Castaway Cottonmouths Depend on Rain to Slake Thirst

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Castaway cottonmouths depend on rain to slake thirst after losing less than 2% mass while others from the same population did not drink until they had lost more than 12%’, says Sandfoss. However, when the scientists compared the two groups of animals, there was no difference in their thirst threshold, so the castaways don’t seem to have altered their thirst tolerance in response to their tropical-island lifestyle.

The pair then investigated how the two populations would react when offered a drink of salt water. ‘We know that even the most well-adapted sea snakes require fresh water for drinking and this made us think that insular cottonmouths would not be able to rely on seawater’,says A cottonmouth snake (Agkistrodon conanti) basking on a Cedar Keys beach. Sandfoss. Discovering quickly that both Photo credit: Mark Sandfoss. populations turned up their noses at full- strength – and even 25% – seawater, Castaways usually meet their fate by Lillywhite’s lab in 2013, Sandfoss began Sandfoss then offered thirsty mainland washing ashore on a desert island, but not comparing Lillywhite’s observations of and island cottonmouths increasingly the cottonmouth pit vipers (Agkistrodon how likely the snakes were to drink with dilute saltwater drinks and found that conanti) of the Cedar Keys, Florida. ‘Our the amount of rainfall on the islands and most of the island snakes (19) refused the understanding is that these islands formed found that the animals that had salty drinks: only 7 chanced a sip. in the recent past as relic sand dunes that experienced the longest drought gulped However, the mainland cottonmouths were connected to the mainland via dry down the largest volume, while snakes were far less discriminating, with land until sea levels rose’, says Mark that had experienced rainfall in recent 6 sipping the saltiest (30%) water and only Sandfoss from the University of Florida, days were less eager to drink. However, 7 holding off until they were offered fresh USA. And when the reptiles found when the scientists compared the island water. Also, when the duo compared the themselves cut off from sources of fresh cottonmouths’ drinking habits with those water preferences of newborns, it seemed water, they really were left high and dry. of their mainland cousins, which live in that the island youngsters had inherited ‘Cottonmouths are almost exclusively swamps and rivers, 64% of the island their parents’ aversion to salt water. found in freshwater habitats’, explains animals were dehydrated while only 24% Sandfoss; which made him and his thesis of their mainland cousins needed a drink. Castaway cottonmouths have evolved the advisor, Harvey Lillywhite, wonder how The duo realised that the island ability to detect and avoid drinking the marooned serpents manage their thirst cottonmouths depend on rainwater and go seawater, even though the prospect may when surrounded by undrinkable thirsty between rainfalls. seem tempting when the rains fail, but do seawater. not supress their thirst, and Sandfoss is The scientists then wondered whether the keen to find out how the marooned snakes ‘Harvey has been collecting drinking data water-deprived castaways were able to sniff out the difference between fresh and on free-ranging cottonmouths for suppress their thirst more than the salt water when feeling parched. decades’, says Sandfoss, describing how mainland population, so Sandfoss Lillywhite picks up the vipers with a allowed island and mainland snakes to go 10.1242/jeb.205526 hook, weighs them and offers them a bowl thirsty and then recorded the animals’ Sandfoss, M. R. and Lillywhite, H. B. (2019). of water to drink. He then reweighs the mass when they decided that they needed Water relations of an insular pit viper. J. Exp. Biol. animals immediately after they have a drink. ‘We found a large variation in the 222, jeb204065. doi:10.1242/jeb.204065 slaked their thirst to determine how much thirst responses of snakes from the two Kathryn Knight water they have consumed. After joining populations, with some snakes drinking [email protected]

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