Carl Zimmer in his excellent blog, The Loom:
Scientists have observed adoption in occurring 120 species of mammals. Other species that are harder to study may be adopting, too. As for kangaroos, scientists have long known that if they put a joey in an unrelated female’s pouch, she will sometimes keep it. But King and her colleagues have now discovered that kangaroos will voluntarily adopt joeys in the wild. All told, they found that 11 of the 326 juveniles were adopted over their five-year study–a rate of about three percent. Given the commitment adoption demands from a mammal mother–a kangaroo mother needs a full year to raise a single joey to weaning–this discovery cries out for an explanation.
Over the years, researchers have proposed a number of different explanations for adoption. Some have suggested that mammals adopt young offspring of their relatives because they are genetically similar. By rearing the offspring of their kin, this argument goes, adoptive parents can ensure that some of their own genes get passed down to future generations.
According to another explanation, unrelated adults may adopt each other’s young because this kind of quid-pro-quo benefits everyone involved. And according to a third explanation, young adults adopt orphaned juveniles as a kind of apprenticeship. They learn some important lessons about how to raise young animals, which they can apply later to raising their own offspring.
These explanations share something in common. They all take adoption to have an evolutionary benefit. In the long run, the genes that make animals willing to adopt become more common thanks to natural selection.
But in the case of kangaroos–and perhaps other species, too–evolution may have instead have made a mess of things. Adoption may not be an adaptation. It may be a maladaptation.