The Generosity of Peacocks

Hamilton’s inclusive fitness may help explain family life not just in ant colonies but among birds and mammals as well. When Marion Petrie began studying peacocks, she was curious not just about their tails but about their leks. Why, she wondered, did peacocks gather together in groups in order to strut before peahens? The less successful males would invariably be passed over for the males with the most resplendent feathers. Wouldn’t it be better for males to look for females on their own so that they wouldn’t suffer by comparison?

At an English zoo called Whipsnade Park 200 peacocks roam freely over the grounds. In 1991 Petrie took 8 Whipsnade peacocks to a farm more than 100 miles away, where she penned each of them with 4 peahens. She collected their eggs each day and hatched the chicks in an incubator. She put rings on their legs for identification, and then mixed them together with chicks from other pens. A year later, she brought 96 of the young peacocks (12 from each of the 8 fathers) back to Whipsnade.

In a peacock’s fourth year he chooses the spot where he begins to display his tail. In 1997 Petrie watched the peacocks she had returned to Whipsnade as they gathered to form their leks. She checked their rings and looked in her charts to see who their parents were. She was surprised to find that brothers and half brothers stayed much closer together than did unrelated peacocks. A peacock’s nearest neighbor was five times likelier to be related than would be predicted by chance. Somehow, even though they had no knowledge of their parents and had never had a chance to get to know their siblings, they had found one another at Whipsnade.

As a family affair, a peacock lek makes sense. A peacock shares many of the same genes as his brother, so if his brother reproduces successfully, the genes they share get carried forward. For some peacocks, it may be more rewarding to help their brother find a mate than to search for one themselves. If a female mates with any of them, their collective genetic legacy is the winner.

Inclusive fitness may even help explain some of nature’s most intricate soap operas. In Kenya, for example, a bird called the white‑fronted bee‑eater lives in what scientists once thought was a sort of utopian commune. They form giant colonies of up to 300 individuals, their nests consisting of holes in a mud cliff packed in as densely as apartments. The adult birds seemed to early ornithologists to live in peaceful monogamy. When the children were grown they often stayed on to help their parents tend to younger brothers and sisters. Sometimes they even helped their neighbors.

In the 1970s Stephen Emlen began visiting the bee‑eaters to figure out just how altruistic the birds really were. He and his colleagues spent years watching the nearly identical birds flitting among the nests or darting away for food and returning not long afterward. They built up genealogies of families, which they confirmed by testing the DNA of the birds. What they found was that what looked like simple altruism was actually a complicated family intrigue.

Bee‑eaters, Emlen discovered, do not live in simple two‑parent nuclear families. They form big multigenerational clans of up to 17 birds, including parents, grandparents, uncles, aunts, cousins, nieces, and nephews. An extended family occupies groups of neighboring nests, and relatives will spend a lot of time visiting one another. If a predator kills a nest of chicks, a son who was helping to raise them would move into a nearby nest to continue helping there. He is not helping a stranger, but an uncle, perhaps, or a sister. Since these relatives share some of his own genes, it makes sense for him to help them if he cannot help his immediate family. And helping, Emlen and his colleagues discovered, can make a huge difference to the success of a nest. The addition of a helper to a nest can double its productivity.

Bee‑eaters live in a web of family conspiracies. Female bee‑eaters do in fact visit the nests of strangers, but not to help raise chicks; instead, they try to lay their eggs in their nests. If the strangers don’t realize that the egg doesn’t belong to them, they will spend the effort raising the chick, effort that the real mother can use for raising even more chicks in her own nest. During the time after a mother has laid her eggs and before they have hatched, families are perpetually on their guard for these intruders. But their own daughters, Emlen discovered, will try to sneak their own eggs into their mother’s nest. Emlen was surprised that the daughters even had eggs to lay, given that they were still living with their family and hadn’t yet been paired off with a male bee‑eater. But he discovered that daughters sometimes fly miles away from their nests to consort with males of other bee‑eater colonies.

The parents conspire as well. If one of their sons pairs up with a female and tries to set up his own nest, his father pays him so many social visits that he can’t start his own family. The son will then be more likely to come back to his parents’ nest to help raise more siblings. What looks like a Utopia is actually a swirling cauldron of inclusive‑fitness‑raising behaviors.