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.
A team led by zoologist Charlie Cornwallis and evolutionary biologist Ashleigh Griffin, both of the University of Oxford in the United Kingdom, set out to test this monogamy hypothesis in birds. Analyzing data from the literature on the breeding habits of 267 bird species, the researchers found a strong negative correlation between promiscuity rates—defined as the percentage of broods with offspring fathered by at least one additional male—and cooperative breeding. The promiscuity rate averaged about 12% in cooperatively breeding birds, but the percentage was about 23% in noncooperative birds. And cooperative breeders with higher promiscuity rates had fewer helpers in their nests, the team reports in the 19 August issue of Nature.
Although this correlation is consistent with the monogamy hypothesis, it doesn't prove that monogamy actually fostered cooperative breeding over evolutionary time. To determine whether it did, the researchers analyzed the evolutionary relationships among all 267 bird species to determine the evolutionary direction of cooperative breeding and promiscuity. They found that over millions of years, cooperative breeding had evolved 33 times and been lost 20 times. The evolutionary transitions fit the predictions of the monogamy hypothesis: The ancestors of today's cooperative breeders were less promiscuous than were the forebears of modern noncooperative breeders. The transition to cooperative breeding was more than twice as rapid for less-promiscuous ancestors than it was for more-promiscuous ancestors.
This "very thorough study leaves little doubt that promiscuity corrupts social life in birds," says Boomsma. But ecologist Kathryn Arnold of the University of York in the United Kingdom says the authors have not made their case that promiscuity rates are the primary driving force behind cooperative breeding. Citing her own research, Arnold argues that cooperative breeding is most closely correlated with life history factors such as low adult mortality rates, which in turn lead to greater levels of care for the offspring.