Thirst and drinking in North American watersnakes (Nerodia spp.)

Abstract

We quantified drinking behavior in three species of North American watersnakes: Nerodia clarkii, which is a marine or brackish water amphibious species, and Nerodiafasciata and Nerodiataxispilota, both freshwater amphibious species. All three species have relatively small and similar thresholds of dehydration (TH, approximately -4% loss of body mass) that elicit thirst and drinking of fresh water. These species have higher thirst sensitivity than several species of hydrophiine and laticaudine sea snakes, which are characterized by much lower TH (greater dehydration, -9% to <-20%). Nerodia clarkii, which is often found in coastal oceanic water, refused to drink seawater, but drank fresh water when dehydrated. In separate trials involving dehydration of N. clarkii and N. fasciata that were concurrently fed fish at regular intervals, snakes eventually refused to eat at TH of approximately -12% of original body mass, but resumed eating after they were allowed to drink fresh water and rehydrate. The drinking behaviors of Nerodia corroborate previous data on the importance of fresh water for drinking, and they complement growing evidence that dietary water does not itself mitigate dehydration in snakes. These new data increase understanding of water relationships in the context of evolutionary transitions from land to sea, and they emphasize the importance of fresh water resources in the conservation of coastal and marine species of reptiles.

Keywords: Dehydration; Osmoregulation; Snakes; Thirst sensitivity; Water; Water balance.

Fig. 1.

Dehydration threshold (TH) in three species of North American watersnakes of the genus Nerodia. (A) Mean dehydration threshold for thirst or freshwater drinking, expressed as a negative percentage of initial body mass that is lost during dehydration. The bars are statistically similar. (B) Mean amount of water that snakes drank at TH, as a percentage of the dehydration deficit. The bars are significantly different. Lines associated with bars represent 1 s.e.m., and further details concerning statistics can be found in Results and Discussion.

Fig. 2.

Plot of relative changes of body mass in a 15.6 g watersnake (Nerodia clarkii). This snake was dehydrated with intermittent feeding but without access to water (left of vertical dashed line). Arrows indicate when the snake refused food; it resumed feeding following rehydration by drinking. Filled circles indicate the mass of the snake while dehydrating; open circles indicate the mass of the snake while it had access to fresh water (right of vertical dashed line); open squares indicate the mass of the snake following voluntary ingestion of live fish. Note that at the end of the dehydration period (vertical dashed line), the snake did not eat immediately (day 48), but began feeding (day 50) after a few days when drinking had restored its starting mass. The mass of the snake was not measured on days without data points.