Bottlenose dolphin mothers typically raise one calf at a time, so researchers took notice when they saw a mother with two calves off the coast of Rangiroa Atoll in French Polynesia. Two calves aren't unheard of, but what really stood out was the difference between them. While one looked like a normal baby bottlenose dolphin, something was unusual about the other one.
Unlike a bottlenose dolphin's namesake snout, this calf had a blunter, rounder face. Led by Pamela Carzon of the Groupe d’Étude des Mammifères Marins (GEMM) de Polynésie, the researchers eventually realized the calf wasn't a bottlenose dolphin, but a baby melon-headed whale, as they report in the journal Ethology. That's not only a different species of dolphin, but a different genus.
As Erica Tennenhouse reports for National Geographic, this is the first known case of a wild bottlenose mother adopting a calf of another species. And it may be just the second confirmed case of any wild mammal adopting a baby from outside its own genus. (Aside from humans, of course, who commonly adopt dogs, cats and other nonhuman mammals as pets.)
Wild mammals sometimes adopt unrelated babies from within their own species, but interspecies adoption is much less common, and cross-genus adoption is even rarer. Until now, the only scientifically documented case was from 2006, Tennenhouse notes, when a group of capuchin monkeys were reported to be raising a baby marmoset.
In this new case, the bottlenose mom already had a young calf — presumably her biological daughter — when she took in the melon-headed whale. That's an extra burden for a species that usually raises one calf at a time, although the researchers think the first calf might have actually made the mother more open to adopting the second.
This photo shows the bottlenose mom with her biological daughter (left) and adopted son (top), a melon-headed whale. (Photo: Pamela Carzon/Groupe d’Étude des Mammifères Marins)
Carzon and her colleagues have been conducting a long-term study of this bottlenose community since 2009. The melon-headed calf first showed up in 2014, when he was about a month old, and quickly grew inseparable from his new mother. Her own daughter had been born the same year, and the trio became a common sight as they swam around the area together. (There was a little sibling rivalry, though, as the adopted calf jockeyed with his sister for a swimming position underneath their mother.)
The adopted calf was even seen nursing from his adopted mother on two occasions, further demonstrating how deep their bond had become. "In mammals, synthesizing milk is very costly — it's a very precious resource," Kirsty MacLeod, a behavioral ecologist at Sweden's Lund University who wasn't involved in the new study, tells Tennenhouse.
In addition to winning over his adoptive mother, the melon-headed calf also proved adept at fitting into bottlenose dolphin society. He often socialized with other bottlenose calves, seemed to communicate with them, and even joined them for recreational surfing and jumping. "The melon-headed whale was behaving exactly the same way as bottlenose dolphins," Carzon tells Tennenhouse.
This family of three lived together for about a year and a half, until the biological daughter disappeared for unknown reasons. It's possible something bad happened to her, although as Meilan Solly notes in Smithsonian Magazine, she might have just moved on to a different social subgroup. The adopted son, however, remained with his mother until April 2018. That's nearly three years after she adopted him, and it's around the age when many bottlenose dolphin calves wean.
A 'slightly wacky situation'
Female bottlenose dolphins have been known to briefly kidnap babies from other species, although those relationships rarely last very long, and the researchers doubt that's what happened here for a few reasons. This mother already had her own biological offspring, for example, which would make her unlikely to kidnap an additional calf of any species. Plus, this adopted calf's dedication to his new family and species suggests he sought out the relationship, or at least didn't enter it against his will.
"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 about the discovery.
One possibility, according to Carzon, is that the mother adopted the calf after he was abandoned by a different bottlenose dolphin who had kidnapped him. Regardless of his backstory, though, why did she make the sacrifice to take him in and raise him?
It was probably due to a lucky combination of factors. For one, the mother had recently given birth to her own daughter, sparking maternal instincts that might have made her more susceptible to the charms of a helpless baby. "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."
On top of that, Carzon and her colleagues cite the mother's personality and inexperience as likely factors. This dolphin was already known for tolerating scuba divers swimming nearby, and that laid-back demeanor might have created an opening for the orphan. She was also a first-time mother, and may not have fully appreciated what a difficult job she faced, even without a second calf.
Finally, the researchers add, we shouldn't overlook the calf's role in sparking this relationship.
"We also propose that the adoptee's persistence in initiating and maintaining an association with the adult female bottlenose dolphin could have played a major role in the adoption's ultimate success," they write.
For more details, including video of the family swimming together, check out this video from GEMM: