From the perspective of prey, movement synchrony can represent either a potent anti-predator strategy or a dangerous liability. Prey must balance the costs and benefits of using conspecifics to mediate risk and the emergent patterns of risk-driven sociality depends on the spatial variation and trait composition of the system. Our literature review outlined the prevailing, but not universal, trend of animals using sociality as an antipredator strategy. Empirically, we then used movement synchrony as a measure of social antipredator response of two ungulates to spatial variation in predator and prey habitat domains. We demonstrated that these responses vary based on prey vulnerability and predator hunting modes. Prey favored asynchrony when calves were present and within habitat domains of ambush predators but not pursuit predators. By unifying community ecology concepts such as habitat domains with movement ecology we provided a comprehensive evaluation of factors mediating prey social response to predation risk.

Density dependence is a fundamental ecological process. Patterns of animal habitat selection and social behaviour are often density-dependent and density-dependent traits should affect reproduction and survival, and subsequently affect fitness and population dynamics. The Ideal Free Distribution and Optimal Foraging Theory present distinct predictions about how the effect of habitat selection on fitness differs across a population density gradient. Using a social ungulate (Rangifer tarandus) as a model system, we test competing hypotheses about how (co)variance in habitat specialization, social behaviour, and fitness vary across a population density gradient. Within a behavioural reaction norm framework, we estimated repeatability, phenotypic plasticity, and phenotypic covariance among social behaviours and habitat selection to demonstrate the adaptive value of these phenotypes across a population density gradient. In support of Optimal Foraging Theory, but not the Ideal Free Distribution, we found that at high density habitat specialists had higher fitness than generalists, but were also less social than habitat generalists, suggesting the possibility that specialists were inhibited from being social. Our findings illustrate that social strength and habitat specialization varied consistently among individuals across a density gradient, but that habitat specialists maximized fitness at high density. Taken together, our study provides preliminary support for Optimal Foraging Theory as the driving mechanism for density-dependent habitat specialization.