Evolution's Rainbow. Joan Roughgarden

Чтение книги онлайн.

СКАЧАТЬ 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, but once in place, the skew sets a baseline for how each individual in the society structures a life plan for reproductive success.

      Cliff swallows (Hirundo pyrrhonota) are perhaps our closest cousins when it comes to life in the big city.³³ Swallows live as monogamous pairs in colonies. Their nests, which look like small pitchers, are arranged side by side. Up to five thousand birds form what amounts to a city of mud huts. Not all swallows live in the big city, though. Some live in small villages of twenty or so nests, and some males hang out outside of town without any nests.

      Cliff swallow life includes many features of our own city life—a hot real estate market, trespassing, robbery, hanky-panky with the neighbors, plus presumably some compensations. Nonetheless, most observers of cliff swallow life emphasize the problems, perhaps because the birds live in the countryside, where the virtues of city living are underappreciated. The nests are packed in so densely that occasionally a bird is trapped inside and dies when a neighbor’s construction project blocks its entrance hole. Even worse, the droppings from a nest above may clog up and bury a nest below—swallows seriously need expertise in civil engineering. Swallows also have a major public health crisis. The colony density promotes the growth of bugs that harm the chicks and adults.

      Birds occasionally “trespass” into one another’s nests by either barging in on the owner unannounced or following the owner in before he or she can turn around and block the entrance. Seventy-five percent of the trespassers are males. Of nest entries considered “successful,” 14 percent of the time, the trespasser stole grass used to line the nest; 9 percent of the time, a male trespasser forced himself on his neighbor’s wife; 7 percent of the time, the trespasser stole some still-wet mud before it had dried; 3 percent of the time, a female laid an egg, or transferred an egg, into her neighbor’s nest; 1 percent of the time, the trespasser tossed one of the neighbor’s eggs or chicks out the window; and in 0.3 percent of the cases, the trespasser evicted the owner. Enough intrigue for a new TV drama, “Cliff СКАЧАТЬ