is between special friends, and the remaining among kin. A hungry bat grooms one who has recently fed to invite a donation of food. To solicit food, a hungry bat licks a donor on her wing and then licks her lips. The donor may then offer food.
The mutual assistance is significant. If they didn’t help each other, the annual mortality of vampires would be about 80 percent, based on the chance of missing a meal two nights in a row. Instead, the annual mortality is around 25 percent because food-sharing tides bats through their bad nights.
Biologists assume that animal species don’t readily cooperate with each other. If natural selection is the survival of the fittest, shouldn’t natural selection reward selfishness and discourage cooperation? Biologists suggest two forms of cooperation that can evolve by natural selection. The first is cooperation restricted to helping kin, and the second is cooperation restricted to helping special buddies—those who regularly reciprocate the cooperative acts.28 Vampire bats help not only kin as do many species, but also unrelated friends—which is what makes vampires so interesting. This mutual helping, called “reciprocal altruism,” takes place primarily between animals who have lived together and gotten to know one another. Each helps the other at various times, and each instance of helping benefits the recipient much more than it costs the donor.
Critics of the idea of reciprocal altruism have argued that natural selection favors the “cheater” who takes food without reciprocating. If cheaters are evolutionarily more successful than food-sharers, the altruism eventually disappears and all the animals wind up being selfish. The vampires solve the problem of cheaters by developing special friendships through what might be considered same-sex courtship. This involves continual mutual grooming and food solicitation using the bat equivalent of kissing, all of which reinforces the pair-bond and promotes long-term survival.
Other species have different tactics to exclude or retaliate against the selfish.29 For instance, Rhesus macaques who find food sources and don’t give food calls telling everyone else about it are subsequently targets of aggression.
Little is known about whether animals acquire a “reputation” that others use to decide whether to include them in cooperative activities. The idea of reciprocal altruism invites thinking in terms of pairs. Yet in my field studies of lizards, whenever I’ve seen two animals interacting with each other using head bobs, pushups, and color changes, all the other lizards in the vicinity were watching too. Do they remember what they’ve just seen? Probably. The lizards can probably remember who won or lost in a showdown over territory, and they can probably remember who cheated and who reciprocated in an instance of cooperation. Animals may talk about each other as well, indulging in animal gossip.30 Animal interactions, from mating to territorial spats, to grooming and food-sharing, are often done out in the open, so that everyone can see and later discuss what happened. Animals with “nice” reputations may be included in cooperative activities and “meanies” left out. Reputations may provide a way for an animal to know whether another is likely to reciprocate, without having to learn the hard way.
Similarly, little is known about animal “generosity.” A social system effective at excluding cheaters promotes evolution of the desire to share. Generosity depends on society’s promise that what goes around comes around. If vampires someday prove to be among nature’s most generous creatures, future children’s comic books may feature vampires as friends rather than foes.
The gold medal for cooperation between mammals is held by small, almost hairless rodents that live underground in parts of Kenya, Ethiopia, and Somalia. Their subterranean families consist of certain individuals specialized for reproduction and others who routinely groom, feed, and protect the offspring. If this society sounds like a colony of bees, with a reproductive queen surrounded by the workers, you’re right. These mammals, called naked mole-rats (Heterocephalus glaber) because of their exposed smooth skin, are the vertebrate counterparts of the social insects.31
A family of naked mole-rats typically consists of about one hundred individuals. Naked mole-rats are underground all the time. Their only aboveground signs are volcano-shaped mounds about one foot high, created by ejecting loose soil from their burrows. Naked mole-rats make these mounds when excavating tunnels, primarily at dawn and dusk, and during the winter rainy season. To find food, the naked mole-rats dig until they bump into a juicy root. The rats can’t see or smell through the dirt, so finding a root is like a miner striking a vein of gold. Because naked mole-rats are so difficult to study in the field, most observations are based on captive colonies in the lab.
Naked mole-rat families are really close—more than 80 percent of the matings occur between brothers and sisters or between parents and offspring. Typically one female and one to three males do the breeding. The breeding female is aggressively dominant over other females. A breeding female gives birth every two to three months, producing a litter of about ten pups. A female produces thirty to sixty offspring per year. Non-breeding naked mole-rats are not sterile. If a breeding male or female dies or is removed, a nonbreeding mole-rat of the same sex steps up to take his or her place.
Breeding is a demanding occupation. Although the breeding female typically remains the largest and heaviest animal in the extended family during her tenure as chief breeder, the breeding males lose weight after they become breeders, quickly shedding 17 to 30 percent of their weight, and appear emaciated after several years. Meanwhile the breeding female becomes not only heavier but also longer, adding vertebrae to her spinal column.
Who becomes a breeder (either male or female) seems to be determined by conflict among the aspiring females. Upon the death of the female breeder, the would-be successors not only attack one another but also target specific males, shoving and biting them. Of seven fights started by females against males, five fights led to the death of the male.32 The males attacked were either mates of the previous breeding female or were pair-bonded to rival females, as indicated by courtship activities such as frequent anogenital nuzzling.
The nonbreeding males and females provide parental care to the offspring of the breeders. From shortly before the pups are born until they’re weaned, the nonbreeders huddle with the pups in the communal nest to provide a stable thermal environment, a warm nursery. The non-breeders regularly nudge, handle, and groom the young; retrieve pups that fall out of the nest; transport pups when the family moves to a new nest site; and evacuate pups from the nest during a disturbance. The nonbreeders also provide food to the pups in the form of caecotrophies, partly digested fecal pellets. The pups routinely solicit and obtain these morsels of candy from the nonbreeders of both sexes, but not from the breeders themselves. After the pups are fully weaned, they are able to eat food that has not been preprocessed. Nonbreeders also defend, maintain, and extend the family’s system of tunnels. They collect and transport food through the tunnels back to the nest, where they feed other family members, including the breeders.
The distribution of reproductive activity throughout a group is called its reproductive skew. A social group where everyone reproduces has low skew. High skew occurs in the naked mole-rats because only two to four individuals in the entire group of one hundred or more reproduce. The reproductive skew in an animal society is the most fundamental attribute a society has from an evolutionary standpoint—the index of a society’s reproductive equity. Little is known about what determines a society’s reproductive skew to begin with,