The small-bat-in-summer paradigm: energetics and adaptive behavioural routines of bats investigated through a stochastic dynamic model

Strong seasonality at high latitudes represents a major challenge for many endotherms as they must balance survival and reproduction in an environment that varies widely in food availability and temperature. Being heterotherms, bats spend long cold winters in hibernation, avoiding the challenges faced by many animals. To avoid energetic mismatches caused by limited foraging time and stochastic weather conditions, bats can also employ this energy-saving state of torpor during summer to save accumulated energy reserves. However, at high latitudes small-bats-in-summer face a particular challenge: as nocturnal foragers they rely on the darkness of the night to avoid predators and/or interspecific competition, but for many the summer involves short nights of mostly twilight, and even a lack of true night at the northernmost distributions of some bat species. To investigate optimal individual behaviour across diurnal cycles, we constructed a stochastic dynamic model of bats living at high latitudes. Using a detailed parameterized model framework with values that are representative for our study system, we show that individual energetic reserves are a strong driver of day-time use of torpor and night-time foraging behaviour alike, with these linked effects being both temperature and photoperiod dependent. We further used the model framework to predict survival probabilities at five locations across a latitudinal gradient (60.1{degrees}N to 70.9{degrees}N), finding that photoperiod is the main limiting factor to bat species distributions. To verify the accuracy of our model results, we compared predictions for optimal decisions with our own empirical data collected on northern bats (Eptesicus nilssonii) from two latitudes in Norway. The similarities between our predictions and observations provide strong confirmation that this model framework incorporates the most important drivers of diurnal decision-making in bat physiology and behaviour. Our model findings regarding state-dependent decisions in bats should therefore contribute to the understanding of how bats cope with the summer challenges at high latitudes.