Why cooperate when you can be selfish? Many animal behaviors are self-centered and apparently evolved to pass on an individual's genes to future generations. Yet cooperative breeding, in which some members of a group help others to raise their young, has evolved independently many times, especially in birds and insects. A new study of birds concludes that parents get more help when they are sexually faithful to each other. Cooperation has been called an evolutionary paradox, and cooperative breeding is relatively rare, with members of only 3% to 10% of bird species helping to raise one another's young. Among the apes, only humans are cooperative breeders, although monkeys such as marmosets and tamarins do it, too.
In the 1960s, British biologist William Hamilton proposed that natural selection could favor cooperation if individuals pass on their own genes by helping relatives raise offspring. But Hamilton argued that cooperation can arise only if such helpers are closely related to recipients and if the benefits outweigh the costs. Over the past few years, Jacobus Boomsma, an evolutionary biologist at the University of Copenhagen, has argued that strict monogamous behavior, such as an ant queen mating for life, spurred the evolution of cooperative breeding in some social insects. Monogamy helps fulfill Hamilton's conditions, because all siblings are equally related to each other and to each parent. Promiscuity, on the other hand, leads to many half-siblings and lowers the relatedness of individuals in a group.