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 defense 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 five-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 bonds, likely for future mating opportunities. 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’ is diminished in this species and possibly other ecologically similar dolphins.

The first file contains group composition data logged from our field sighting logs with dolphin identities collected from underwater still photos and video. These data were used to conduct association analyses. The second file contains tactile contact events logged from underwater video specifically from mixed sightings (sightings including resident and immigrant dolphins). These data were used to conduct chi-square tests using basic code in R. See DCP_SIGHTING_TACTILE_DATAreadme.txt file for further details.

See DCP_SIGHTING_TACTILE_DATAreadme.txt file.