Standard Evaporative Water Loss and Metabolism of Juvenile Varanus mertensi (Squamata: Varanidae)

Abstract

Evaporative water loss (EWL) varies considerably both within and between reptile species (Dmi'el, 1985; Eynan and Dmi'el, 1993; Lahav and Dmi'el, 1996) and can also be affected by acclimation (Hillman et al., 1979; Kobayashi et al., 1983). EWL data are available for a wide variety of reptiles from various habitats (see review by Mautz, 1982) but are limited for goannas under controlled conditions (Warburg, 1965; Green, 1969; Thompson and Withers, 1997a). For lizards, such as goannas, body water is lost by evaporation from the skin (including the eyes) and the respiratory passages and as a mixture of urine and feces (and in some species from nasal salt glands; Green, 1969; Saint Girons et al., 1981). The rate of cutaneous evaporative water loss (CEWL) is largely determined by the exposed surface area of the skin and eyes, the resistance of these surfaces to evaporation, and the difference in water vapor concentrations across the surfaces. The respiratory evaporative water loss (REWL) is determined mainly by the lung ventilation rate and body temperature but may be reduced by nasal countercurrent heat and water exchange (Mautz, 1980). Environmental conditions such as ambient temperature, relative humidity, and wind velocity also affect EWL (Thorpe and Kontogiannis, 1977; Mautz, 1980, 1982). EWL is generally higher for mesic reptiles than arid species (Snyder, 1971; Dmi'el, 1972; Shoemaker and Nagy, 1977), and we would expect the EWL of a semiaquatic goanna to be higher than for comparable terrestrial species. However, there are no data for EWL of semiaquatic goannas under controlled conditions. The semiaquatic goanna Varanus mertensi has a much higher field water turnover (WTR) than predicted for lizards from arid and tropical zones (Christian et al., 1996), but it is not clear whether their high WTR is associated with a high cutaneous EWL, respiratory EWL, water excretion, or a combination thereof. This study examines the standard EWL (EWL,td) and standard metabolic rate (VO2std) for juvenile V mertensi. The primary objectives were to measure total evaporative water loss and calculate resistance to water evaporation under standardized laboratory conditions (TEWLtd, rstd), and to partition TEWLStd and rstd into its respiratory and cutaneous components. Thompson and Withers (1997a) report TEWL,td and resistance to total water loss (rtota) for a number of terrestrial and arboreal Western Australian