Reaction Norms in Natural Conditions: How Does Metabolic Performance Respond to Weather Variations in a Small Endotherm Facing Cold Environments?

Reaction norms reflect an organisms' capacity to adjust its phenotype to the environment and allows for identifying trait values associated with physiological limits. However, reaction norms of physiological parameters are mostly unknown for endotherms living in natural conditions. Black-capped chickadees (Poecile atricapillus) increase their metabolic performance during winter acclimatization and are thus good model to measure reaction norms in the wild. We repeatedly measured basal (BMR) and summit (Msum) metabolism in chickadees to characterize, for the first time in a free-living endotherm, reaction norms of these parameters across the natural range of weather variation. BMR varied between individuals and was weakly and negatively related to minimal temperature. Msum varied with minimal temperature following a Z-shape curve, increasing linearly between 24°C and −10°C, and changed with absolute humidity following a U-shape relationship. These results suggest that thermal exchanges with the environment have minimal effects on maintenance costs, which may be individual-dependent, while thermogenic capacity is responding to body heat loss. Our results suggest also that BMR and Msum respond to different and likely independent constraints.

反应规范（reaction norms）体现了生物体调整表型以适应环境的能力，可用于识别与生理极限相关的性状值。然而，对于自然环境中的恒温动物（endotherms）而言，其生理参数的反应规范大多仍未被探明。黑顶山雀（Poecile atricapillus）在冬季驯化过程中会提升代谢机能，因此是野外测量反应规范的理想实验模型。我们对该山雀的基础代谢率（basal metabolic rate, BMR）和极限代谢率（summit metabolism, Msum）进行了重复测量，首次在自由生活的恒温动物中，刻画了这两类代谢参数在自然天气变化范围内的反应规范。个体间基础代谢率存在差异，且与最低气温呈微弱负相关。极限代谢率随最低气温呈Z型曲线变化：在24℃至-10℃区间内呈线性上升，同时随绝对湿度呈U型关系变化。上述结果表明，与环境的热交换对维持代谢成本的影响极小，而维持代谢成本可能存在个体差异；与此同时，产热能力会随机体散热情况发生响应。此外，我们的研究结果还显示，基础代谢率与极限代谢率受到不同且大概率相互独立的约束因素调控。