Water and electrolyte budgets of a free-living desert lizard,Sauromalus obesus

Abstract

1. Seasonal water metabolism in the herbivorous desert lizardSauromalus obesus (chuckwalla) was measured with tritiated water (HTO). Electrolyte budgets were obtained from field and laboratory measurements. These data describe the roles of various excretory organs in maintaining balance and permit evaluation of some aspects of the physiological ecology of these lizards. 2. When vegetation was succulent, chuckwallas obtained more than enough water from their food to meet their needs (Table 5). The excess was excreted rather than being stored (Table 1). 3. Although water requirements of chuckwallas maintaining constant weights are low in comparison to some other desert vertebrates, metabolic water production is insufficient to balance losses. These lizards must eat succulent vegetation to maintain water balance. 4. When vegetation was dry (summer), chuckwallas remained in crevices most of the day and did not eat. This behavior resulted in a considerable reduction in water loss. 5. These lizards did not drink rain water. 6. During drought, chuckwallas became dehydrated (Fig. 2), but fractional fluid volumes were maintained at normal levels (Table 1), as were plasma electrolyte concentrations (Table 2). 7. Chuckwallas entered winter dens in October weighing only two-thirds as much as in April (Fig. 1). They did not grow in length during 1970. 8. The diet was always hyperosmotic with K+ predominating, but this load was excreted by the nasal salt glands and as precipitated potassium urate (Table 7). These avenues of electrolyte loss require little water (Table 6), resulting in effective separation of water excretion from electrolyte excretion.