The partial merger of two dolphin societies

Abstract

Interactions between mammalian social groups are generally antagonistic as individuals in groups cooperate to defend resources from non-members. Members of the family Delphinidae inhabit a three-dimensional habitat where resource defence is usually impractical. Here, we describe a long-term partial fusion of two communities of Atlantic spotted dolphins ( Stenella frontalis ). The northern community, studied for 30 years, immigrated 160 km to the range of the southern community, observed for 20 years. Both communities featured fission–fusion grouping patterns, strongest associations between adult males, and frequent affiliative contact between individuals. For the 5-year period following the immigration, we found members of all age classes and both sexes in mixed groups, but there was a strong bias toward finding immigrant males in mixed groups. Some association levels between males, and males and females, from different communities were as high as the highest within-community associations. Affiliative contacts indicate that these individuals were forming social relationships. The mixing of two separate social groups with new bond formation is rare in terrestrial mammal groups. Such mixing between spotted dolphin groups suggests that adaptations to respond aggressively to ‘outsiders’ are diminished in this species and possibly other ecologically similar dolphins.

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“…Humans are unusual for the range of our intergroup relationships which can include affiliation and altruism toward strangers as well as destructive large-scale wars. While other social species such as dolphins and bonobos may have affiliative relationships between groups (Danaher-Garcia, Connor, Fay, Melillo-Sweeting, & Dudzinski, 2022;Elliser, Volker, & Herzing, 2022), sustained positive-sum relationships that cross pronounced group boundaries are exceedingly rare among nonhuman animals likely appearing only in a few eusocial insect species. Our cousins the bonobos often have affiliative interactions with other bonobo groups that include grooming, sex, and sometimes food sharing .…”

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