Bottlenose dolphins are doting mothers, nursing, protecting, and playing with their youngsters for up to six years.

Now, new research has revealed the first known case of a wild bottlenose mom adopting a calf of another species.

In 2014, researchers spotted a bottlenose mother caring for an unusual-looking male calf, along with what was presumed to be her biological calf, in coastal waters off French Polynesia.

While bottlenose dolphins have slender beaks, the mysterious one-month-old’s beak was short and blunt. Eventually, the scientists identified the orphan as a melon-headed whale—an entirely different species and genus of dolphin.

“We were really excited to be able to witness such a rare phenomenon,” says study lead author Pamela Carzon, scientific leader of the Groupe d’Étude des Mammifères Marins (GEMM) de Polynésie, based in Tiputa, French Polynesia.

Adoption is uncommon among wild mammals, with most occurring between related members of the same species. The only other scientifically documented case involving an adopted orphan of a different species and genus was in 2006, when University of São Paulo primatologist Patrícia Izar observed a group of capuchins caring for a baby marmoset. “At the time, we were really, really astonished,” she says.

Scientists are intrigued by adoption in the animal world because it was once considered a uniquely human behavior, adds Izar, who was not involved in the study. (Read why adoptions are more common among domestic animals.)

Sibling rivalry

Carzon and her team filmed and photographed the dual-species dolphin family from land, boat, and underwater as part of a long-term study of this community of around 30 bottlenose dolphins that began in 2009.

Although the mother already had a baby, once the lone melon-headed calf entered the picture, he rarely left his new mom’s side. The trio were frequently seen swimming together—an unusual sight, as dolphin mothers normally care for a single infant at a time.

It wasn’t always familial bliss—the melon-headed calf would repeatedly shove his adoptive “sister” out from her place under their mother’s abdomen.

The ever-persistent orphan was not only intent on integrating himself into the family unit; he also figured out how to fit into the broader group of dolphins.

“The melon-headed whale was behaving exactly the same way as bottlenose dolphins,” says Carzon, who reported the observations in June in the journal Ethology.

For instance, he regularly socialized with other youngsters and would even join in on their favorite pastime of surfing and leaping into the waves. (Read about a deformed adult dolphin that was accepted into a new family.)

Dedicated mom

Female bottlenose dolphins have been known to “steal” babies of other species for brief periods during conflicts, but the adoptee’s enthusiasm and the mom’s dedication show this was no kidnapping.

In this case, the mother made an enormous time commitment to the orphan—the two were spotted together for nearly three years, disappearing around April 2018, the time he would have weaned. Their union continued long after her biological calf vanished for unknown reasons at one-and-a-half years old.

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The bottlenose female was also seen nursing her adoptive calf on two occasions, which suggests she was highly invested, says Kirsty MacLeod, a behavioral ecologist at Lund University in Sweden, who was not involved in the study. “In mammals, synthesizing milk is very costly—it’s a very precious resource.”

Why adopt?

But a big question remains: Why would a bottlenose dolphin bother to invest in an infant to which she has no genetic ties?

One possibility is that the recent birth of her calf triggered her maternal instincts. “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,” says MacLeod, “and it led to this slightly wacky situation.” (Failure to launch: Learn about animals that stay with their moms for years.)

Personality could have been another driving factor, as this particular dolphin was already well known for her tolerance of scuba divers in the area. Her easygoing attitude may be what kept her from displaying typical bottlenose aggression toward non-offspring.

Then there was the melon-headed whale orphan himself. The researchers believe his determination to join the bottlenose family—and act like them—played a key role in the successful adoption.