Social structure in northern bottlenose whales: Stable relationships without stable groups

Sam Froman Walmsley, Laura J Feyrer, Hal Whitehead

Uncovering the drivers and function(s) of social relationships across a wide range of species is key to understanding the ecology and evolution of social behaviour. Northern bottlenose whales have been identified as a particularly important case study for social evolution. Unlike other large, toothed whale species which live in kin-based social units, northern bottlenose whales appear to form fluid, fission-fusion communities. However, this understanding has not been revisited in over 20 years, and new lines of evidence suggest that female relationships may have been underestimated in previous work. Drawing on 35 years of photo-identification data, we used Bayesian social network and binomial mixture models to quantify social relationships between individuals. Northern bottlenose whale social networks had very low modularity but high social differentiation, indicating strong social preferences in the absence of stable groups or community partitions. Contrary to previous work, we found evidence for strong, long-lasting relationships among females, not just males. Though kinship had little influence on social relationships, age differences and residency influenced patterns of association. Broadly, this analysis supports the hypothesis that northern bottlenose whales exhibit a social system more similar to smaller, less sexually dimorphic toothed whales, such as dolphins. This work highlights the value of long-term ecological research for quantifying the social lives of long-lived species.