Meal consumption is ineffective at maintaining or correcting water balance in a desert lizard, Heloderma suspectum

Many xeric organisms maintain water balance by relying on dietary and metabolic water rather than free water, even when free water may be available. For such organisms, hydric state may influence foraging decisions, since meal consumption is meeting both energy and water demands. To understand foraging decisions it is vital to understand the role of dietary water in maintaining water balance. We investigated whether meal consumption was sufficient to maintain water balance in captive Gila monsters (Heloderma suspectum) at varying levels of dehydration. Gila monsters could not maintain water balance over long time scales through meal consumption alone. Animals fed a single meal took no longer to dehydrate than controls when both groups were deprived of free water. Additionally, meal consumption imparts an acute short-term hydric cost regardless of hydration state. Meal consumption typically resulted in a significant elevation in osmolality at 6?h post-feeding, and plasma osmolality never fell below pre-feeding levels despite high water content (̃70%) of meals. These results failed to support our hypothesis that dietary water is valuable to Gila monsters during seasonal drought. When considered in conjunction with previous research, these results demonstrate that Gila monsters, unlike many xeric species, are heavily reliant on seasonal rainfall and the resulting free-standing water to maintain water balance.