Familiarity and aggression shape long‐term associations and mortality risk in a solitary ungulate

Abstract

Periodic social interactions are important to animal fitness, even in solitary species. For solitary species, these interactions can be unexpected and shaped by previous encounters. Despite being aggressive and largely solitary, black rhinoceroses Diceros bicornis are commonly seen in groups, suggesting they may engage in more social behaviours than previously recognised. However, their social behaviour, and how those behaviours influence survival, are largely unknown. We evaluated association behaviour of black rhinos at different life stages (i.e. calf, subadult, adult) from 69 920 sightings from 14 reserves in southern Africa. Associative behaviours varied significantly by age class and sex, with subadult males associating with specific individuals at the highest rate and adult females associating with the greatest number of individuals. Rate of association was influenced by previous association (i.e. familiarity). However, our results illustrate that social associations can be costly for black rhinos. Specifically, mortality risk was higher for calves that associated at higher rates, and higher for adults that associated with more males, or more individuals close in age, likely due to aggressive interactions since fighting conspecifics is a common cause of death. In contrast, association might provide benefits. In adults that associated with at least one unrelated conspecific, males displayed lower mortality risk than females. To our knowledge, this is the first study documenting long‐term association behaviour in a solitary ungulate, providing evidence that social interactions in solitary species are more common, and more important with regards to the impact on long‐term survival, than previously thought.