Thermal adaptation of pelage in desert rodents balances cooling and insulation

Phenotypic convergence across distantly related taxa can be driven by
similar selective pressures from the environment or intrinsic constraints.
The roles of these processes on physiological strategies, such as
homeothermy, are poorly understood. We studied the evolution of thermal
properties of mammalian pelage in a diverse community of rodents
inhabiting the Mojave Desert, U.S.A. We used a heat flux device to measure
the thermal insulation of museum specimens and determined whether thermal
properties were associated with habitat preferences while assessing
phylogenetic dependence. Species that prefer arid habitats exhibited
significantly lower conductivity and thinner pelage relative to species
with other habitat preferences. Despite having thinner pelage, the low
conductivity imparted comparable insulation to species with other habitat
preferences. Thus, arid species retain insulative pelage while
simultaneously benefitting from thin pelage that promotes heat loss via
convective cooling. We found no evidence of intrinsic constraints or
phylogenetic dependence. Heat flux models designed to simulate body
temperature regulation demonstrated that arid specialists saved 14.5% of
their annual energy required for homeothermy by evolving lower
conductivity, providing support for adaptive evolution of pelage. Our
study indicates that selection for lower energetic requirements of
homeothermy has shaped evolution of pelage thermal properties.

远缘类群间的表型趋同（phenotypic convergence）可由环境相似的选择压力或内在约束所驱动。目前学界对于这些过程在恒温性（homeothermy）等生理策略演化中所扮演的角色尚不明晰。我们以栖息于美国莫哈韦沙漠的多样啮齿类群落为研究对象，探讨了哺乳动物被毛（pelage）热特性的演化规律。我们借助热通量装置（heat flux device）测量馆藏标本的隔热性能，并在评估系统发育依赖性（phylogenetic dependence）的同时，探究被毛热特性与栖息地偏好是否存在关联。相较于其他栖息地偏好的物种，偏好干旱生境的物种其热导率显著更低，被毛也更为稀薄。尽管干旱生境物种的被毛更为稀薄，但其较低的热导率赋予的隔热性能，与其他栖息地偏好的物种相当。因此，干旱生境物种既保留了具备隔热功能的被毛，同时又可从稀薄被毛中获益——稀薄被毛可通过对流散热（convective cooling）促进热量散失。我们未发现内在约束或系统发育依赖性的相关证据。旨在模拟体温调控的热通量模型显示，干旱特化物种通过演化出更低的热导率，每年可节省恒温所需能量的14.5%，这为被毛热特性的适应性演化提供了支持。本研究表明，针对恒温性更低能量需求的自然选择，塑造了哺乳动物被毛热特性的演化历程。