When you spend six years watching kangaroos, you start to see some strange things. From 2008 to 2013, Wendy King, a doctoral student at the University of Queensland, and her colleagues studied wild grey kangaroos in a national park in Victoria, Australia.
All told, King and her colleagues studied 615 animals–194 adult females, and 326 juveniles, known as joeys. The first time King and her colleagues captured each kangaroo, they took a number of measurements and then marked it so they could recognize it later.
From time to time, they’d find a juvenile kangaroo in the pouch of a different mother. Sometimes it would climb out, but then it would climb back into the new pouch, getting milk and protection from the adult female for months, until it was ready to live on its own.
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.