Allometry of evaporative water loss in marsupials: implications of the effect of ambient relative humidity on the physiology of brushtail possums(Trichosurus vulpecula)

Abstract

SUMMARYTo better understand the effects of ambient relative humidity (RH) on physiological variables and the implications of RHcorrecting evaporative water loss (EWL) data for marsupials, we examined the effect of RH on EWL, body temperature (T b ), metabolic rate (MR) and thermal conductance (C) of the brushtail possum (Trichosurus vulpecula), a medium-sized marsupial. Correcting EWL data for 27 species of marsupial for water vapour pressure deficit (ΔWVP) in the chamber during measurement significantly increased, rather than decreased, the variability of the allometric relationship for EWL. For the brushtail possum, both ambient temperature (T a ) and RH significantly affected EWL. At T a =25°C, EWL was independent of RH at ≤63% RH, but decreased linearly at higher RH values. At T a =30°C, EWL was significantly related to RH from 26% to 92% RH. There was a significant effect of T a on T b and dry thermal conductance (C dry ; higher at 30°C), but no effect of RH. For MR and wet thermal conductance (C wet ) there was a significant effect of T a (MR higher and C wet lower at 25°C), and RH at T a =30°C (MR higher and C wet lower at the lowest RH) but not at 25°C. Our results indicate that brushtail possums do not necessarily show the linear relationship between ambient RH and EWL expected for an endotherm, possibly because of behavioural modification of their immediate microclimate. This may account for the failure of WVP deficit correction to improve the allometric EWL relationship for marsupials. Chamber RH is an important environmental factor to be considered when measuring standard physiological variables such as MR and C wet .

Physiological regulation of evaporative water loss in endotherms: is the little red kaluta (Dasykaluta rosamondae) an exception or the rule?

“…For heterothermic male and post-lactating female little brown bats, there was no wvp effect on EWL at T a of 288C or 338C [13], suggesting that EWL is a controlled rather than a physical process, and there was an unexpected linear relationship between EWL and wvp at T a ¼ 378C. We [14] previously reported that EWL of brushtail possums was constant at low RH, at a thermoneutral T a (258C); we attributed this EWL constancy at low RH to postural changes and a body core to extremity thermal gradient. Our data for kalutas, showing an unexpected constancy of EWL under environmental conditions that would be expected to perturb water balance, suggest that EWL was under active physiological regulation, reducing their EWL at low RH (high Dwvp).…”

“…Various studies have reported that EWL changes inversely with RH and wvp (or linearly with Dwvp) for small endotherms [5][6][7][8][9][10][11][12][13], consistent with a simple physical model. However, some studies have reported a lower or no relationship between EWL and RH or Dwvp, generally at low or moderate T a [12][13][14]. EWL is independent of wvp at 208C for rock pigeons; adjustments in respiratory ventilation or expired air temperature might account for this [12].…”

“…It is customary to calculate Dwvp as the difference between wvp at 100% RH and ambient T a and wvp at ambient RH and T a [7,14,20]. Skin temperature is more appropriate than T a to calculate Dwvp [16], but its measurement is more difficult, so ambient temperature is generally used as a proxy.…”

“…For example, wvp may affect T b for kangaroo rats, particularly at high T a [37]. For brushtail possums [14], there was a significant RH effect on MR at T a ¼ 308C, sufficient to maintain a constant T b when EWL (and thus EHL) was reduced at higher RH. However, RH does not affect T b or MR for some rodents [6,7,38].…”

Size, shape, and the thermal niche of endotherms

“…The LCT was determined as described. We approximated the upper critical temperature of the TNZ for brush-tailed possums as the environment requiring metabolic rate to be Ͻ60% of basal (i.e., Ͼ40% of BMR required to be lost by evaporation), based on published observations of thermoregulation in this species (50,51).…”