Birds do it, so do mammals and fish, but why animals adopt unrelated offspring is an evolutionary puzzle
ONE SUMMER DAY on Sanibel Island, Florida, a female raccoon led her young out of a patch of woods toward a large dumpster parked behind a resort hotel. Following her were three healthy-looking young raccoons and, bringing up the rear, one tabby kitten. Ducking under a fence around the dumpster, this unorthodox family stuck together long after it had joined a couple dozen other cats and raccoons that were feeding on the bounty of edible garbage there.
The raccoon's generosity seems to contradict the basic tenets of Darwinian theory. By adopting an unrelated offspring, she not only risked decreasing her own reproductive success, she invested precious food and energy in aiding someone else's. In evolutionary terms, the animal appeared to be committing an incredible blunder.
But if adoption is simply a mistake, natural selection ought to have weeded it out by now. That hasn't happened. And adoption is not just a quirk among human beings and the occasional eccentric raccoon. From gulls, geese and bats to seals, coyotes and dolphins, all kinds of creatures have been known to take in and raise another animal's young. According to Eva Jablonka, an evolutionary biologist at Tel-Aviv University who describes the behavior in the book Animal Traditions, adoption "is certainly more common than previously thought." She and her coauthor, zoologist Eytan Avital, report that several hundred bird and mammal species at least occasionally adopt. And while in some cases there seem to be practical advantages for the animals involved--a chance to gain parenting experience, for example--"adoption remains a poorly understood behavior," says Jablonka.
Adoption is far more common in chaotic breeding colonies of animals such as seals, bats and gulls than it is among solitary breeders. Studies of elephant seals and Hawaiian monk seals, for example, show that the denser a colony, the more likely a mother and pup will become permanently separated during the ruckus caused by bull seals barging through the group, scuffles between neighboring females or high surf. More separations, in turn, lead to more adoptions.
In one colony of Hawaiian monk seals, Daryl Boness, senior scientist in the Smithsonian Institution's Department of Conservation Biology at the National Zoological Park, found that about 90 percent of mother seals took in foster pups after they became confused and switched offspring by accident. Monk seals seem less adept at recognizing their own young than do other seal species, which use sight, smell and the unique sound of each pup's cry to tell one from the other. While most monk seal mothers care for only one pup at a time, they may cycle through two to four different ones by the time they're done nursing for the season. "The pups that were separated from their true mothers did about as well as those that were not," says Boness. In this case, caring for unrelated offspring "does not affect a female's own reproductive success, in part because of the willingness of most monk seal females to pick up a persistent pup and feed it."
ng northern elephant seals, which breed in crowded, rowdy colonies off the coast of California and Mexico, pups frequently become separated from their mothers--and would die if they were not adopted by another female. Fortunately, such pups are often taken in by first-time mothers who have lost their own young. According to Boness, the mother seals' behavior makes evolutionary sense. Practice raising a stranger's offspring can help them learn how to keep their own pups alive and healthy the following breeding season.
for other colonial breeders, caring for the wrong youngster can have devastating consequences. Gulls and terns, for example, hatch more chicks each season than they can successfully raise to adulthood. Still, the birds sometimes adopt unrelated chicks, even if it means that one or more of their own may starve to death.
in Brown, a biologist at Brock University in Ontario, spent several summers studying a crowded colony of ring-billed gulls on Lake Erie. He believes that adoptions among these birds result from a desperate tactic devised by hungry chicks themselves. If chicks are starving in their own nest, they may sneak into a neighboring nest and try to pass as a member of the family. Sometimes the interlopers pull it off--surviving by taking food out of the mouths of their foster nest-mates.
Photo: ©; ANUP SHAH
ALL IN THE FAMILY: Among lionesses in Africa (above), communal care of young by a group of related individuals--or pride--is the rule. Wolves and coyotes, which live in packs of related animals, also share the rearing of offspring. Each year, only one or two females in a pack will give birth to pups. These mammals' behaviors can be explained by the theory of kin selection, which holds that helping a close relative reproduce is the next best thing to having offspring of one's own.
The ruse may work, says Brown, because "gulls don't begin to recognize their young until five to seven days post hatch, when the chicks become mobile. Until that time they follow simple rules: If the chick is in my nest, it must be mine; if the chick behaves like it belongs to me, it must be mine. These rules work most of the time and lessen the chance of rejecting one of their own, a more costly error than accepting a foreign chick." Still, it's a risky business for the wandering youngster. If adults recognize it as an intruder, they'll attack or even kill the outsider.
Geese, which also breed in colonies, can afford to be more easy-going about letting strangers into the family. Their young are ready to feed themselves as soon as they hatch, and all adults need to do is lead goslings to food and watch over them. This may explain why lesser snow geese and barnacle geese often sneak successfully into a neighbor's nest to lay a few eggs on the sly, leaving their offspring for someone else to nurture. But some mother geese also seem willing to adopt another bird's newly hatched young. Researchers who used DNA fingerprinting to track relationships in families of barnacle geese in Sweden found that 17 percent of the goslings were adopted.
Biologists theorize that by adopting goslings, adult geese may help their own young in a variety of ways. In a larger group, for instance, each gosling's chance of falling prey to a predator is likely to be lower. Later, the adoptees can help their host family defend good foraging sites against other geese. Jablonka and Avital also suggest that geese may adopt in order to set their own offspring up with future mates; geese that couple with individuals they've known since chickhood seem to have more stable relationships than those who mate with strangers. All such possible advantages are difficult to prove, of course. It may be that adoption persists in geese simply because it has little impact one way or another. Research has shown that goslings whose parents raise outsiders survive just as well as those who live only among their genetic brothers and sisters.
Other cases of parental generosity are strictly a family affair. Among several bird and mammal species that live in groups--often with close relatives--caring for one another's offspring is the norm. In packs of wolves or coyotes, for example, only one or two females give birth to pups each year, and adults of both sexes help care for the young. Mothers sometimes nurse another female's pup, and in a few cases have been known to adopt orphans. Lionesses on the African savanna also care for cubs communally, freely nursing each other's offspring. Feral house cats do the same thing in farmyards and suburban woodlots worldwide. And young acorn woodpeckers often spend their first year of adulthood hanging around the home nest, helping to raise younger siblings rather than hatching their own chicks.
Such examples of familial devotion can be explained by an evolutionary theory known as kin selection. The idea is that genes that lead to an altruistic behavior--such as adoption--will spread as long as the behavior significantly enhances the survival and reproduction of the self-sacrificing animal's close kin. After all, an individual is just as related to a full sibling as it would be to its own offspring. In terms of passing on genes, then, caring for a close relative is the next best thing to having young of one's own--or perhaps a better option if an animal's offspring is unlikely to survive.
But even bears, solitary animals that normally have little contact with other adults, have been known to become adoptive parents. To test the possibility that polar bears take in orphans born to a close a relative--a mother or a sister--Nicholas Lunn of the Canadian Wildlife Service analyzed the DNA of mothers and cubs involved in three known cases of adoption. He found no evidence of genetic relatedness between any of the mothers and their adopted cubs.
It turned out that two of the polar bear mothers had swapped sets of twin cubs, and each was raising the other's young. In this case, confusion during a tense encounter with another family of bears is the most likely explanation. In the vastness of the Arctic, polar bears rarely encounter others of their own kind, so they have not needed to evolve a strong system for recognizing their own offspring.
Photo: ©; MICHIO HOSHINO
MOSTLY MISTAKES:In the Arctic, an edgy encounter between two polar bear families can cause so much confusion that mothers accidentally switch their cubs. Because these encounters are relatively rare, the bears have not evolved a foolproof way to recognize their own offspring.
Both grizzly and Alaskan brown bears have been known to enlarge their families by adopting single or twin orphans, although the practice is uncommon. Among grizzlies, adoption happens most often at busy fishing spots where bears congregate to feast on huge salmon runs. The hubbub surrounding such food bonanzas may make it easier for orphans to seek out adoptive moms, and the availability of abundant food might make mothers more likely to accept a stranger. Yet cases of grizzly and brown bear adoptions remain unexplained. Bears do not share childcare with members of a pack like wolves do. Why would they take on extra cubs, and decrease their own offsprings' chances?
Even more difficult to explain is why an animal--such as that Sanibel Island raccoon--would adopt the offspring of a different species. Boness and Brown both suggest that the raccoon may have taken in the kitten by mistake while her own babies were very young. In captivity, dogs with young puppies have been induced to suckle cats, and cats have nursed rats. Mothers in these experiments accepted the alien offspring before their own young were old enough to move around, the point at which powerful recognition systems usually kick in.
Still, it's hard to interpret such adoptions as anything other than reproductive bloopers. According to Jablonka, biologists understand very little about cross-species adoptions in the wild. "There are few reports of this behavior," she says, "and I suspect its occurrence is underestimated."
Sometimes, when the urge to nurture overwhelms, animal parents can end up in bizarre situations. In the mid-1970s, a biologist working in Alaska observed a pair of arctic loons, which had lost their own chicks, raising five spectacled eider ducklings that might otherwise have made a decent lunch. More recently, a lioness in Kenya's Samburu National Park took in a newborn Beisa oryx--normally a prey species--then attempted to adopt a second baby oryx after game wardens took away the first. Despite such occasional parenting blunders, the persistence of adoption suggests that even in the unsentimental calculus of natural selection, it may be better to err on the side of compassion.
Sharon Levy writes about science and nature from northern California, where many of her favorite people are adoptees and their wonderfully well-adapted parents.