Social bonds decrease epigenetic age in male bottlenose dolphins.

Ageing is a universal process characterised by the deterioration of functional traits over an individual's lifespan. Differing degrees of age-related decline between individuals of the same chronological age suggest varying rates of ageing. Identifying factors influencing these inter-individual differences in 'biological age' is central to understanding ageing. In social mammals, social variables affect lifespan and are therefore likely to affect biological age. In Shark Bay, Western Australia, male bottlenose dolphins forge persisting intrasexual social bonds that affect their reproductive success and, therefore, their evolutionary fitness. We investigate the relationship between cumulative social bond strength, group size, and biological age of individual male dolphins in this population. Biological age is inferred using a species-specific epigenetic clock, the current gold standard for such an inference. We find a significant negative effect of cumulative social bond strength and a significant positive effect of group size. This implies that the negative effect of social bonds on epigenetic age cannot be attributed solely to group-living, but to benefits of the social bonds themselves. As established in humans, we find that the strength of social relationships affects epigenetic age, indicating that sociality may be linked to biological ageing more broadly across social mammals.