Discussion
We found that social situations differ in their contribution to the social structure of griffon vultures. There was no single individual that ranked the highest in all centrality measures or across all social situations (Figs. 4, S1). Furthermore, the centrality of individuals in the aggregate network did not always reflect the centrality of individuals in each of the social situations (Fig. 5). Finally, social situations differed in their contribution to the different centrality measures, with some being more important for the number of individuals a vulture interacted with and others being more important for the strength of the social interaction (Fig. 6).
Individuals differed in their social importance across social situations. While some individuals had a similar centrality rank in all three social situations, others did not. Thus, the social role of an individual does not necessarily carryover across situations. The differential social importance of individuals across social situations may have implications for population-level processes. For example, individuals that interact with many others (high degree) while co-feeding might spread pathogens broadly during feeding interactions, however if they do not interact with many individuals while roosting, their impact on pathogen spread in a roosting situation would be smaller. Such differences in social position may emerge from differences in behavior while feeding and roosting, with certain individuals engaging in more interactions during the day than at night. Investigating the relationship between an individual’s spatial and social behavior (Webber et al. 2021; Spiegel & Pinter-Wollman 2022), may shed light on the mechanisms that underlie individual variation in interactions across different social situations, and consequently on variation in exposure to pathogens (Hughes et al.2002; Vanderwaal et al. 2016) and information (Cortés-Avizandaet al. 2014; Spiegel & Crofoot 2016) and the tradeoffs among them (Romano et al. 2021). Studies of mammals have revealed differences across systems in whether or not the social centrality of individuals is maintained across situations (Smith-Aguilar et al.2019; Kulahci et al. 2018; Gazda et al. 2015). Because of the potential implications of the relationship between an individual’s social position across situations for population level processes it is important to understand in which systems and when (e.g., different seasons or life history stages) such relationships occur and when they do not.
The contribution of each social situations to the population-level social structure differed. Co-flight and diurnal ground interactions are important for determining the number of individuals a vulture interacts with and both nocturnal and diurnal ground interactions are important for determining the intensity of interactions. Our finding that vultures seem to repeatedly interact with few individuals while on the ground (e.g., while roosting and feeding), but may have brief interactions with many partners while co-flying supports our prediction that co-flight interactions have a lower influence on the strength of interactions than other situations. Indeed, overall, the strength of interactions while flying was lower than in the other two situations (Table S1) suggesting that interactions on the ground are longer and provide more social information for establishing social relationships, compared to co-flying interactions. The strong contribution of co-flying interactions to the number of individuals one interacts with (degree) relative to nocturnal ground interactions suggests that movement patterns in different situations can influence their impact on social structure, with longer ranging movements resulting in interactions with more unique individuals relative to short distance movements (Webber et al. 2021; Spiegel & Pinter-Wollman 2022). However, we found that diurnal ground interactions contribute to the number of unique individuals one encounters, suggesting that other mechanisms, in addition to movement patterns, determine who one interacts with. For example, it is possible that carcasses, being an ephemeral resource, attract larger crowds than roosting sites, providing more opportunities to interact with new individuals. It is also possible that social preferences differ across situations with preference for interactions with particular individuals in one situation and different preferences in another situation. Examining social preferences and the relative abundance of feeding and roosting locations and how their distribution in space might influence social interactions can help uncover the mechanisms that underlie the differences we observed in the impact of different social situations on population social structure. While studies of other animals have found that degree (number of interactions) and strength (intensity of interactions) are not always aligned, the impact of interactions in different social situations on society structure are limited (but see Gazda et al. 2015; Lehmann et al. 2016, Roberts & Roberts 2016; and Dragić et al. 2021). How each social situation impacts population level structure and what are the consequences of the differential impact of each situation on population dynamics, remains to be examined.
Uncovering how different social situations impact population dynamics can be crucial for conservation and wildlife management actions. In the vulture system for example, carcasses can serve as a site for disease spread. Supplementing food at multiple sites simultaneously might spread the vulture population and reduce the number of individuals each one interacts with, potentially slowing the spread of disease and reducing exposure to poisoning. However, further investigation is needed to determine the population density that will facilitate vultures utilizing multiple spatially dispersed carcasses simultaneously (Spiegel et al. 2013a). In contrast, stronger bonds established while on the ground can facilitate the acclimation of introduced vultures and benefit long-term breeding programs. While our work is based on a single population during the breeding season, it already shows that ecological context matters. Future research that examines more populations and different seasons may help map more precisely when each situation is most important and link explicitly the importance of each situation with different fitness consequences.
Uncovering the mechanism by which global population processes emerge from individual interactions can help unravel how societies balance the trade-off between the costs and benefits of sociality. Considering social interactions within their ecological situation and incorporating the differential impact that each situation has on social structure can uncover previously overlooked causes and consequences of animal social behavior and aid wildlife management actions.