How temperature, humidity, and burrow selection affect evaporative water loss in desert tortoises
Susan J. Bulova
Department of Zoology, 430 Lincoln Drive, University of Wisconsin–Madison, Madison, WI, 53706, USA
Received 1 March 2001; accepted 10 July 2001 Available online 2 April 2002.
By combining field behavior and microclimate measurements with biophysical models, I assessed the value of underground burrows as thermal refuge for desert tortoises (Gopherus agassizii) and their additional advantage for water conservation. Between 1000–1200 h, humidity was significantly higher and temperature and predicted evaporative water loss (EWL) lower inside burrows than on the surface. Greater burrow length and smaller entrances were correlated with greater burrow humidity. Furthermore, the range of variation in humidity, temperature and EWL over 24 h was greater on the surface than inside burrows. Thus, surface conditions would be more favorable during certain times of day.
Water Balance in Neonate and Juvenile Desert Tortoises,
Dawn S. Wilson, Kenneth A. Nagy, C. Richard Tracy, David J. Morafka, Rebecca A. Yates
Herpetological Monographs, Vol. 15, 2001 (2001), pp. 158-170
We examined evaporative water loss of neonate (<1 yr old) tortoises in laboratory experiments designed to evaluate the dependence of evaporation on humidity, and of juvenile (1-4 yr old) tortoises in field experiments designed to reveal the influence of burrow microclimate on water gain and loss. In controlled laboratory conditions, rates of body mass loss which reflect net evaporative water losses, were independent of the difference in vapor density between the animal and its environment. Changes in skin permeability and respiratory parameters may account for this. Sleeping tortoises lost body mass half as fast as did active tortoises and hibernating individuals lost body mass 1/20th as quickly as active animals. Juveniles confined to short (20 cm) or long (70 cm) burrows in the field lost body mass faster in the drier and warmer short burrows. Doubly labeled water was used in tortoises residing in different burrow types to measure total (unidirectional) rates of evaporation, vapor influx and metabolic water production, to partition net water loss (as reflected in body mass loss) into its parts. Total evaporation rate was independent of burrow conditions, but tortoises in the longer, more humid burrows had higher rates of water vapor input and total water input than did those in shorter burrows. Thus, tortoises in long burrows lost body mass more slowly in response to a higher humidity, in contrast to neonates under laboratory conditions. Rates of body mass loss due to evaporation from neonates were relatively high in the laboratory (0.4 to 0.8% of body mass per day) and the field (0.7 to 1.1%/d) compared to those of adults in the laboratory (0.17%/d) or the field (0.1%/d). Thus, young tortoises apparently are obliged to rely on behavioral means (drinking pooled rain, withdrawing into their shell, seeking long, deep burrows) to avoid lethal dehydration in relatively hot, dry seasons.