Many animals will adopt orphaned infants of their own kind (or even, in rare cases, of other species) but their motivations are still up for debate. Is a dog that adopts a squirrel evidence of deep altruism in the animal kingdom, or just evidence of a very confused dog? To answer that question, we need to tackle the slightly-less-confusing topic of same-species adoption.
A new study in PLoS One reports on the adoption behaviors of chimpanzees in the wild. Previous work has been difficult, because infant mortality rates tend to be very high in chimpanzee communities. With so many of the children dying, it was difficult to get a good sample of adopted orphans to observe.
But over 20 years of observation of the Sonso chimpanzee community in Uganda allowed researchers to gain some insight into the practice. The majority of adoptions, they reported, occurred within families.
Chimpanzee fathers don't actively raise their young, so a mother's death makes a chimp an orphan. But if they had older maternal siblings, the researchers found, those siblings would almost certainly step in to raise the helpless young ones - even if they were still years away from adulthood themselves.
Non-sibling adoption only seemed to occur when no siblings were present, and even then it took much longer. While orphans with siblings would be cared for immediately by brothers and sisters, an orphan on her own would wait weeks or even months to be adopted by an adult chimp in the community.
Why the wait? Young chimps form strong social bonds with their mothers and maternal siblings, but not with their fathers or the rest of the community. It's possible that adoptive parents can only be persuaded to care for an orphan once the child has succeeded in bonding with them. Unfortunately, many orphans die quickly, and ones who are never adopted rarely make it to adulthood.
Scientists still aren't sure why adoption takes place in the animal kingdom. Adopted orphans are much more likely to survive, but adopting them can be burdensome for the new parent. It could be that adoptive parents are fulfilling a biological urge to help their species survive, especially if the orphans they adopt are closely related to them. In some cases, females may be physiologically primed to care for a baby and unable to give birth to one.
Alternatively, adoptive parents may see orphans as potential future allies - especially if the child belongs to a stronger species. That's one explanation that scientists provided for a deformed bottlenose dolphin, who could have been seen as protection against killer whales by the sperm whales that seemed to adopt it. But scientists have also observed much smaller, weaker species being adopted. In that case, it's possible that the orphans were able to tag along because (compared to their adoptive siblings) they just weren't enough trouble to cause a stir.