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An unlikely friendship has been observed beneath the waves of French Polynesia. For the first time, scientists have documented a case of a wild bottlenose dolphin mother adopting a melon‐headed whale calf. Not only are these two animals different species, they even belong to different genera.
As reported in the journal Ethology, the curious duo was first seen by researchers led by the Marine Mammal Study Group of French Polynesia around the Rangiroa Atoll off the coast of the Tuamotu Archipelago in June 2014.
At first glance, they assumed it was just a typical bottlenose (Tursiops truncatus) caring for her juvenile son. However, a closer look revealed that the youngster had an unusually blunt nose. This was no bottlenose, it was actually a melon‐headed whale (Peponocephala electra), another member of the dolphin family.
The two were recorded together on 11 different days between April and October 2015, then on 24 different days between April 2016 and August 2017, after which time the adoptee would have been weaned and “fled the nest.”
While very uncommon in the wild, adoptions have been reported in a number of mammals. Just this month, researchers reported that a wild group of endangered Barbary macaques adopted an injured juvenile from another group.
However, it’s not so common to see adoption between species. It’s even rarer to see it between genera, as seen here. One of the only other times the phenomenon has been scientifically documented was when a marmoset (Callithrix jacchus) was adopted by a group of wild capuchin monkeys (Cebus libidinosus) in 2004. However, for two different genera of oceanic dolphins, this is extremely rare.
“To our knowledge, the case reported here is thus unique in wild mammals,” the study authors write.
"Considering the high cost of lactation, long postnatal investment for delphinids, and absence of life‐threatening behaviors toward the new calf, it is reasonable to assume that the female Tursiops adopted the Peponocephala calf," they add.
Melon‐headed whales can be found on the outskirts of the bottlenose dolphin’s range, which would explain how these two came into contact. However, it’s less certain why the mother put her energy and resources into rearing the calf despite having relatively little genetic relation. Other explanations of interspecies adoption range from a simple mistake made through inexperience to pre‐parental training or even "babysitting" behavior.
In this case, the researchers suggest it was the result of the “mother’s inexperience and personality.” As you can see from the silhouette of the images above, melon-headed whales do appear fairly similar to bottlenose dolphins, if you ignore their characteristic nose.
