The geography of nocturnality: emergence patterns of bats on a latitudinal gradient

Animals often attempt to resolve the trade-off between foraging and predation by regulating their activity timings and patterns. Driven by individual energetic requirements and the local environment, onset of activity in a species varies across space and time. These differences are particularly enhanced on a latitudinal gradient, influenced by varying lengths of daylight, temperature and seasonality. For a nocturnal mammal like the bat, such patterns translate to shorter activity windows at higher latitudes, both on a nightly and a seasonal basis. This constructs the timing of activity onset as a crucial variable for a successful night of foraging. While roost exit timings and influence of local conditions on bat activity have been extensively studied across species inhabiting a variety of locations around the world, our understanding of bat emergence behaviour on a global scale remains limited. To investigate emergence patterns in bats across a latitudinal gradient, we conducted a meta-analysis spanning 91 species from across 13 Chiropteran families. We evaluated how timing and luminosity (i.e., sun altitude) at exit varied across latitudes and species. Results revealed that bats across the globe tended to emerge under conserved thresholds of brightness, a pattern that was facilitated by a temporal delay in roost exits at higher latitudes. By maintaining emergence within the confines of nautical twilight (sun altitude between 0{degrees} to -12{degrees}), bats across the phylogeny thus seemed to balance the dilemma of predation versus foraging. This pattern was mirrored across the two broad dietary types (frugivores and insectivores) as well as different social structures (maternal v. non-maternal colonies). Furthermore, our models indicated that while local conditions dictate final exit decisions, shared ancestry was potentially influencing the extent of plasticity seen in emergence patterns. Taken together, these results highlight how broad-scale latitudinal gradients of light can influence the foraging strategies in a globally distributed nocturnal mammal.