Darwinian Family Life

Once an animal is born, it may grow up in a big family or find itself an orphan. Among mayflies, both parents are dead by the time their eggs hatch. Among black bears, the mother takes care of her cubs for a year while the father offers no help at all. Among barn swallows, the father will work just as hard as the mother to bring food to his fledglings until they’re ready to fly. And elephants may live with brothers, sisters, aunts, uncles, and grandmothers in clans that endure for decades.

Raising children can be just as crucial to an animal’s reproductive success as finding a mate. If a male dung beetle has sex with thousands of female dung beetles but all his children die within a week of hatching, all his sexual conquests have been, evolutionarily speaking, for nothing. In many species, mothers and fathers work together to raise children. But their conflict of interest can threaten their family bond. Males who spend their time raising another male’s babies are less likely to pass on their own genes. As a result, males in some species have gotten wise to their cheating partners. Andrew Dixon at the University of Leicester observed how much care reed buntings took in feeding and protecting their young. When he sampled the DNA of the birds to see which were actually related, Dixon found that when fewer fledglings in a nest belonged to the father, he put less effort into bringing food to the nest.

But in many species, ranging from mice to langur monkeys to dolphins, males do not simply neglect young animals that don’t belong to them. Sometimes they turn homicidal. This disturbing behavior has been particularly well studied in lions. A pride of lions consists of a dozen or more lionesses and up to four males, along with their cubs. When the male cubs grow to maturity, they are driven away by the older males. Together with other outcasts, they search for another pride where the males seem weak. They fight with the residents, and if the residents run away, they take over the pride. Cubs are in grave danger at that moment: the new males are likely to take them in their jaws and crush them to death. Of all the lion cubs who die in their first year, adult male lions kill one out of four.

What looks like cruelty to us humans, a number of zoologists argue, is evolution’s logic at work once again. A male lion’s ultimate goal in taking over a harem is to father cubs of his own. Nursing prevents a lioness from coming into estrus, so the presence of cubs in a pride means a male lion may have to wait for months before mating with their mothers. Given that a male lion may be overthrown in a couple years–and his own cubs possibly killed if they are too young at the time–he has no time to waste as a stepfather.

Lionesses do what they can to keep their cubs alive. The roar of alien males will make them stand up, snarl, and gather together to put up a fight. The more lionesses defending a cub, the better its chances of surviving. That may be why lionesses form prides to begin with, as a way to fight against infanticide.

But lionesses are not always able to defend their cubs, and when new males take over they try to start families again. After a male lion has killed a lioness’s cubs she comes into estrus and soon becomes sexually insatiable. The dominant male lion of the pride will mount a lioness in estrus nearly a hundred times a day, and after a day or two he will become exhausted. Now the lower‑ranking male lions step in and mate with her for a few days more. When she gives birth four months later, the paternity of her cubs is a mystery. That uncertainty may explain why male lions don’t kill cubs in their own pride. There’s always a chance that the cub they kill is their own–a disaster for their own evolutionary legacy.

When the first reports of infanticide in animals emerged in the 1970s, many researchers were skeptical. Surely, they said, these brutal males must be pathological. Besides, how could they possibly know which infants were their offspring and which weren’t? But more cases kept pouring in. Stephen Emlen, a Cornell University ornithologist, found a particularly elegant way to test the infanticide hypothesis. In 1987 he was studying jacanas, a Panamanian bird that is, like the pipefish, a species in which the usual sex roles are switched. Male jacanas sit on eggs and raise the young, while the females rove around their territories, mating with many males and fighting off intruding females. Sometimes the attacker may drive away the resident female jacana and take over her males.

If a male lion taking over a new pride should benefit by killing cubs, Emlen reasoned, then a female jacana taking over a harem of males would benefit by killing their chicks. Emlen needed to shoot some jacanas to get their DNA, and he decided to choose two females whose male partners were caring for nests of babies. He shot one of the females one night, and by the next morning a new female was on its territory, pecking at the chicks and hurling them to the ground until they were dead. The male jacana looked on helplessly. Within hours she was soliciting him to mate, and he mounted her. The next night Emlen shot a second female, and the following morning the same violence played itself out again.

“If we’re thinking about individuals leaving genes to the next generation,” says Emlen, “this makes sense, despite its grisliness.”