Abstract
We tested the hypothesis that interspecific variation in chiropteran heat tolerance and evaporative cooling capacity is correlated with day-roost microclimates, using three vespertilionid bats that occur sympatrically during summer in Saskatchewan, Canada. We predicted that hoary bats (Lasiurus cinereus (Palisot de Beauvois, 1796); ∼22 g) would have higher heat tolerance than little brown bats (Myotis lucifugus (Le Conte, 1831); ∼7 g) and silver-haired bats (Lasionycteris noctivagans (Le Conte, 1831); ∼13 g), as the latter two species roost in tree crevices or cavities that are more thermally buffered than the foliage roosts of hoary bats. We measured core body temperature (Tb; passive integrated transponder tags), evaporative water loss, and resting metabolic rate (flow-through respirometry) while exposing individuals to a stepped profile of increasing air temperature (Ta) from ∼30 °C in ∼2 °C increments. Experiments were terminated when individuals became hyperthermic (Tb ≈ 42.5 °C), with maximum Ta (Ta,max) ranging from 42.0 to 49.7 °C. As predicted, hoary bats had the highest heat tolerance and evaporative cooling capacity, reaching Ta,max ∼2.4 and 1.2 °C higher than little brown and silver-haired bats, respectively. Our results are consistent with the hypothesis that heat tolerance of bats is correlated with roost microclimates, although interspecific variation in body mass and phylogeny may confound these conclusions.
Highlights
Free-ranging endotherms frequently encounter environmental temperatures above their normothermic body temperature (Tb), for example, while foraging in sunlit microsites or occupying hot roosts during the day (e.g., Henshaw and Folk 1966; Wolf and Walsberg 1996)
We predicted that hoary bats (Lasiurus cinereus (Palisot de Beauvois, 1796); $22 g) would have higher heat tolerance than little brown bats (Myotis lucifugus (Le Conte, 1831); $7 g) and silver-haired bats (Lasionycteris noctivagans (Le Conte, 1831); $13 g), as the latter two species roost in tree crevices or cavities that are more thermally buffered than the foliage roosts of hoary bats
Individual Ta,max ranged from 42.0 °C in a little brown bat to 49.7 °C in a hoary bat
Summary
Free-ranging endotherms frequently encounter environmental temperatures above their normothermic body temperature (Tb), for example, while foraging in sunlit microsites or occupying hot roosts during the day (e.g., Henshaw and Folk 1966; Wolf and Walsberg 1996). When environmental temperature exceeds Tb, endotherms can dissipate heat only by increasing evaporative water loss (EWL) from respiratory and cutaneous surfaces, giving rise to a trade-off between conserving water and avoiding lethal hyperthermia (reviewed by McKechnie and Wolf 2019). Day roost selection is likely an important factor in determining bat survival during severe heat stress, for example, a free-ranging little broadnosed bat (Scotorepens greyii (Gray, 1843)) abandoned its poorly insulated roost when outside Ta exceeded 48 °C during a heat wave in Australia, whereas inland freetail bats (Mormopterus petersi (Leche, 1884)) remained in their better-insulated roosts (Bondarenco et al 2014). Foliageroosting bats likely experience hot microclimates during summer as a result of little buffering from external Ta and high solar heat loads (Willis and Brigham 2005; Klu€g et al 2012), which can co-occur with high humidity levels that impede evaporative heat dissipation
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