Thermal constraints on energy balance, behavior, and spatial distribution of grizzly bears

1. Heat dissipation limit theory posits that energy available for growth and reproduction in endotherms is limited by their ability to dissipate heat. In mammals, endogenous heat production increases markedly during gestation and lactation, and thus female mammals may be subject to greater thermal constraints on energy expenditure than males. Such constraints likely have important implications for behavior and population performance in a warming climate. 2. We used a mechanistic simulation model based on first principles of heat and mass transfer to study thermal constraints on activity (both timing and intensity) of captive female grizzly bears (Ursus arctos) in current and future climate scenarios. We then quantified the relative importance of regulatory behaviors for maintaining heat balance using GPS telemetry locations of lactating versus non-lactating female bears from Yellowstone National Park, and assessed the degree to which costs of thermoregulation constrained the distribution of sampled bears in space and time. 3. Lactating female bears benefitted considerably more from behavioral cooling mechanisms (e.g., partial submersion in cool water or bedding on cool substrate) than non-lactating females in our simulations; the availability of water for thermoregulation increased the number of hours during which lactating females could be active by up to 60% under current climatic conditions and by up to 43% in the future climate scenario. Moreover, even in the future climate scenario lactating bears were able to achieve heat balance 24 hrs/day by thermoregulating behaviorally when water was available to facilitate cooling. 4. The most important predictor of female grizzly bear distribution in Yellowstone, regardless of reproductive status, was elevation. However, variables associated with the thermal environment occurred with greater frequency in rules for predicting the distribution of lactating than non-lactating female bears. 5. Our results suggest that the costs of heat dissipation, which are modulated by climate, may impose constraints on the behavior and energetics of large endotherms like grizzly bears, and that access to water for cooling will likely be an increasingly important driver of grizzly bear distribution in Yellowstone as the climate continues to warm.

1. 热耗散极限理论（Heat dissipation limit theory）提出，内温动物（endotherms）可用于生长和繁殖的能量，受限于其自身的热耗散能力。在哺乳动物中，妊娠（gestation）与泌乳（lactation）期间内源性产热会显著升高，因此雌性哺乳动物在能量消耗上可能面临比雄性更强的热约束。这类约束对于气候变暖背景下的动物行为与种群表现可能具有重要影响。
2. 本研究基于传热传质（heat and mass transfer）的基本原理，构建机理模拟模型（mechanistic simulation model），以探究当前及未来气候情景下，圈养雌性灰熊（Ursus arctos）的活动（包括活动时序与活动强度）所受的热约束。随后，我们利用黄石国家公园内泌乳与非泌乳雌性灰熊的GPS遥测（GPS telemetry）定位数据，量化了维持热平衡所需的调控行为的相对重要性，并评估了体温调节成本在空间与时间维度上对受试灰熊分布的约束程度。
3. 模拟结果显示，相较于非泌乳雌性灰熊，泌乳雌性灰熊从行为性降温机制（如部分浸入冷水或卧于凉爽基质）中获益更多。在当前气候条件下，可用于体温调节的水资源可使泌乳雌性灰熊的可活动时长增加至多60%；在未来气候情景下，这一增幅可达43%。此外，即便在未来气候情景中，只要有可供降温的水资源，泌乳灰熊通过行为性体温调节即可实现每日24小时的热平衡。
4. 黄石国家公园内雌性灰熊的分布，无论繁殖状态如何，其最重要的预测因子均为海拔。但与热环境相关的变量，在预测泌乳雌性灰熊分布的模型规则中出现的频率高于非泌乳雌性灰熊。
5. 本研究结果表明，受气候调控的热耗散成本，可能会对灰熊这类大型内温动物的行为与能量代谢施加约束；而随着气候持续变暖，可获取降温用水资源，或将逐渐成为黄石公园内灰熊分布格局的关键驱动因子。