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. 散热极限理论（Heat dissipation limit theory）指出，恒温动物（endotherms）用于生长与繁殖的可利用能量，受限于自身的散热能力。在哺乳类动物中，妊娠期与泌乳期的内源性产热会显著升高，因此雌性哺乳类可能比雄性面临更强的能量支出热约束。这类约束或对气候变暖背景下的动物行为与种群表现产生重要影响。
2. 本研究基于热质传递（heat and mass transfer）的基本原理构建机理模拟模型（mechanistic simulation model），用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，研究团队借助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，量化了维持热量平衡的调节性行为的相对重要性，并评估了体温调节成本在多大程度上限制了采样个体的时空分布。
3. 模拟结果显示，相较于非泌乳雌性灰熊，泌乳个体能从行为性降温机制（如部分躯体浸入冷水或趴卧于低温基质）中获得更显著的收益；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的可活动时长最多提升60%，而在未来气候情景中该提升幅度可达43%。此外，即便在未来气候情景下，只要存在可用于降温的水资源，泌乳雌性灰熊通过行为性体温调节即可实现每日24小时的热量平衡。
4. 无论繁殖状态如何，海拔高程均为影响黄石地区雌性灰熊分布的最重要预测因子。不过，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频率更高。
5. 本研究结果表明，受气候调控的散热成本，或许会对灰熊这类大型恒温动物的行为与能量代谢施加约束；随着气候持续变暖，可获取降温用水资源将愈发成为黄石地区灰熊分布格局的重要驱动因子。