Energy management of heterothermic bats at northern latitudes: Understanding the physiological flexibility of bats and how this enables them to live in the northern edge of their distribution

Abstract

Understanding how bats manage their energy budgets under different thermal conditions is an important component in understanding the limits of distribution and their ability to cope with environmental changes, such as ongoing climate change. In particular, as multiple species of bats are predicted to increase their distribution northward, this thesis aimed to estimate energy consumption (i.e., metabolic rate) in bats currently inhabiting high latitudes in the northern hemisphere. Metabolic rate was estimated using indirect calorimetry, while bats were exposed to an increasing temperature profile during daytime. This was supplemented with reviewing previously published papers for comparison.

Interestingly, energy consumption in bats at high northern latitudes is affected by a multitude of factors. For instance, the extent to which torpor is used can be highly dependent on energy availability, as bats with higher body mass delay torpor entry, and perform more costly arousals at lower ambient temperatures to achieve normothermic body temperatures. Despite being exposed to near-freezing temperatures, increased metabolic rate, indicating a propensity to prevent body temperature from passing past a critical level, was only evident within half the bats in brown long-eared bat (Plecotus auritus), and none in northern bat (Eptesicus nilssonii). Length of time needed to enter torpor, can also differ between species, primarily as a result of differences in whole-animal metabolic rate, and thermal effects on cooling.

To date, basal metabolic rate has only been estimated in 3.4% of all species of the Vespertilionidae family. This limited dataset contains too few data points to perform analysis on how basal metabolic rate differs between sexes, season, and reproductive state at a grander scale. Accordingly, body mass is the main predictor describing variation in basal metabolic rate between species. Similarly, on a global scale, the minimum torpor metabolic rate is fairly universal across different habitats and species. Minimum torpor metabolic rates are first and foremost affected by ambient temperature. However, some species of bats inhabiting more arid climate zones present minimum torpor metabolic rates at higher ambient temperatures, thus indicating a need to defend a higher body temperature.

In its totality, this thesis provides further insight into the physiological flexibility of bats living at high latitudes in the northern hemisphere. With an opportunistic use of torpor to manage their energy budgets, and a particularly wide thermoneutral zone, bats can sustain long periods of limited food supply. On a global scale, energy consumption in bats has been largely overlooked, as work so far has been limited to a few species, often focused around small geographic areas. Thus, to better predict, manage, and conserve species in the face of climate change, more work is needed to understand the physiological and behavioral responses of bats to different environmental conditions in bats.