Desert dreams: seeking the secret mammals of the salt pans

Natural History, Nov, 2003 by Michael A. Mares

On several forays into the Salinas Grandes--one of Argentina's great salt deserts--we had dug up mysterious burrows, but we had never discovered what kind of animal made them. Our team of biologists was looking for a rare mammal, a salt-pan specialist with a store of unique genetic information. Our failure was like a weight I carried with me every day. The bad news, I told my three co-workers, was that we would go back one more time. There were groans; it was a long, hard field trip. The good news, I said, was that we wouldn't dig anymore. Moving tons of earth had already proved to be no way to find our quarry, presumed to be a rodent.

The year before, I had brought some Conibear traps to Argentina, spring-loaded traps (named for their inventor, Frank Conibear) that can seize and kill animals without the need for bait. The problem had been figuring out where to place the traps so that animals would step into them. I had never been able to catch anything in Conibear traps before, but now I pinned my hopes on them. "There must be some reason I carted them down here," I told myself.

Back we went, to the Gran Chaco area of Central Argentina, the road by now familiar, and made camp. That first day I spread out my two dozen Conibear traps, while the others set our standard array of baited traps. There was nothing to do now but wait. In the frigid winter night, I dreamed that a new, rare, salt-desert rodent got caught in one of my traps. In my dream, though, the animal was dragged away by a predator before I could get to it. The thought jolted me wide-awake. It was 3:00 A.M., and the Sun wouldn't be up for hours. The cold stillness hardly beckoned me out of my sleeping bag. I rationalized: How could I find the trap in the dark, anyway? I decided to wait for dawn.

Salt deserts or salt pans--"salinas" in Latin America--are among the most extreme habitats on Earth. In some cases they formed where ancient seas once intruded on the land, then retreated and evaporated, leaving crystalline salts--mainly sodium chloride--behind. In other areas, where arid mountains surrounded enclosed basins, salts weathered from the uplands by seasonal precipitation were carried with the runoff into the valley below; again, when the water evaporated, the salts remained. But in whatever way the salts accumulated, eventually they covered the soil with a blindingly white patina. Few species, plant or animal, can survive in the forbidding habitat.

Among the exceptions are certain halophytic, or salt-tolerant, plant species in the goosefoot family (the Chenopodiaceae, or chenopods), which thrive in hot salt deserts throughout the world. The salt concentrations in the stems and leaves of these plants are many times what they are in seawater; the salts keep precious fluids in the plants' cells from being drawn by osmotic pressure into the salty soil. At the same time, as the water in their cells evaporates, the plants must prevent their internal salt concentrations from getting too high. Many halophytes compensate for evaporation by depositing salt crystals on the surfaces of their leaves, giving the leaves a grayish silver cast.

Halophytic chenopods, such as saltbushes, are green throughout the year, and their lush color might mislead one into thinking they are an ideal food source for plant-eating mammals. In fact, almost no mammals can process their high concentrations of salts. In three widely separated deserts, however, a few remarkable exceptions have been discovered, some only quite recently. All the species are rodents that have evolved highly specialized features to overcome the challenge of surviving on the salt-filled vegetation of the salt pans.

The first such species to be discovered was the chisel-toothed kangaroo rat (Dipodomys microps). Kangaroo rats belong to a North American family of rodents well known for living in arid habitats, where they forage almost exclusively for seeds. They seldom have access to drinking water, but instead get most of their moisture from digesting the seeds.

Even among kangaroo rats, though, the chisel-toothed species is unusual. Unlike other species in the family, which have pointed upper and lower incisors, the upper incisors of D. microps have a broad cutting edge, and the lower incisors have the bladelike profile of a wood chisel. Investigators who studied the animal in the Great Basin Desert of the western United State in the 1970s discovered that it scrapes saltbush leaves against the lower incisors, thereby peeling away the crystalline layer of salt. The succulent green leaf tissues that remain still contain high levels of salt, but the rat's specialized kidneys produce a highly concentrated urine that removes those salts. Those adaptations enable the rodent to rely on the evergreen saltbush plants for sustenance throughout the year. But for many years ecologists thought it was the only mammal to have solved the riddle of living on the poisonous saltbush.

Then, in the 1980s, biologists began to study another desert denizen, the fat sand rat (Psammomys obesus) of the northern Sahara. The species belongs to the same family as the house mouse and the Norway and black rats, but it is only distantly related to the kangaroo rat. Even the basic body forms of the two desert rodents underscore the remoteness of their evolutionary kinship. The kangaroo rat has long hind legs on which it hops--a bit like its Australian namesake in miniature. The fat sand rat looks more like a gerbil. Yet, just like the chisel-toothed kangaroo rat, the fat sand rat inhabits saltbush areas in a desert; it feeds on the saltbush; it scrapes salt crystals from leaves with its lower incisors; and its kidneys excrete a highly concentrated urine. Having embarked long ago on distinct evolutionary journeys in widely separated arid areas of the planet, the two species have converged in certain remarkable details of their anatomy, physiology, behavior, and ecology.