Sandhill Crane with adopted Canada Goose gosling. Mark Graf / Alamy
Animal adoptions make no evolutionary sense, so why do they happen?
Isabelle Catherine Winder, Vivien Shaw, Bangor University
April 28, 2021 5.14pm BST
Scientists used to think that humans are special because we have larger brains than other animals. However, some experts in human evolution have suggested that it isn’t how we think that makes the difference, but how we feel. They suggest that humans may have evolved to be kinder to one another, or at least to suppress their tendency to lash out in anger.
If so, true generosity might be a uniquely human trait. Generosity is usually defined in evolutionary terms as altruism. This is when an individual acts in a way that costs valuable resources or time, with no expectation of repayment.
There are occasions, however, when our animal kin surprise us. For instance, scientists working with gorillas in Rwanada recently found the gorillas band together to take care of orphans. In these cases, young peers and (surprisingly) dominant adult males can be key to immature orphans’ survival. Perhaps it really does take a village to raise a child.
Meanwhile researchers in DR Congo found that bonobos (apes closely related to chimpanzees) go even further, and sometimes adopt babies from a different social group. We even have examples of cross-species adoption, such as the dolphin who adopted and nursed a melon-headed whale, and a group of capuchin monkeys who raised a marmoset.
Bonobos can sometimes adopt babies from entirely different social groups. Nature Picture Library / Alamy
Adoptions like these puzzle biologists. From an evolutionary perspective, what matters is how many copies of your genes make it into the next generation. Adopting a niece or nephew might therefore make sense. A biological sibling shares on average 50% of your genetic material, and their offspring inherits about half of that (25%).
If the cost of raising your niece is less than the cost of having another baby, you may ensure more copies of your genes survive by helping your niece. The idea that an animal might be altruistic (helpful) to members of their family to increase their own evolutionary success is called kin selection.
Altruism beyond kinship
Adopting an unrelated child, however, can’t be explained by kin selection. It’s a choice that incurs significant cost, but does not result in more copies of the adoptive parent’s genes being passed on.
Until quite recently, apparently altruistic acts were explained in various ways. First, researchers noted that many adoptions between different species happened among captive animals kept in homes, farms, or zoos. Animals living closely together might think they are part of the same family group even if they are in fact of different species.
Another idea is that we breed our domestic animals to be docile and caring. Perhaps they are simply more likely to help because of our influence?
Finally, living in captivity means you never have to worry about where your next meal comes from, so captive animals might just be better placed to be generous. Either way, cute stories of cats adopting ducklings (or ugly ducklings turning out to be swans), are not going to convince sceptics that animals genuinely behave selflessly.
In some wild animals, the answer may be that adoptive parents are biologically programmed to care for young animals, and are not very good at telling babies apart. This has been called “misplaced reproductive function” – in other words, a mistake.
Among elephant seals, who breed in huge colonies, mothers and pups often get separated and adoption seems to be common. Adoption might offer a young female the chance to practice rearing young, or to replace her lost pup.
Elephant seal pups can get lost in the crowd. Iryna Savina / shutterstock
We might assume that more intelligent creatures like monkeys, apes and dolphins are able to recognise their young. In some of these however, any male in the group can mate with a fertile female. For example, chimpanzees, bonobos and some gorilla troops have more than one male. This makes it unlikely that they will know which infants they sired.
Dominant males sometimes try to “guard” the female from competitors to ensure that she mates exclusively with him. In these cases, individuals only know their maternal kin: mother, siblings and perhaps aunts and cousins. Within a social group, a male has no way to know which babies might be his, so will generally be tolerant – but he might commit infanticide when he takes over a new group and knows they definitely are not his children.
Infanticide by a newly dominant male can also be explained by the fact that milk production acts as a natural contraceptive. So when a female stop nursing, she becomes fertile again. This makes her available to bear the new dominant male’s offspring much sooner than if she were to wait until her existing baby is weaned.
Humans are not so unique after all
We would argue, however, that the adoptions described above by gorillas, bonobos and dolphins don’t fit any of these models. Perhaps humans are not unique in their capacity for generosity after all?
After all, animal behaviour specialists have suggested that spontaneous generosity is actually well-known and it is well documented among other large-brained, highly-social animals, such as apes, dolphins and elephants. In addition to adoptions, chimpanzees routinely comfort the loser of a fight. They are also occasionally known to commit heroic acts of selflessness, like dying to rescue an infant from drowning.
Attempts have been made to look for “hidden” relationships between helper and recipient, to make altruism “fit” with evolutionary selfishness. Perhaps instead we may just have to accept that humans are not unique in their capacity to care for and help each other.
