Cross-genus adoptions in delphinids: One example with taxonomic discussion

Although relatively rare, adoptions have been reported in a number of mammals, involving almost exclusively individuals of the same species, and hardly ever between species or across genera. Adoption remains poorly documented and its proximate causes are controversial. Here, we describe a unique case of a cross-genus adoption within a small community of common bottlenose dolphins (Tursiops truncatus) at Rangiroa Atoll in French Polynesia. It involves a foster female adopting a presumed melon-headed whale (Peponocephala electra) calf while already mothering its presumed biological offspring. While the inclusive fitness hypothesis can be rejected for this adult female mother, acquisition of parental skills is also unlikely to have driven adoption in parallel to natural motherhood. We argue that the primiparous foster mother's inexperience and personality may have contributed to factors driving such non-adaptive behavior. 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, as well as the persistence of this cross-genus adoption after the disappearance of the biological offspring. A brief discussion of adoption and hybridization within the Delphinidae taxon is included to identify how this cross-genus adoption fits into context of marine mammal parental care.