The Quest to Quench
Animals have evolved some remarkable adaptations to wring water from even the most inhospitable habitats
- Doug Stewart
- Jun 01, 2004
THE TEXAS HORNED LIZARD is one of the fiercer-looking inhabitants of the arid American Southwest. When threatened, the reptile splays its legs wide, lowers its head menacingly and arches its spike-covered back like a tank ready for war. Oddly, the reptile assumes much the same posture during summer rains.
"It’s a rain-harvesting stance," explains biologist Wade Sherbrooke, director emeritus of the American Museum of Natural History’s Southwestern Research Station in Arizona. In the desert, a light rainfall normally disappears into the sand like water into a sponge. And Phrynosoma cornutum’s unusually wide body is perfectly shaped to catch the moisture before it’s lost. The reptile draws the water—by capillary action—through hair-thin channels between scales on its body that extend all the way to its mouth. The creature does not have to move, just swallow.
Sherbrooke has observed horned lizards doing something stranger still: "When the rain stops, they drop down and rub their bellies on the moist sand." He assumed at first the creatures were scent-marking. But based on his studies of the equally bizarre-looking Australian thorny devil, Sherbrooke believes horned lizards use their water-channeling hide to suck moisture right out of the sand and, in defiance of gravity, carry water up to their mouths.
The lizard’s water-gathering technique is just one of the adaptations animals have evolved to wring water from even the most inhospitable habitats. From a purely physiological point of view, life for most wild creatures is an unending effort to take in water. They manage to do so in an astounding variety of ways, while also struggling to avoid losing water too rapidly through evaporation, respiration or elimination.
Every animal is, in essence, a watery solution inside a not-always-watertight bag of skin. And maintaining that solution at just the right strength is crucial for life itself. Starting in the 1950s, Duke University physiologist Knut Schmidt-Nielsen carried out a series of groundbreaking studies on how different creatures—from birds to camels—maintain their water balance. For land animals, the key struggle is avoiding dehydration; freshwater fish, meanwhile, have to keep water out, not in.
Where Schmidt-Nielsen looked at organisms as a whole, in habitats wet and dry, research in today’s more high-tech era usually proceeds on a finer scale. Earlier naturalists would ask themselves, "How does this animal solve its problems?" But now researchers are more apt to investigate which control mechanism turns on a salt gland or turns down a kidney. Underlying every insight is the constant theme of how certain adaptations, both physical and behavioral, allow an animal to flourish in its particular habitat.
Follow the Water
For birds and mammals exposed to the fully dehydrating effects of sun and wind, the most obvious adaptation is nomadism. Unlike seasonal migrants, nomads are opportunists. Some strong-flying birds, perhaps lured by distant clouds, will fly for miles just to drink from a pond of rainwater. In dry areas, large grazing animals often depend on widely spaced watering holes and so must be able to go for days or longer without drinking. Finding water, they will drink their fill more as a precaution than out of thirst. But a huge number of species in the world’s arid regions, from sand rats to giraffes, can get all the moisture they need from plants. Carnivorous snakes and lizards take in most of their water from the flesh of their prey.
That sounds plausible enough (even a meal of ants is almost two-thirds water), but what about flour beetles, which can spend their lives in bone-dry flour bins? How do they find moisture? In part, they manufacture it. The H (hydrogen) and O (oxygen) in H20 are, along with carbon, chemical constituents of all carbohydrates. Rearranging the molecules to make water takes only energy. Humans actually perform this feat as part of metabolism. Flour beetles, clothes moths and termites convert carbohydrates to water too, but far more efficiently than we do. (Moreover, even "bone-dry" flour normally is 5 to 10 percent water.)
WINGING ACROSS Hawaiian waters near Midway Atoll, this Laysan albatross spends several months at a time at sea, drinking saltwater freely. Rather than having to urinate constantly to release the excess salt in its body, the seabird has glands in its head that shed hyper-salty tears through its nostrils.
Insects: Keeping the Water In
However they obtain water, terrestrial insects are stingy in letting it go. Their hard outer covering, which is more watertight than a mammal’s hide, cuts down on evaporation. Also, like birds and reptiles, the insects reclaim most of the moisture in their wastes before releasing them. Isopods such as wood lice actually reverse the usual direction of water flow: They absorb moisture through the anus, which means they quench their thirst just by sitting down.
Amphibians: Soaking Up Water and Waterproofing
While an insect’s "skin" is relatively watertight, an amphibian’s is highly porous. Amphibians actually use their skin, not their mouths, to take in water. Although they require moist surroundings to avoid death by dehydration, a surprising number of amphibians do quite well in arid deserts. Couch’s spadefoot toads, for example, spend most of the year buried like stones, cool and moist, in their Southwest habitat. Only heavy summer rain clattering on the hard desert above the toads prompts the creatures to emerge. In shallow ponds of rainwater that appear, the toads noisily mingle and mate. Their progeny’s youth is fleeting. A tadpole needs only ten days to mature into a four-footed, air-breathing metamorph capable of hopping away, but many desert ponds dry up well before then. The tadpoles are left in smaller and smaller ponds until ultimately the desert is dotted with pancakes of former amphibians. Still, enough survive in larger pools to keep populations going.
IN THE SONORAN DESERT of Arizona, a Couch’s spadefoot toad digs itself into the mud after a seasonal rainstorm. The amphibian spends most of the year buried beneath the surface, where it absorbs moisture out of the desert soil.
Though adult spadefoots and other frogs and toads can absorb water while buried, another amphibian, the siren, has evolved a different mechanism for staying moist during dry spells. Sirens are salamanders that live in wetlands in the Southeast called Carolina bays, which are prone to drying up. These large eellike creatures are so slimy that field biologists put dry socks on their hands before trying to grip them.
"When their habitat dries up completely, instead of traveling overland to find water, the sirens burrow down into the mud," explains herpetologist Whitfield Gibbons of the University of Georgia’s Savannah River Ecology Lab. "They secrete a mucus coating that hardens underground to form a cocoon that’s impermeable to water." The sirens remain moist but inert for months and perhaps years, he says, "until it rains again."
Mammals: Conserving Water, Keeping Cool
Mammals, of course, are incapable of waterproofing themselves, so they have developed other strategies. The camel can lose more than a quarter of its body weight in water without suffering. By comparison, humans who lose one-tenth their weight in water can become deaf and delirious.
Many large grazing animals keep evaporation low by seeking shade and staying quiet during midday. Still, they need water to drink. The oryx, by contrast, doesn’t need a drop. The East African antelope doesn’t even seek shade during the hottest part of the day. How the oryx pulls off this neat trick was discovered in the late 1960s by physiologists C. Richard Taylor, now at Harvard, and David Robertshaw, now at Cornell. As the day heats up, they found, the creature slows its metabolism and stops sweating altogether. It simply gets hot rather than fighting the heat. "The oryx lets its body temperature rise during the day and then dumps the heat after the sun goes down," says Robertshaw. It waits, in other words, for the night air to cool its body, rather than wasting water on sweat during the day. As for water intake, the oryx grazes before dawn when even dead grass turns moist with dew.
If any mammal has completely kicked the water habit, it’s the tiny kangaroo rat, which lives in arid regions of the American Southwest. Everything about the creature is water-conserving. Ounce for ounce, its urine holds 14 times as much material in solution as does its blood. "It’s almost like syrup," says zoologist Jack Cranford of Virginia Polytechnic Institute. "In several species, the urine almost crystallizes when it hits the ground."
A kangaroo rat spends most of its life in a burrow, which it plugs up so the air within stays cool and humid—so humid that the seeds it caches there can swell with one-third their weight in water before the animal devours them. The rat’s nasal passages are narrow, so water vapor condenses on their linings when the animal breathes out. Like some other animals in dry habitats, the rat also converts some of its food into metabolic water. Cranford has observed captive kangaroo rats living for two years on a diet of dried seeds and no water.
Water, Water Everywhere
If the desert is the most challenging environment for obtaining water, is the ocean the easiest? Hardly. There is indeed water everywhere—but only one known vertebrate, the primitive hag fish, can drink seawater without somehow removing the excess salt. Still, oceanic birds such as albatrosses can spend more than a year at sea, drinking saltwater freely. So if the kidneys are removing excess salt, they must be producing ten gallons of urine for every gallon of seawater consumed—a seemingly impossible feat. But in a series of studies, Schmidt-Nielsen discovered that the birds’ secret turned out to be a set of salt glands in their heads, a bit like tear ducts that release hyper-salty tears, but through the nostrils. Reptiles such as sea turtles have similar glands.
Animals that live in saltwater have to avoid becoming pickled in the stuff. The fluids inside an oceanic fish are only about a third as salty as the ocean itself. Osmotic pressure acts constantly to suck water out of the fish through its skin and gills. To avoid drying out, the fish opens its mouth and drinks seawater. The problem is getting rid of unwanted salt. The kidneys of bony fish are not up to the task, so the gills take over much of the job.
For freshwater fish, the osmotic pressure is reversed: The fluids inside the fish are saltier than those outside (even freshwater contains some salt), so water tends to seep in through the fish’s skin and gills.
Whales and other aquatic mammals avoid dehydration in part by producing extremely concentrated urine, saltier than seawater itself. Aquatic mammals are even sparing in the amount of water they release when nursing. Schmidt-Nielsen once analyzed milk he had coaxed from a seal in Antarctica and found it had twice the fat content of whipping cream and less than half the water content of lean hamburger. Humans can’t hope to compete with that kind of water-conservation effort, no matter how many bricks they place in their toilet tanks.
Massachusetts journalist Doug Stewart wrote about butterfly behavior in the April/May issue.
Facts of Life
The Truth About Camels
Despite all of the stories, a camel doesn’t store water for its legendary pan-desert treks—not in its humps, which contain fat, nor in its stomach, which holds a mundane amount of liquid. Rather, according to Duke University physiologist Knut Schmidt-Nielsen, one of the camel’s most impressive water-conserving organs is its oversized nose.
"A camel that has been deprived of water under hot desert conditions is actually able to withdraw water from its own exhaled air," he explains. As with other mammals, the inside surface of a camel’s nostrils consist of whorled tissue called turbinates. But the similarity ends there. Rolled up like a papyrus scroll, the camel’s turbinates have a huge surface area that is also unusually dry and cool. Consequently, the camel’s nose effectively recaptures most of the moisture contained in the warm, water-saturated air moving out of its lungs.
In addition, camels have the unusual ability to allow their body temperature to creep several degrees above normal without breaking a sweat or suffering heat stroke. Instead, the camel simply permits the excess heat to dissipate when air temperatures drop during the nighttime hours.
How Birds Drink
Doves and pigeons are the only birds that can swallow water directly after drinking it. Others have to dip their bills and then flip their heads back to let the water run down their throats.
Coping with Heat and Drought
Plants face many of the same challenges as do animals when it comes to surviving drought and drying heat. They must breathe—that is, exchange atmospheric gases—which they do through tiny pores called stomata in their leaves and stems. Because plants lose moisture every time they open their stomata, a barrel cactus and many other desert plants minimize the loss by essentially holding their breath until nightfall, says Stanley Smith, a University of Nevada plant ecologist.
During the day, notes the scientist, the plants’ green leaves and stems perform the first half of the photosynthetic reaction—the capture of light energy—with stomata sealed tight. But at night, when temperatures drop and air humidity increases, these plants open their stomata to gulp in air and complete the photosynthetic reaction, assembling carbon dioxide gas into carbohydrates.
Some desert plants excel at obtaining and storing large amounts of water. The mesquite—the only tree to survive in the Mojave Desert—sends down roots 50 to 100 feet beneath the surface to tap deep groundwater flows. The spindly creosote bush has evolved small cells that can survive near-complete dehydration. Under conditions when all other desert plants totally shut down, the creosote bush continues photosynthesizing. How it does so remains one of botany’s great mysteries and the subject of much biochemical research.
At sea, a salmon drinks and desalinates seawater to stay hydrated. But after entering freshwater to spawn, the fish stops drinking altogether and absorbs water through osmosis.