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 nocturnal mammals like bats, 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 bats’ emergence behaviour on a global scale remains limited. To investigate emergence patterns in bats across a latitudinal gradient, we conducted a meta-analysis spanning 92 species 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 emerged under conserved thresholds of brightness, a pattern that was facilitated by a temporal delay in roost exit at higher latitudes. By maintaining emergences within the confines of nautical twilight (sun altitude between 0° to -12°), bats in known predatory environments appear to balance the dilemma between risk and food. This pattern in brightness at emergence was mirrored across different social structures (maternal v. non-maternal colonies) and two broad dietary types (frugivores and insectivores). 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. Lastly, on comparing exit patterns of bats in regions with and without predators, we found that species in locations without any known predators emerged from their roosts before sunset, and under consistently brighter conditions.