The social structure of a captive group of Rüppell's vultures (Gyps rueppellii)
Studies on the evolution of intelligence have so far mainly focused on mammals, primates in particular, and only few avian taxa, mainly corvids and parrots. Vultures, however, have relatively large brains too, and those of the Gyps genus are highly social. Their obligate scavenging lifestyle and sociality make them an excellent taxon for investigating the role of social and ecological selective pressures on the evolution of intelligence. However, little is known of Gyps vultures, particularly regarding their social complexity (sensu Social Brain Hypothesis). To investigate the complexity within their social structure, we studied the dominance hierarchy and social bonds of a captive group (n {equiv} 23) of Ruppell's vultures (Gyps rueppellii) over two consecutive years. Our results revealed that dominance ranks and affiliative bonds were stable, although there were some differences between breeding and non-breeding seasons. During the breeding season, pair-bonded individuals were more dominant and less social than unpaired individuals. Pair-bond status, age and season influenced dyadic bond strength. Most notably, pair-bonded dyads had substantially stronger bonds, but maintained multiple other strong bonds, too. Our results highlight that multiple forces shape the dominance hierarchy and affiliative network of captive Ruppell's vultures. Individual characteristics shape social bonds, social relationships are differentiated, and changes between the breeding and non-breeding season can result in social uncertainty, adding to the complexity of this system. The complexity of Ruppell's vultures' social structure seems comparable to that of some social corvid and parrot species, where it has been linked to their sophisticated cognition. Future socio-cognitive work on this and other less social vulture species could further identify the social challenges they face, and reveal whether a complex social environment may have been a selection pressure on the evolution of intelligence, both in general, and in this promising new taxonomic group for cognition studies specifically.