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.