Seasonal Water Turnover Rates and Body Water Volumes in Desert Chukars

Abstract

-Tritiated water was used to estimate water turnover rates and total body water volumes (TBW) in free-living Chukars (Alectoris chukar) in the Negev Desert of Israel during late autumn and winter. Water turnover rates varied widely, ranging from 66.8 to 420.1 mlkg-' I day-' (n = 25). They were lowest before winter rains, averaging 15.0% (SD = 1.0; n = 17) of TBW in the dry autumn and early winter, and 45.3% (SD = 10.9; n = 8) in late winter. This seasonal increase in water turnover rate may reflect a dietary shift from dry foods to succulent green vegetation, the growth of which was induced by winter rains. Water turnover was much more variable among wild Chukars than in captive birds from the same population kept in a controlled environment. TBW averaged 68.5% (SD = 4.1; n = 58) of body mass in free-living birds and was significantly elevated during cold, wet weather. Birds and other homeotherms inhabiting deserts face behavioral and physiological challenges in maintaining favorable water balances under severe and widely fluctuating environmental regimes (Bartholomew and Cade 1963, Bartholomew 1972). Two physiological variables that are sensitive to environmental fluctuations and that are, therefore, useful for examining animal adaptations to extreme environments are the rate of water turnover and total body water volume (TBW). These variables have received increasing attention because of their biological importance and because they can be readily estimated using tritiated water (TOH; Nagy and Costa 1980). Indeed, TOH measurements made under similar ambient conditions demonstrate that xeric mammals generally have lower water turnover rates than species in mesic and humid regions (Macfarlane and Howard 1972, Nicol 1978). Fewer data are available for birds, but a recent comparison suggests that a similar relationship exists in 14 avian species (Degen et al. 1982). Water turnover rate is sensitive to shortterm fluctuations in ambient meteorological conditions, amounts and kinds of food, and available drinking water (Degen 1977, Grubbs 1980, Withers et al. 1980). TBW responds to environmental stresses also, and is considered a useful indicator of body condition. Specifically, elevated TBWs may reflect a loss of body solids (principally fat) and thus indicate poor body condition (e.g., Campbell and Leatherland 1980). To date, the TOH method has rarely been used in studies of free-living birds. In the present study we employed TOH to measure the rate of water turnover and TOH space (as an estimate of TBW) of wild Chukars (Alectoris chukar) in the Negev highlands of southern Israel. Measurements were made before, during, and after winter rains. Our aim was to determine if seasonal fluctuations in the birds' water content corresponded with major changes in climate and phenological patterns.