Homophily around specialized foraging underlies dolphin social preferences

Abstract

Individuals often associate socially with those who behave the same way. This principle, homophily, could structure populations into distinct social groups. We tested this hypothesis in a bottlenose dolphin population that appeared to be clustered around a specialized foraging tactic involving cooperation with net-casting fishermen, but in which other potential drivers of such social structure have never been assessed. We measured and controlled for the contribution of sex, age, genetic relatedness, home range and foraging tactics on social associations to test for homophily effects. Dolphins tended to group with others having similar home ranges and frequency of using the specialized foraging tactic, but not other traits. Such social preferences were particularly clear when dolphins were not foraging, showing that homophily extends beyond simply participating in a specific tactic. Combined, these findings highlight the need to account for multiple drivers of group formation across behavioural contexts to determine true social affiliations. We suggest that homophily around behavioural specialization can be a major driver of social patterns, with implications for other social processes. If homophily based on specialized tactics underlies animal social structures more widely, then it may be important in modulating opportunities for social learning, and therefore influence patterns of cultural transmission.

Keywords: Tursiops truncatus; behaviour specialization; fission–fusion; network; social structure.

Figure 1.

Dolphin social preferences at the individual and population level across behavioural contexts. Nodes representing photo-identified individuals are proportional to the frequency of use of the specialized foraging tactic and colour-coded by social modules; individuals are connected by links whose thicknesses are proportional to SRI in the association networks and to GAI removing confounding factors (electronic supplementary material, tables S1–S3) in the affiliation networks (for better visualization, only positive GAIs were plotted). In the density plots, red dots denote statistically significant observed values; grey dots indicate non-significant values; shaded distributions indicate null expectancy; and blue whiskers indicate 95% confidence intervals. The specialized foraging tactic underlies social patterns as shown by (a) significant standard deviations (s.d.) of SRI and GAI indicating social preferences; (b) significant modularity (Q) indicating social division; and significant assortativity by both (c) frequency of foraging with fishermen and (d) home range size. Values in the scales are rounded. (Online version in colour.)