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.

In other words, these kangaroos had been adopted.

View Images A marked kangaroo and her adopted joey. King et al PLOS One 2015 http://bit.ly/1dxRljP

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.

To understand why some kangaroos adopted joeys, King and her colleagues looked for evidence that adoption provides an evolutionary benefit. They found that adoptive mothers didn’t pick out closely related juveniles to adopt. That finding weighs against kinship as an explanation.

King and her colleagues also didn’t find evidence to support the adoption-as-practice explanation. For one thing, only one out of eleven adoptive mothers was a young female that hadn’t yet had joeys of her own. In fact, some of the mothers swapped their babies.

Remarkably, King and her colleagues never observed an orphaned juvenile being adopted. In some cases, a mother adopting one joey would, in the process, abandon her own. These abandoned joeys were not then taken up by another adult female. Instead, they disappeared, presumably killed by foxes or other predators in the park.

All in all, adoption seems like a pretty bad move for mothers and joeys alike. King and her colleagues propose that adoptions happen not because natural selection favors it, but because kangaroos aren’t very good at recognizing their own joeys.

When a joey climbs out of its mother’s pouch and then tries to hop back in, its mother gives it a sniff. If the joey doesn’t smell like it belongs to her, she will push it away. King and her colleagues propose that in an emergency–such as when a predator turns up, prompting joeys to rush into pouches and adults to hop away–mothers may not have time for this inspection. An unrelated joey may leap in their pouch and stay there as the mother flees for safety. Once in the pouch, the joey will take on the same odor as her own offspring had. Now it will pass the sniff test–and become officially adopted.

One observation that King and her colleagues made supports this explanation: adoptions happened more often when the kangaroo population was high. When a mother is surrounded by a big crowd of joeys, there may be more opportunities for an unrelated joey to leap into her pouch.

I asked Kirsty MacLeod, a biologist at the University of Cambridge, what she thought about the new study. She found the evidence compelling, if bizarre. “It’s pretty weird,” she said. “It makes very little evolutionary sense to stop investing in your own young, and instead divert all your resources to another offspring, especially one that isn’t related to you.”

MacLeod thinks King is probably right that kangaroos adopt by accident during a crisis. “But that doesn’t necessarily mean it’s intrinsically nonadaptive,” she added.

Grabbing the closest joey in an emergency may be a good strategy for kangaroo mothers, since the closest joey will probably be her own. “It’s likely better to take that chance (and one out of ten times, face the consequence of raising another female’s young),” said MacLeod, “than to risk separation and offspring death, a catastrophe for a mother that spends over a year rearing a joey.”