by Whit Gibbons

September 3, 2006

Chris Winne, a University of Georgia graduate student, studies the ecology of snakes at the Savannah River Ecology Laboratory. I asked him to describe some of the strategies that aquatic animals use to respond to the natural phenomenon of drought. Here is his response.

"Everyone with a lawn knows that when it doesn’t rain for a long time you must water the grass to keep it alive. Have you ever thought about what aquatic animals do when it doesn’t rain and their water source disappears? The animals with the most extreme challenges may be aquatic species that live in isolated wetlands, those not connected to a permanent water source. Wild animals can’t turn a sprinkler on themselves or refill a pond that has dried up. But some species do have various strategies for coping with a severe drought. These fall into two main categories: skedaddle or hunker down, more commonly known to scientists as migrate or estivate.

"Scientists have long known about the ability of some aquatic species to migrate overland. Perhaps the most striking example, familiar to anyone who has seen a nature show on this topic, is the walking catfish, which uses its pectoral fins to scoot out of a drying body of water and then travel overland to a deeper pool. Numerous animals, including many species of semiaquatic turtles, migrate to more permanent wetlands during drought, with some migrations being well over a mile. Similar long-distance overland migrations occur in some watersnakes, frogs, and salamanders. Aquatic mammals, such as beavers or otters, will also travel to another area to avoid drying conditions. Birds and winged insects dependent on water can obviously fly to more permanent wetlands when the water disappears. Interestingly, some wading birds actually prefer to forage in drying wetlands because aquatic prey that are incapable of migrating make easy prey for them to feed on.

"However, not all species are equipped to move overland for any distance. The most aquatic species in a wetland, especially the great majority of fish, are generally the least suited for overland migration. Some species of animals have highly permeable skin that causes them to lose too much body water if they venture onto land. Others have inefficient terrestrial locomotion (most fish) or such small body sizes that they would be unable to reach a faraway aquatic site. For such species the only possible outcomes are to die (fish again) or to reduce activity and wait out the drought, that is, to hunker down.

"The scientific term for the hunker down strategy is 'estivation,' defined as a state of inactivity and reduced metabolism. (For those of you who thought it meant the opposite of 'hibernation,' you're right. But English is a living language and, like other living things, words evolve.) In any case, some species, especially insects, fish, amphibians, turtles, and snakes, use this strategy to survive droughts. Many invertebrate organisms have life history stages that remain dormant during droughts and then awaken when the environment becomes wet again. A few fish, such as the lungfish of Africa and Australia, secrete a cocoon to protect them from water loss and then reduce their metabolic rate.

Several species of large-bodied salamanders are capable of secreting cocoons, reducing their metabolism, and surviving for at least two years buried under dried wetlands. Some freshwater turtles, such as chicken turtles and mud turtles, burrow in the dried wetland or in the surrounding terrestrial uplands to survive drought. They do not form cocoons but do decrease their metabolic rate to conserve energy. Recent studies have demonstrated that some aquatic snakes estivate during long-term droughts. Black swamp snakes, a southeastern species and the most aquatic of U.S. snakes, were able to estivate and survive the drought of 2000-2003.

"Lawns can be tended and cared for by those who value them. Creatures in the wild must rely on other tactics to ensure their survival. And they have, indeed, developed some effective methods of self-preservation. For some species, two such strategies--migration and estivation--often prove to be successful means of surviving drought.

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