Linking digestive efficiency and climate to basal metabolic rate in bats of the family vespertilionidae.

Animals consume, process, use, and allocate energy to survive. The rate at which individuals use energy depends on how they lose heat to the environment, which is influenced by both climate and body mass (Mb), as well as their ability to process food. Apparent digestibility of dry matter (D*m) is commonly used to estimate digestive efficiency and has been positively linked to mass-independent basal metabolic rate (hereafter, BMR) in some birds and mammals. In vespertilionid bats, D*m has been reported to be inversely related to Mb. Since Mb is negatively associated with BMR, a positive relationship between D*m and BMR is expected. However, bats from cold climates tend to exhibit lower BMRs. We measured BMR in 10 bat species from the family Vespertilionidae and gathered information from 29 additional species across a range of climates and Mb (3.4 g to 30.3 g). Using phylogenetic and additive approaches, we tested the relationship between BMR, climate and D*m mathematically modeled. We found that BMR was positive associated with D*m and that this relationship differed by climate. BMR was constant in bats with Mb ≥ 13–17 g and D*m ≤ 74–76%, while BMR increased in smaller species. Our results suggest that D*m could be critical in smaller bats from warm climates due to their high rates of energy use. The different responses we found with other studies can be explained by trade-offs in energy allocation among organs and other metabolically active tissues. We discuss morphological, physiological, and ecological mechanisms underlying D*m variation.