DOI: 10.5061/dryad.0zpc8671g
title: Thermal adaptation of pelage in desert rodents balances cooling and insulation

description: 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.
description: We measured the thermal insulation of pelage from specimens
 curated in the Museum of Vertebrate Zoology (MVZ) at the University of
 California, Berkeley. We developed our technique to non-destructively
 measure the thermal conductance of museum specimens, while also assessing
 the potential effects of making measurements on dried specimens collected
 over the last century. We built a custom device to measure the thermal
 conductance of mammal pelage. Our approach was inspired by similar
 techniques that measured temperature gradients and heat flux across pelage
 in a controlled laboratory setting. After the conductance measurements, we
 recorded length of the hairs and thickness of the pelage from museum
 specimens, where we define length as the distance from the outer surface
 of the skin to the tip of the hair and thickness as the vertical distance
 from the outer surface of the skin to the outer surface of the hair. Prior
 to taking measurements, we ensured that the pelage was not laying
 irregularly. We measured the length of hairs at three locations spanning
 the dorsal side of the specimen at the approximate locations of the
 thoracic, lumbar, and sacral vertebrae. We placed the base of a
 Fisherbrand<sup>TM</sup> 150 mm ruler at the base of the hair
 and pressed the hair against the ruler to measure the length to the
 nearest 0.5 mm. We also measured thickness at the same three locations
 spanning the dorsal side of the specimen. To evaluate
 support for adaptive evolution of thermal conductivity of pelage, we used
 heat flux simulations (Endoscape) designed and validated for this
 community of small mammals in the Mojave Desert (see Riddell et al. 2021
 for a detailed explanation of the model and validation). These simulations
 use biophysical principles to estimate the rate at which mammals lose or
 gain heat to their environment.
description: These data files and code require R and Python to open and
 run.