The Relationship between Cutaneous Water Loss and Thermoregulatory State in Kuhl’s PipistrellePipistrellus kuhlii, a Vespertillionid Bat

Abstract

Total evaporative water loss is the sum of respiratory water loss (RWL) and cutaneous water loss (CWL) and constitutes the main avenue of water loss in bats. Because bats fly and have large surface-to-volume ratios, they potentially have high rates of RWL and CWL. Most species of small insectivorous bats have the ability to reduce their body temperature (T(b)) at rest, which substantially reduces energy expenditure and water loss. We hypothesized that bats reduce evaporative water loss during bouts of deep hypothermia (torpor) by decreasing RWL and CWL. We measured T(b), RWL, CWL, and resting metabolic rate (RMR) in Kuhl's pipistrelle Pipistrellus kuhlii, a small insectivorous bat. In support of our hypothesis, we found that RWL decreased with decreasing RMR. We found that CWL was lower in torpid individuals than in normothermic bats; however, bats in deep torpor had similar or higher CWL than bats in shallow torpor, suggesting that they exert a less effective physiological control over CWL when in deep torpor. Because insectivorous bats spend most of their lives in torpor or hibernation, the regulation of CWL in different heterothermic states has relevant ecological and evolutionary consequences.

Partitioning of Evaporative Water Loss into Respiratory and Cutaneous Pathways in Wahlberg's Epauletted Fruit Bats (Epomophorus wahlbergi)

“…We separated CEWL and REWL using a mask (Tieleman and Williams 2002;Muñoz-Garcia et al 2012). Masks were constructed using liquid latex painted onto a mold made of modeling clay, the dimensions of which were determined by measuring male and female E. wahlbergi skulls.…”

“…We used these measurements to test whether the masks had any effect on TEWL, T b , or RMR, in light of the observation by Muñoz-Garcia et al (2012) that TEWL was significantly higher in Pipistrellus kuhlii when bats were restrained and wearing a mask than when they were not. They interpreted the increased TEWL as an effect of stress associated with restraint (Muñoz-Garcia et al 2012).…”

“…Rates of CEWL and REWL were estimated using the same approach as Tieleman and Williams (2002) and Muñoz-Garcia et al (2012). CEWL was estimated from the water vapor pressure in the chamber and the incurrent flow rate (corrected for the addition of water vapor).…”

“…Most recently, Muñoz-Garcia et al (2012) partitioned TEWL in the insectivorous Kuhl's pipistrelle (Pipistrellus kuhlii) using a mask and found that REWL decreased with resting metabolic rate (RMR) and that CEWL was less precisely regulated when P. kuhlii was in deep torpor compared with normothermy or shallow torpor. In that study, bats were restrained during measurements of REWL and CEWL, and the significantly higher TEWL of restrained bats (i.e., those wearing a mask) compared with unrestrained individuals raised the possibility that observed REWL/CEWL ratios were influenced by stress arising from the experimental setup (Muñoz-Garcia et al 2012).…”

“…We hence predicted that both REWL and CEWL in E. wahlbergi should increase sharply at high T a , contrary to the pattern typical of many birds, where either REWL or CEWL, but generally not both, increase rapidly as T a approaches normothermic T b (reviewed by Wolf and Walsberg 1996). We tested this prediction by training individuals to wear masks, so that we could obtain CEWL and REWL measurements while they were hanging in a natural posture within respirometry chambers, presumably reducing restraint-associated stress (Muñoz-Garcia et al 2012). Although the focus of our study was evaporative water and heat fluxes, our experimental design also permitted an evaluation of recent evidence that a significant fraction (5%-10%) of respiratory gas exchange occurs across the wings in E. wahlbergi (Makanya and Mortola 2007).…”