Sex differences in the thermoregulation and evaporative water loss of a heterothermic bat,Lasiurus cinereus,during its spring migration

Abstract

SUMMARYThis study quantifies sex differences in thermoregulation and water loss of a small (20-35 g) insectivorous heterothermic mammal, the hoary bat Lasiurus cinereus, during its spring migration. We measured body temperature, metabolic rate and evaporative water loss, and calculated wet thermal conductance, for bats exposed to air temperatures ranging from 0 to 40°C for periods of 2-5 h. Pregnant females maintained normothermic body temperatures (35.7±0.7°C; mean ± s.e.m.)independent of air temperature. In contrast, males became torpid during the majority (68%) of exposures to air temperatures <25°C. The thermal neutral zone (TNZ) ranged between approximately 30°C and 34°C in both sexes and, within the TNZ, females had lower mass-specific metabolic rates(6.1±0.2 mW g-1) than males (9.0±0.9 mW g-1). Wet thermal conductance values in torpid bats (0.7±0.5 mW g-1 deg.-1) were lower than those of normothermic individuals (1.1±0.3 mW g-1 deg.-1). Mass-specific rates of evaporative water loss in males were consistently higher than in females at most air temperatures and rates of water loss in torpid bats were 63±6% of normothermic values. These results suggest that male and pregnant female L. cinereus employ different thermoregulatory strategies during their spring migration. Females defend normothermic body temperatures, presumably to expedite embryonic growth, while males use torpor, presumably to minimize energy and water deficits. These variable thermoregulatory strategies may reflect continental differences in the summer distribution of the sexes.

Obese super athletes: fat-fueled migration in birds and bats

“…Temperate bats are consummate heterotherms, and are well known for their extensive use of daily and seasonal torpor. Migrating hoary and silver-haired bats use torpor facultatively during stopovers (Cryan and Wolf, 2003;Jonasson, 2017;McGuire et al, 2014); in silverhaired bats, the depth and duration of torpor are adjusted depending on ambient temperature, such that fat loss and energy expenditure during daytime roosting at a stopover is independent of temperature (Baloun, 2017;McGuire et al, 2014). Female silver-haired bats appear to use less torpor and have greater roosting energy expenditure than males during spring stopovers, when they are pregnant, which may explain their greater fat loads in spring (Baloun, 2017;Jonasson, 2017).…”

“…Stopovers may be quite different from those experienced by birds because whereas most migratory birds (excepting species such as owls and nightjars) feed during the day and travel at night, bats are inherently constrained by their strict nocturnality to both refuel and/or travel during the night. Migrating bats use torpor to reduce energy expenditure during daytime roosting periods at stopovers (Cryan and Wolf, 2003;Jonasson, 2017;McGuire et al, 2014), whereas migratory birds, apart from a few exceptions (e.g. hummingbirds and nightjars), cannot use torpor.…”

Phenotypic flexibility in migrating bats: seasonal variation in body composition, organ sizes and fatty acid profiles

“…In summer the sexes are largely segregated, with males more commonly found in mountainous regions in the western part of North America, whereas females are widespread throughout the eastern part of the continent (Cryan, 2003). Mating occurs during autumn migration (Cryan, 2008) and females delay pregnancy until spring, migrating north while pregnant (Cryan and Wolf, 2003). The winter distribution is not well understood, though it is thought that most individuals overwinter in southern California and Mexico (Cryan, 2003).…”

“…However, another review suggested that a high n-6:n-3 ratio is important for maintaining membrane function at low body temperatures (Ruf and Arnold, 2008). Male hoary bats are more likely to use torpor during spring migration than females (Cryan and Wolf, 2003), and therefore we predicted that sexes would differ in muscle PL composition during migration. Specifically, we predicted that males would have either greater total PUFAs or greater n-6:n-3 if the Munro and Thomas (Munro and Thomas, 2004) or Ruf and Arnold (Ruf and Arnold, 2008) hypotheses were supported, respectively.…”

Low metabolism in a tropical bat from lowland Panama measured using heart rate telemetry: an unexpected life in the slow lane

“…Most work in this context has focused on thermoregulation by bats in the temperate zones (e.g. Chruszcz and Barclay, 2002;Cryan and Wolf, 2003;Hamilton and Barclay, 1994;Kurta, 1991;Kurta et al, 1990;Lausen and Barclay, 2003), but subtropical or even tropical bats are also known to use torpor and even hibernate (e.g. Bartels et al, 1998;Cory Toussaint et al, 2010;Geiser and Brigham, 2000;Hosken and Withers, 1997;Kelm and von Helversen, 2007).…”