Skin temperature data from northern bats (Eptesicus nilssonii) in Norway

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. To avoid energetic mismatches caused by limited foraging time and stochastic weather conditions, bats employ the 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 at night to avoid predators and/or interspecific competition, but live in an environment with short, light summer nights, and even a lack of true night at the northernmost distributions of some bat species. To predict optimal behaviour in relation to latitudinal variation in diurnal cycles, we constructed a stochastic dynamic programming model of bats living at high latitudes. Using a stochastic dynamic programming framework with values that are representative for our study system, we show that individual energetic reserves are a strong driver of daytime use of torpor and night-time foraging behaviour alike, with these linked effects being both temperature and photoperiod dependent. We further used the model to predict survival probabilities at five locations across a latitudinal gradient (60.1°N to 70.9°N), finding that combinations of photoperiod and temperature conditions limited population distributions in the model. To verify 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 evidence that this model framework incorporates the most important drivers of diurnal decision-making in bat physiology and behaviour. Comparing empirical data and model predictions also revealed that bats facing lighter night conditions further north restrict their mass gain, which strengthens the hypothesis that predation threat is a main driver of bat nocturnality. Our model findings regarding state-dependent decisions in bats should contribute to the understanding of how bats cope with the summer challenges at high latitudes.

高纬度地区强烈的季节节律对多数恒温动物（endotherms）而言是一项重大挑战，这类生物必须在食物可获得性与温度波动剧烈的环境中，平衡生存与繁衍需求。为规避因觅食时长受限与随机性天气引发的能量失配，蝙蝠会在夏季进入节约能量的蛰伏（torpor）状态，以留存累积的能量储备。不过，高纬度地区的夏季小型蝙蝠面临一项特殊挑战：作为夜行性觅食者，它们依赖夜间黑暗来躲避天敌或种间竞争，但所处环境的夏季夜晚短促且光照充足，部分蝙蝠种群的最北分布区甚至不存在真正的黑夜。为预测与昼夜周期纬度变化相关的最优行为模式，我们构建了适用于高纬度蝙蝠的随机动态规划（stochastic dynamic programming）模型。我们采用贴合本研究系统参数的随机动态规划框架开展研究，结果显示：个体能量储备是支配白日蛰伏与夜间觅食行为的关键因素，且这类关联效应同时受温度与光周期（photoperiod）调控。我们进一步利用该模型，对纬度梯度（60.1°N至70.9°N）上5个地点的种群存活概率进行预测，结果发现光周期与温度的组合条件限制了模型中种群的分布范围。为验证模型结果，我们将最优决策的预测结果与自身采集的挪威境内两个纬度位点的北棕蝠（Eptesicus nilssonii）实证数据进行比对。预测结果与实际观测结果高度吻合，有力证明该模型框架涵盖了蝙蝠生理与行为中昼夜决策的核心驱动因素。实证数据与模型预测的比对还显示：栖息于高纬度、夜间光照更强地区的蝙蝠会限制自身体重增长，这一结果进一步支持了“捕食威胁是蝙蝠夜行性的主要驱动因素”这一假说。本研究中关于蝙蝠状态依赖型决策的模型结果，将有助于加深学界对蝙蝠如何应对高纬度夏季生存挑战的认知。