Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011, USA.
Faculty of Life Sciences, School of Zoology, Tel Aviv University, Tel Aviv 6997801, Israel.
J Exp Biol. 2024 Jul 1;227(13). doi: 10.1242/jeb.247357. Epub 2024 Jul 2.
Scholander-Irving curves describe the relationship between ambient temperature and metabolic rate and are fundamental to understanding the energetic demands of homeothermy. However, Scholander-Irving curves are typically measured in dry air, which is not representative of the humidity many organisms experience in nature. Consequently, it is unclear (1) whether Scholander-Irving curves (especially below thermoneutrality) are altered by humidity, given the effects of humidity on thermal properties of air, and (2) whether physiological responses associated with Scholander-Irving curves in the lab reflect organismal performance in humid field conditions. We used laboratory experiments and biophysical models to test the effects of humidity on the thermoregulatory physiology of tree swallows (Tachycineta bicolor). We also tested whether physiological responses measured under lab conditions were correlated with field body temperatures and nestling provisioning rates. We found that humidity reduced rates of evaporative water loss but did not have large effects on body temperature or metabolic rate, suggesting that swallows can decouple evaporative cooling, body temperature and metabolic rate. Although the effect of humidity on metabolic rate in the lab was small, our biophysical models indicated that energetic costs of thermoregulation were ∼8% greater in simulations that used metabolic rates from birds in humid compared with dry conditions. Finally, we found mixed evidence that physiological responses measured in the lab under humid or dry conditions were associated with body temperature and nest provisioning rates in the field. Our results help clarify the effect of humidity on endotherm thermoregulation, which may help forecast organismal responses to environmental change.
施莱德-欧文曲线描述了环境温度与代谢率之间的关系，是理解恒温动物能量需求的基础。然而，施莱德-欧文曲线通常是在干燥空气中测量的，而这并不代表许多生物在自然界中所经历的湿度。因此，目前尚不清楚（1）湿度是否会改变施莱德-欧文曲线（尤其是在热中性以下），因为湿度会影响空气的热特性；（2）与施莱德-欧文曲线相关的生理反应是否反映了生物体在潮湿野外条件下的表现。我们使用实验室实验和生物物理模型来测试湿度对树燕（Tachycineta bicolor）体温调节生理的影响。我们还测试了在实验室条件下测量的生理反应是否与野外体温和育雏提供率相关。我们发现，湿度降低了蒸发失水的速度，但对体温或代谢率没有太大影响，这表明燕子可以将蒸发冷却、体温和代谢率分离开来。尽管湿度对实验室代谢率的影响很小，但我们的生物物理模型表明，在模拟中使用在潮湿条件下鸟类的代谢率时，恒温动物的体温调节能量成本比在干燥条件下高约 8%。最后，我们发现有混合证据表明，在实验室中在潮湿或干燥条件下测量的生理反应与野外的体温和育雏提供率有关。我们的研究结果有助于阐明湿度对恒温动物体温调节的影响，这可能有助于预测生物体对环境变化的反应。
