COS 10-1 - Testing the influence of social context on the occurrence of mixed-species groups of Serengeti grazers

Abstract: Despite continued interest in mixed-species groups, we lack a unified understanding of how ecological and social processes lead to the formation of single versus mixed-species groups. Recent work has revealed ecological correlates of mixed-species group formation, but the mechanisms by which concomitant social dynamics produce these patterns, if at all, are unknown. Here, we use camera trap survey data for six mammalian grazer species in Serengeti National Park. We used generalized linear mixed models to test: 1) the mechanisms that influence the availability of 1) heterospecific potential social partners for individuals of a focal species, and 2) how the availability of social partners shapes the likelihood of mixed-species group formation. We found that ecological variables, and especially primary productivity, influenced the chances of species overlap over small spatiotemporal scales (i.e., on the scales of several meters and hours). Migratory species (gazelle, wildebeest, and zebra) were significantly more likely to have heterospecific partners available in sites with higher primary productivity, but the opposite was true for resident species (buffalo, hartebeest, and topi). These findings illuminate the circumstances under which mixed-species group formation is even possible. Next, we found that greater heterospecific availability was significantly associated with an increased probability of mixed-species group formation in gazelle, hartebeest, wildebeest, and zebra, but ecological variables did not further shape these patterns. Overall, our results are consistent with a model whereby ecological and social drivers of group formation are species-specific and operate on different spatiotemporal scales.