Abstract
We investigated social structure and association patterns for a small population of Mediterranean bottlenose dolphins, Tursiops truncatus, inhabiting the Aeolian Archipelago (southern Italy). Specifically we evaluate the role of sex and age composition, residency patterns and interaction with trammel nets on this social organization. Association data for 23 regularly sighted individuals were obtained from summer photoidentification surveys collected from 2005–2012. Using a combined cluster and social network analysis approach, we found associations between dolphins were hierarchically structured, where two mixed-sex social units were subdivided into smaller temporarily dynamic groups. We found non-random and long-term preferred associations in the population; however, the degree of social cohesion, residence pattern and interaction with trammel nets differed considerably between the two social units. Six of eight females occurred in the more resident social unit-1; in addition, social unit-1 individuals had significantly stronger associations, higher preferred associates, lived in larger groups and occurred less frequently with trammel nets. Nine of eleven males were clustered in social unit-2 and five of these males, interacting with trammel nets, formed small groups and preferred associations. We propose that female and male groups associate in the study area during the breeding season and that some males choose to interact with reproductive females forming a distinct but interrelated social unit. Other males may be associating in a larger fission-fusion network, which consists of dolphins that appear to temporarily join the network from the coastal population. We cannot exclude that some males specialized in trammel net foraging, suggesting that this foraging technique may favor a solitary lifestyle. Large group sizes and high degree of social cohesion for females could be an indication of greater protection and more efficiency in detecting, deterring or repelling anthropogenic pressures. Most likely dolphins' social organization depends on a combination of socio-ecological, demographic and anthropogenic factors.
Highlights
Bottlenose dolphins (Tursiops truncatus), like many mammal species [1], [2], [3], live in fission–fusion societies where individuals join and leave groups on a flexible basis, with group composition and size changing frequently on small spatial and temporal scales [4]
The purpose of this study is to evaluate the socio-ecological, demographic, and anthropogenic factors that influence the social structure of bottlenose dolphins in the Aeolian Archipelago (2005–2012)
Our results represent the first documentation on the association patterns and social structure for this small bottlenose dolphin population
Summary
Bottlenose dolphins (Tursiops truncatus), like many mammal species [1], [2], [3], live in fission–fusion societies where individuals join and leave groups on a flexible basis, with group composition and size changing frequently on small spatial and temporal scales [4]. Sexual segregation is believed to be the basic social framework of some bottlenose dolphin populations (e.g. T_aduncus in eastern Australia) where grouping patterns reflect sex-specific behavioural strategies [23]. Sexually mature male bottlenose dolphins form first-order alliances in pairs, trios, and second-order super-alliances, which cooperate by pursuing distinct alliance strategies to monopolize females in reproductive condition [15], [16], [17], [18]. Other studies have shown that male alliances may not occur in all bottlenose dolphin populations and the degree of sexual segregation and the proportion of mixed-sex groups can be geographically very variable [24], [25], [26], [27]. In Sarasota Bay (Florida), female strategies appear to be driven by calf protection (from predators and/or conspecifics) with adult females and their calves forming strongly bonded bands [4], [28]
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