Feeding begets drinking: insights from intermittent feeding in snakes.

An important question related to the survival of dehydrating animals is whether feeding provides a net gain of water - contributing postprandial free water and metabolic water - or, alternatively, whether digestion and assimilation of ingested food incur a net loss of water because of requirements for digestion and the excretion of resulting metabolic wastes. Here, I address the question whether voluntary drinking increases or decreases following the ingestion of food. Increased postprandial drinking implies that food consumption increases rather than decreases the requirement for free water, whereas decreased postprandial drinking suggests there is a net profit of water from food. Snakes are ideally suited for such inquiry because they feed intermittently, and the temporal separation of meals allows relatively clear examination of the associated patterns of pre- and postprandial drinking. Voluntary drinking associated with meal consumption was quantified during consecutive feeding trials in four species representing two families of snakes. Postprandial relative to preprandial drinking increased in all four species, indicating that eating increases the physiological requirement for water. These data add to a growing literature pointing to some generality that eating can have negative rather than positive consequences for fluid homeostasis in some dehydrating animals.