Fine-scale sociality reveals female–male affiliations and absence of male alliances in bottlenose dolphins (Tursiops truncatus) in the Shannon Estuary, Ireland

Abstract

Knowledge of social behavior at an individual level is central to our understanding of complex mammalian societies. In this study, we analyzed the fine-scale sociality of wild bottlenose dolphins in the Shannon Estuary, Ireland, by examining associations between members of the whole population and between specific female and male dolphins. We carried out 51 boat-based individual focal follows on 18 identifiable bottlenose dolphins over 90.8 hr (39 days) between 2014 and 2016. Additionally, we conducted 353 boat-based surveys (with 607 sightings) between 2012 and 2015, and identified 121 distinct adult/juvenile dolphins. The mean group size of focal dolphin groups was 7.2 ± 4.1 (range = 1–20) and the mean fission-fusion rate was 3.06 ± 1.35 changes/hr. The most frequent composition of within-group affiliate pairs was female–male. Focal males spent more time with female nearest-neighbors than male nearest-neighbors (p = .013). Differences between female and male activity budgets were not strongly supported (p = .13). There was no evidence for male alliance formation in the Shannon Estuary population, and all of the known-sex top-ranked associates (7) of known males (n = 10) were females. This research reveals a distinct bottlenose dolphin society with female–male affiliations and an absence of male alliances.