Until recently, the only scientifically documented case of interspecies adoption among wild mammals dated to 2006, when primatologist Patrícia Izar spotted a group of capuchin monkeys raising a baby marmoset as one of their own.

Now, a new study published in the journal Ethology offers a second example of the rare phenomenon. As Erica Tennenhouse reports for National Geographic, scientists led by Pamela Carzon of the Groupe d’Étude des Mammifères Marins (GEMM) de Polynésie observed a bottlenose dolphin caring for a young melon-headed whale over a period of more than three years. This apparent adoption, unusual in and of itself, was made all the more striking by the fact that the bottlenose already had a biological baby; typically, dolphin mothers only care for one calf at a time.

The intimate interspecies relationship began when the male calf was roughly one month old and ended when he presumably weaned in April 2018. Interestingly, Carzon and her colleagues note, the dolphin mother’s attachment to her adopted son endured long beyond her bond with the slightly older biological daughter. This bottlenose baby, born around September 2014, lived alongside its mother and whale sibling for a year and half before abruptly vanishing—perhaps indicative of an early death or, on a more positive note, a shift to a different social subgroup.

Per the study, the melon-headed calf actively competed for his mother’s attention, repeatedly pushing his adoptive sister out of her place below the dolphin’s abdomen. Whereas the female bottlenose regularly socialized with peers prior to disappearing, the male rarely left his mother’s side. Still, eager to ingratiate himself into both the family unit and the wider dolphin community, he later adopted bottlenose behaviors such as surfing, jumping and socializing with other young males.

“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,” Carzon says in a video published by GEMM Polynésie.

Female bottlenoses have been known to kidnap calves of other species—likely in an attempt to foster unfulfilled maternal instincts—but such relationships rarely last. Given the fact that the mother in question already had her own biological offspring, it’s unlikely she personally kidnapped the whale calf. Instead, Carzon explains in the video, the dolphin may have adopted the calf after another female kidnapped but lost interest in him.

Kirsty MacLeod, a behavioral ecologist at Sweden’s Lund University who was not involved in the new research, tells National Geographic’s Tennenhouse that the mother, seen nursing the calf on two occassions, appeared to be highly invested in his well-being.

MacLeod adds, “In mammals, synthesizing milk is very costly—it’s a very precious resource.”

The unexpected attachment likely stemmed from a surprisingly straightforward series of events: A persistent, seemingly orphaned calf latched onto a tolerant, curious dolphin whose recent birthing experience had sparked her maternal instincts, and the pair hit it off.

“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,” MacLeod says, “and it led to this slightly wacky situation.”

Rather than rejecting the whale or displaying aggression toward him, the dolphin—already known for tolerating scuba divers in the area—simply adapted to the change in circumstances. The male calf, meanwhile, showed a marked determination to integrate himself within the bottlenose group.

As Carzon concludes, “The young melon-headed whale was certainly the main initiator of this adoption, [but] the mother’s remarkably permissive personality could have played a crucial role in the process.”