Sighting data and tactile events logged from mixed-group underwater interactions

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.