In the animal kingdom, parents adopt babies that aren’t their own, and even other species, says Jason G Goldman. Why do they do it?

Is adopting a child a benevolent or a foolish act? If you were taking a cold evolutionary perspective, it would appear to be the latter.

For foster parents, there are huge costs involved, with no promise of passing on genes. Scientists have long been interested in adoption, because it seems to be wholly altruistic. But this makes it especially perplexing in animals, who do not have the cultural influences we do. So could taking a closer look at adoption in non-humans shed any light on why it’s so common?

One of the more striking places to see adoption in the animal kingdom is Ano Nuevo Island, rising from the sea less than one kilometre off the rocky California coast. Once a year, it is host to the breeding of hundreds of northern elephant seals.

From 1976 onwards, marine scientist Marianne Riedman, together with her colleague Burney Le Boeuf, studied adoption among the seals – and why it was happening. It's a crowded beach, with bad weather, high tides and rough surf, which perhaps explains why one-quarter to two-thirds of pups each year were separated from their mothers at least once – some permanently.

The researchers counted a total of 572 orphaned pups over the course of the four consecutive breeding seasons. Riedman charmingly referred to the ones that were adopted as "alien young".

Intriguingly, some adult seals were more likely than others to become foster parents. For one thing, all the foster parents were female. That's perhaps unsurprising, since part of what drives orphans to seek out care is the need to nurse. Yet among females, the most common foster seal was a mother who had lost her own pup. Why might this be? One possible reason is that fostering helped these females reproduce later on. Regular nursing may induce ovulation, which in turn could make a female more likely to give birth to her own pup the following season. The evidence supporting this explanation is tenuous, but the hypothesis is at least reasonable.

Another possibility is that mothers are behaviourally and physiologically prepared to care for their pups immediately following birth. Given the absence of their own young, the motivation towards maternal care may be so great that they redirect their attention onto other, unrelated pups. Biologist George C Williams called this phenomenon "misplaced reproductive function”.

One other common form of adoption occurred when a female who had never given birth still cared for an unrelated infant. Riedman speculated that those females might gain valuable maternal experience, increasing their own parenting competence. So perhaps there is something in it for foster parents after all.

Elephant seal pups aren't the only ones to win adoptive parents either. It's actually fairly common among birds. Many chicks intentionally abandon the nests in which they hatched to seek out temporary or full adoption by foster parents. This "nest switching" has been seen in seabirds like gulls and terns, as well as storks, raptors, egrets and herons.

Take white storks. In one breeding season, biologists from Spain's Universidad de Cordoba found nest switching in 40% of broods across three distinct white-stork breeding colonies. That infant birds seek out new digs actually makes sense, since they could benefit from a longer period of parental care. If they moved into nests containing fewer or younger chicks than their previous homes, then they could also receive more food by more easily outcompeting smaller adoptive siblings.

But why would the adoptive parents allow the intruder into their nest, especially to the detriment of their own young? It could simply be that evolution has not endowed the parents with the ability to discriminate their own chicks from strangers.

This lack of discriminatory ability is seen particularly starkly in Lake Erie's ring-billed gulls. Nest invasions are common, but unlike for white storks, there is a significant cost incurred by the foster parents: half as many of their own chicks grow to fledging age than the gulls that did not adopt.

Given such high risks for adoption, why hasn't evolution endowed these birds with a better ability to identify – and reject – intruders? Biologist Kevin Brown of York University thinks that the costs of better chick discrimination could be even higher. "If the cost of rejecting one’s own offspring is greater than accepting an alien chick," he writes in the journal Animal Behaviour, "selection will favour universal acceptance of young in the nest." In other words, it may be better to needlessly waste resources on alien infiltrators than to accidentally reject one of your own hatchlings.

Adoption is also fairly common among non-human primates. It's been seen in red howler monkeys in Venezuela, black-fronted titi monkeys and woolly spider monkeys in Brazil, and even among chimpanzees. But in all of those cases the adoption has always been within species. Storks adopt stork chicks, howler monkeys adopt infant howlers.

In captivity, there are occasionally cross-species adoptions, such as between rhesus and Japanese macaques or among different types of marmoset. But these species are closely related, and have similar behaviour.

Given that, perhaps the most striking instance of adoption in the wild was between completely different types of monkeys. In 2004, researchers noticed an infant marmoset travelling with a group of capuchin monkeys at the Green Wing Valley wildlife reserve in Brazil. For at least 14 months, the marmoset was raised by its adoptive capuchin group, alternating between two primary female foster mothers.

One reason the adoption was so surprising is because marmosets and capuchins are so different. For one thing, a fully-grown capuchin weighs perhaps 3-4kg (7-8lbs), but a fully-grown marmoset maxes out at less than 500g (1lb). The two species also have different feeding patterns and different parenting styles.

Despite those differences, the juvenile marmoset became wholly integrated into its adoptive social group. "It travelled and fed with the group, responded to alarm vocalisations given by members of the group, and played," the researchers wrote in the American Journal of Primatology. During social play with their unlikely friend, the juvenile capuchins actually adjusted the force of their movements to account for the puny marmoset's size and strength. And the adult capuchins, including the dominant male, were extremely tolerant of the impostor. The marmoset would patiently watch the adults crack nuts between two rocks and sneak an occasional snack, much as a young capuchin would.

But he (or she) wasn't simply a marmoset in a capuchin suit. Capuchins travel by leaping from tree to tree. Given its size, the marmoset often struggled to keep up. The capuchins all but ignored the marmoset's distress cries, despite being within hearing distance. Like many human adoptions, the match wasn't always perfect.

How did it happen? It's likely that the adult capuchins were simply predisposed to care for young primates. They're also extremely tolerant of infants in the first place. In addition, given how small the marmoset is compared to the capuchins, they didn't need to give up too much of their own food so that it might survive. A female capuchin would barely notice a tiny marmoset clinging to her fur, making it reasonable to assume that the infant didn't slow her down at all.

It seems as if the drive to care for helpless infants is fairly universal among species that care for their own young – and even between different animals. What else could explain our own species' obsession with puppies, kittens and other baby animals?

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