Abstract
Social systems are characterized by the pattern of sex-specific relationships which are, in turn, determined by the pattern of interactions between individuals over time. Long-term underwater behavioral studies offer the opportunity to investigate association (i.e., presence in the same group) and affiliative interaction (e.g., tactile exchanges) between individuals. A population of Atlantic spotted dolphins (Stenella frontalis) off Bimini, The Bahamas, has been studied by the Dolphin Communication Project since 2003. A 16-year archive (2003 – 2018) of systematically collected underwater video of these dolphins was used to quantify association among these spotted dolphins, as well as to document body contact between individuals. These dolphins exhibit non-random associations, showing a preference for same-sex associates of similar age. In particular, a subgroup of five male dolphins of similar age maintained the highest association overall throughout the study period. Similarly, these spotted dolphins exchanged the most contact with others of similar age and same sex, and juvenile dolphins initiated the most contact of any age class. In addition to association and interaction between dolphins, the arrival in 2013 of a novel group of spotted dolphins, confirmed as a subset of a population from the northern Bahamas, is described. This new group was composed mostly of subadult and adult males and females and was observed to assimilate peacefully into the resident population. High coefficients of association were recorded between members of both groups, especially between males. Additionally, affiliative contact exchanges between resident and new dolphins were documented and no aggression was observed. Finally, a comparative social network analysis was conducted to compare the network metrics and individual position in an association network versus one composed of behavioral exchanges. Due to the difficulty of directly observing marine mammal interactions underwater, it is common for delphinid association networks, in which association in the same group is used as a proxy for interaction (i.e., “gambit of the group”), to be compared directly to networks describing terrestrial animal interactions (e.g., grooming). Using the same individuals, this study compared the metrics of an association network with an interaction network (i.e., number of documented pectoral fin contacts). The results of this analysis show that individuals hold different positions depending on the measure used to construct the network. This study presents the first comprehensive account of associations and tactile exchanges for any dolphin population, as well as the first detailed account of the merger of two previously separate social networks coming together peacefully and exchanging affiliative behaviors. Additionally, this study is one of the first to test the “gambit of the group” for wild cetaceans.