Common dolphin ( Delphinus delphis ) fission–fusion dynamics in the south coast of Portugal

Animals with fission–fusion dynamics live in fluid societies with varying party sizes. The joining (fusion) and separation (fission) of these parties is considered a response to costs and benefits associated with grouping. This study investigates which factors influence grouping and fission–fusion dynamics in common dolphins ( Delphinus delphis ) in Portugal. Between June and October of 2016 and 2017, 94 boat-based group focal follows were conducted, resulting in 2410, 2-min behavioral samples. Using generalized linear models, we examined how mother-calf pair presence, behavioral state, and temporal variability influence party size. We used generalized estimating equations to model behavior and fission–fusion dynamics, testing the responses to mother-calf presence and time. Parties were smaller when mother-calf pairs were absent or when animals were resting. Socializing parties without mother-calf pairs were more likely to be observed than traveling parties with mother-calf pairs. Compared to traveling, foraging was less likely in September and October than in June, and resting was more likely in September. Compared to stable state, parties without mother-calf pairs were more likely to fission than parties with mother-calf pairs and fusion events became less likely by advancing hour. We revealed that common dolphins in Southern Portugal exhibit a high degree of fission–fusion dynamics that are predominantly influenced by mother-calf presence, confirming the importance of this region as a nursery ground. By assessing fine-scale social dynamics in common dolphins, this study advances understanding of the evolutionary drivers shaping grouping patterns in wild cetaceans while also providing an important comparative counterpoint to terrestrial fission–fusion dynamics. Significance statement Dolphin species are known for their fission-fusion societies with extensive intra- and interspecific variation in their social organization. This work provided an opportunity for new insights into the social lives of common dolphins in the South of Portugal and how social factors such as mother-calf pair presence, behavioral state, and temporal variability shape the social organization of this species. We found that common dolphins present a highly fluid organization and that the presence of mother-calf pairs plays a major role, affecting party size, behavior, and fission-fusion rate. Our work also confirms the relevance of Southern Portugal as an important calving and nursery area for this species, and therefore, it is essential that wise management practices are implemented in order to maintain a healthy population.