The human tendency of generalized kindly feelings toward babies is less natural to other animals, who as often as not regard the young of others, and even sometimes their own, as food. Parents knowingly caring for the offspring of others is out of the ordinary, especially toward genetic strangers. But it does happen.
Some birds are so stimulated by the calls of chicks in another nest that they respond by feeding the hungry others. A walrus may adopt an orphaned calf. A female monkey may care for another’s baby. Several instances of chimp adoption have been recorded.
A pack of African wild dog males adopted 9 5-week-old pups when the pups’ mother died. The adult males went to the pups’ den each day, delivering food; caring for the younglings until they were able to join the pack on hunting trips. Food sharing is common among these dogs.
Female burrower bugs lay their nest of eggs near others. Mothers tend to developing eggs before they hatch, then feed their offspring nutlets from mint plants. Finding mint nutlets is a competitive business. If a young bug decides mom is not provisioning to satisfaction, it may join a better-fed brood.
Colonial, ground-nesting gulls occasionally adopted unrelated chicks. Chicks may abandon poor parents who fail to provide enough food and take refuge in the nest of a neighbor. That desperate tactic works less than 10% of the time, but a lucky few improve their prospects in life.
The situation is different with goslings. There are widespread adoptions in the 1st week of a gosling’s life. The chicks themselves decide to throw in their lot with parents of higher social rank.
The chick decides when to leave the family, and which family it wants to join. ~ Dutch ornithologist Jan Komdeur
A chick’s freedom to switch parents is abetted by the fact that geese find it hard to distinguish their own goslings before they are ~9 days old. This is an adaptive inability. Young goslings are easy pickings for Arctic foxes and gulls. The grief of caring parents from such loss is lessened by adoption. And larger families fare better than small ones.
While goslings may choose new parents, other younglings have little say. Faced with chronic predation, convict cichlids like their families large. So, cichlid parents kidnap little ones to act as a living shield for their own offspring.
Cichlid parents are always actively searching around, and if they see babies, and they don’t eat them, they will lure them into their brood. ~ American ichthyologist Brian Wisenden
Cichlid adoptees tend to be the smallest of the brood, and the least likely to survive. Predators choose the easy prey.
Among tessellated darter fish, males tend the young on their own. A prospective father sets up his territory: cleaning the underside of a rock, waiting for a female to lay her eggs there, and then fertilizing them.
At some point, the father moves on; whereupon a smaller, unrelated male typically takes over the territory – picking up where the other male left off: keeping the rock clean, aerating the eggs, and defending them. Females prefer a doting dad. So, despite his size, an adoptive dad may get lucky and fertilize some eggs of his own.
Male hamadryas baboons willingly adopt juvenile females. Such adoption is far from selfless, as it helps in accumulating a harem. But it is caring, and for a goal far removed in time.