Interspecies adoption isn’t really a thing in the animal kingdom. The only creatures who routinely take in babies of other species are, well, humans. But recently, a real case of interspecies adoption was observed in the wild: a bottlenose dolphin who adopted a whale calf.

Scientists in French Polynesia observed a wild bottlenose dolphin caring for a melon-headed whale for a period of over three years. The dolphin first adopted the calf when he was about one month old.

The story is incredibly unique. Bottlenose dolphins often “kidnap” babies of other species, but those relationships don’t usually last anywhere near this long. Also, bottlenose females typically only kidnap when they don’t have a baby of their own.

In this case, the mom already had a biological daughter. Dolphin moms usually only care for one baby at a time, but this one went out of her way to raise the whale calf alongside her own.

“It is very difficult to explain such behavior, especially since we have no information on how the melon-headed whale newborn was separated from his natural mother,” lead scientist Pamela Carzon explained in a video.

The whale calf stuck to his mom’s side like glue, often vying for her attention and pushing his adoptive sister out of the way. He adopted many typical bottlenose dolphin behaviors, like surfing and socializing with other young males.

The researchers believe that the whale calf likely came along when the dolphin had recently given birth, and her maternal instincts took over.

“Most likely, it was just a perfect moment for this calf to come along, when [the mother] was at a very receptive period to forming those bonds with her own offspring,” behavioral ecologist Kirsty MacLeod told Smithsonian Mag. “And it led to this slightly wacky situation.”