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First published online August 22, 2008
Journal of Experimental Biology 211, 2759-2766 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.019463

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

C. E. Cooper^1,2,* and P. C. Withers^1,2

¹ Centre for Ecosystem Diversity and Dynamics in the Department of Environmental Biology, Curtin University of Technology, PO Box U1987, Perth, Western Australia, 6845
² Zoology, School of Animal Biology, University of Western Australia, Crawley, Western Australia, 6009

^* Author for correspondence (e-mail: c.cooper{at}curtin.edu.au)

Accepted 21 May 2008

To better understand the effects of ambient relative humidity (RH) on physiological variables and the implications of RH-correcting 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 (WVP) 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.

Key words: evaporative water loss, respirometry, relative humidity, marsupial, brushtail possum, methodology, water vapour pressure deficit

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