Environmental, social, and morphological drivers of fission-fusion dynamics in a social ungulate

Abstract

Social groups exist because individuals within the group accrue a net benefit from sharing space. The profitability of sociality, however, varies with ecological context. As ecological context varies, tension emerges among the costs and benefits of social grouping. Fission-fusion societies are fluid in their group dynamics across spatial and temporal contexts, permitting insights into how context affects whether animals choose to join or depart a group. We tested four non-mutually exclusive hypotheses driving variation in fission and fusion in caribou: the risky places, environment heterogeneity, activity budget, and social familiarity hypotheses. The risky places hypothesis predicts animals are unlikely to diffuse when habitats are open and risk of predation is elevated. The habitat heterogeneity hypothesis predicts that fission is more likely in a heterogeneous landscape due to the rising conflicts of interest between group members. The activity budget hypothesis predicts dyads associate by body size due to similar food passage rates. The social cohesion hypothesis predicts that familiar individuals are less likely to fission. We tested the hypotheses using time-to-event (time before fission) analyses and a linear model that assesses spatial, social, and body size relationships among female caribou (n = 22) on Fogo Island, Newfoundland, Canada. Contrary to our prediction for risky places, probability of fission was not influenced by habitat openness. The hypothesis of environmental heterogeneity was partially supported, as caribou remained less cohesive in environments with a higher richness of habitats. No direct evidence emerged to support the activity budget hypothesis. However, it appears that caribou maintain the strongest social bonds among variably sized individuals and these social bonds do decrease the propensity to split. Collectively, our findings showed that social interactions may depend not only on individual identity and characteristics, but also the spatial context in which these interactions occur.

Do mixed-species groups travel as one? An investigation on large African herbivores using animal-borne video collars

“…Here, we found that the cohesion of mixed-species group was lower when zebras herded with wildebeest or buffalos than with impalas or giraffes. Conflicts of interests in decision making lower the cohesion of single-species group, such as observed in red deer (Cervus elaphus) or woodland caribou (Rangifer tarandus) (Conradt and Roper 2000;Sueur et al 2011;Le Goff et al 2024), and can be expected to constrain even more strongly the cohesion of mixedspecies group. Although zebras benefit from a lower risk of predation by herding with wildebeest than impalas (Schmitt et al 2014), the lower difference of habitat selectivity with impalas than wildebeest may explain the higher cohesion of zebras herding with impalas.…”

Predicting fine-scale distributions and emergent spatiotemporal patterns from temporally dynamic step selection simulations

“…In part this was a decision made with respect to our data, as individuals from mostly separate clans were GPS-tagged, resulting in little observable influence between individuals. However, interacting covariates relating to conspecifics with temporal dynamics may lead to realistic patterns of social behaviour throughout the time-scale of interest, as there are likely to be fluctuations of attraction and repulsion to other individuals, for instance those that produce fusion-fission dynamics (Haydon et al, 2008;Silk et al, 2014;Le Goff et al, 2023). These fluctuations of social dynamics may relate to longer time-scales such cycles of breeding, although there may be fine-scale temporal dynamics that influence movement within a daily period.…”