The apparent altruism is sometimes actually part of a mutual-aid system in which favors are given because they will almost certainly be repaid. One chimpanzee will groom another, removing parasites from areas the receiver could not reach, because later the roles will be exchanged. Such a system, however, requires that animals be able to recognize one another as individuals, and hence be able to reject those who would accept favors without paying them back. A second kind of altruism is exemplified by the behavior of male sage grouse, which congregate into displaying groups known as leks. Females come to these assemblies to mate, but only a handful of males in the central spots actually sire the next generation. The dozens of other males advertise their virtues vigorously but succeed only in attracting additional females to the favored few in the center. Natural selection has not gone wrong here, however; males move further inward every year, through this celibate and demanding apprenticeship, until they reach the center of the lek. The altruism of honey bees has an entirely genetic explanation. Through a quirk of hymenopteran genetics, males have only one set of chromosomes. Animals normally have two sets, passing on only one when they mate; hence, they share half their genes with any offspring and the offspring have half their genes in common with one another. Because male Hymenoptera have a single set of chromosomes, however, all the daughters have those genes in common. Added to the genes they happen to share that came from their mother, the queen, most workers are three-fourths related to one another-more related than they would be to their own offspring. Genes that favor a selfless sterility that assists in rearing the next generation of sisters, then, should spread faster in the population than those programming the more conventional every-female-for-herself strategy. This system, known as kin selection, is widespread. All it requires is that an animal perform services of little cost to itself but of great benefit to relations. Bees are the ultimate example of altruism because of the extra genetic benefit that their system confers, but kin selection works almost as well in a variety of genetically conventional animals. The male lions that cooperate in taking over another male's pride, for example, are usually brothers, whereas the females in a pride that hunt as a group and share food are a complex collection of sisters, daughters, and aunts. Even human societies may not be immune to the programming of kin selection. Anthropologists consistently report that simple cultures are organized along lines of kinship. Such observations, combined with the recent discovery that human language learning is in part a kind of imprinting-that consonants are innately recognized sign stimuli, for instance-suggest that human behavior may be more of a piece with animal behavior than was hitherto imagined.
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