The bottlenose dolphin mom had a biological calf when she had adopted the male melon-headed whale, which did not leave her side.
We know that humans adopt. But wild mammals do too, apparently. It's a once-in-a-blue-moon occurrence. Whatever adoption happens in wild mammals is between creatures of the same species. For a mammal to adopt another species is a rare thing entirely. The only other time a cross-species adoption was scientifically documented was in 2006. The University of São Paulo primatologist Patrícia Izar observed a group of capuchins caring for a baby marmoset, according to National Geographic.
So, when researchers found a dolphin mom had adopted a whale calf, they were really surprised. “We were really excited to be able to witness such a rare phenomenon,” study lead author Pamela Carzon, scientific leader of the Groupe d’Étude des Mammifères Marins (GEMM) de Polynésie, based in Tiputa, French Polynesia, told the magazine.
In a first, the researchers found a bottlenose dolphin mom adopting a calf of another species. In 2014, they saw the mother dolphin with an unusual looking male calf and her presumed biological calf in coastal waters off French Polynesia. The one-month-old male's beak was short and blunt whereas these dolphins have slender beaks. The scientists realized later that the adopted orphan was a melon-headed whale— a different species and genus of dolphin.
The adopted calf stayed close to his new mom the entire time and the trio was seen swimming often, which is unusual since dolphin mothers care for only one infant at a time. The female bottlenose dolphins are also known to "steal" infants of other species for a short period. However, the adopted calf and the new mom's affection towards each other showed their love, which made the scientists believe that this was no kidnapping.
She was dedicated to him for a long period of time. They were seen together for almost three years, until the time he probably weaned off. The adopted mom and son were together longer than she was with her biological calf. The sister vanished when she was one-and-a-half years old for unknown reasons.
The male calf, not unlike any human sibling, tried to shove the sister out from under the mother dolphin's abdomen, the scientists observed. The adopted calf did not leave his mother's side and also learned to act like the dolphin family unit. The male calf socialized with the other youngsters of the pod and participated in their favorite pastime of surfing and leaping into the waves.
“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,” Kirsty MacLeod, a behavioral ecologist at Lund University in Sweden, told the magazine, “and it led to this slightly wacky situation.” She was not part of the study, which was published in the journal Ethology.
"We argue that the primiparous foster mother’s inexperience and personality may have contributed to factors driving such non‐adaptive behavior," said the group of scientists in their paper, Cross‐genus adoptions in delphinids: One example with taxonomic discussion.