Sam Froman Walmsley, Laura J Feyrer, Claire Girard, Elizabeth Zwamborn, Hal Whitehead

How social behaviour changes as individuals age has important consequences for the health and function of both human and non-human societies. However, the extent of inter-individual variation in social ageing has been underappreciated, especially in natural populations of animals. Here, we leverage a photo-identification dataset spanning 35 years to examine social ageing in an Endangered population of northern bottlenose whales (Hyperoodon ampullatus), an oceanic beaked whale that exhibits fission-fusion social dynamics. Patterns of social ageing were sex-specific: males experienced a decline in social behaviour with age, shedding direct social connections and holding less central social roles. Conversely, female social traits were generally stable throughout adulthood, while the average strength of their relationships increased with age. Beyond sex, additional inter-individual variation played a large role in how animals responded socially to age. This study shows that inter-individual variation can play a key role in how animals experience behavioural senescence.