Thermal energetics of bats of the family Vespertilionidae: An evolutionary approach

Thermal energetics define the way animals spend energy for thermoregulation. In this regard, numerous studies have determined that body mass (M_b) is the most influential morphological trait affecting the thermal traits in different species of birds and mammals. However, most of the studies have been focused on the basal metabolic rate (BMR), while other thermal traits have been less studied. We addressed this gap by examining thermal variables on bats of the family Vespertilionidae. Using open-flow respirometry, we measured BMR, absolute thermal conductance (C'), lower and upper critical temperatures (T_LC and T_UC), and breadth of the thermoneutral zone (TNZ_b) of 16 bat species ranging in M_b from ∼ 4.0-21.0 g from central Mexico. We: 1) combined our empirical data with information gathered from the literature and conducted phylogenetic analyses to investigate the relationship between M_b and thermal traits, 2) tested the relationship between mass independent C' and mass independent BMR with T_LC and T_UC of bats, and the relationship between critical temperatures and TNZ_b, and 3) mapped the thermal energetic traits along the phylogeny to explore their evolutionary trends. We found a positive relationship between M_b and BMR and absolute C' but not to T_LC, T_UC and TNZ_b of bats. Mass independent BMR and mass independent C' were positively related to T_LC and T_UC. Finally, T_LC showed a negative relationship with TNZ_b while T_UC exhibited a positive relationship with this thermal trait. The phylogenetic approach indicates that over the evolutionary history, BMR and C´ have decreased while T_LC, T_UC and TNZ_b have increased. Our results suggest that: 1) differences in the limits of the TNZ and C' may have helped bats to avoid the constraints on heat dissipation imposed by ambient temperatures, and 2) adaptive changes in M_b and thermal traits may have influenced the geographical distribution and energy-saving strategies of bats. These findings contribute to an understanding of how small endotherms cope with thermal challenges, shedding light on the physiological and evolutionary mechanisms that shape species' ecological niches and biogeographic patterns across diverse environments.