Interspecies pectoral fin contact between bottlenose dolphins and Atlantic spotted dolphins off Bimini, The Bahamas

Affiliative interspecies interactions are important to community structure within an ecosystem yet understudied relative to antagonistic relationships. Mixed-species associations provide important benefits via foraging, predator avoidance and social interactions. Tactile behaviours help with social bond formation and group establishment among conspecifics during social interactions, but little is known about the role of contact between heterospecific animals in mixed-species groups. Our objective was to quantify characteristics of pectoral fin contact between individuals of two sympatric species of wild dolphins, spotted dolphins, Stenella frontalis, and bottlenose dolphins, Tursiops truncatus, off Bimini, The Bahamas, in order to assess possible benefits of heterospecific interactions. Using underwater video recordings from mixed-species dolphin encounters from 2011 to 2017, we observed 74 pectoral fin contacts, of which 27 were between heterospecific animals. Encounters were either affiliative (61.5%) or sociosexual (38.5%). Conspecific and heterospecific dolphins had similar rates of initiating and receiving contacts. This suggests that during mixed-species groups, the cost-benefit trade-off in choosing a social partner is similar for conspecific and heterospecific individuals. Heterospecific contacts initiated by S. frontalis were primarily affiliative whereas heterospecific contacts initiated by T. truncatus were sociosexual. Both sociosexual behaviour and pectoral fin contact in dolphins have been suggested as mechanisms of bond formation. We conclude that mixed-species interactions with pectoral fin contact may be a mechanism for social bond formation between heterospecifics and that these bonds may help to re-establish groups, as needed, for predator avoidance.