The Kangaroo Rat

FOCUS NATURE OH, RATS! THERE GOES THE DESERT

Somewhere out in the desert dark, kangaroo rats scurry and scamper and scratch: seed stealers, snake scoffers, and the hinge on which a whole biologic world turns. Who would suspect it to look at them? Furry creations from a Steven Spielberg flick, the cute little hoppers shun the daylight and lavish their lives in the ceaseless gathering, hiding, and rearranging of seeds. But somehow in that seed-obsessive task, they spell the difference between lush grassland and desert scrub, according to the very latest kangaroo-rat research.

The kangaroo rat stands as the latest and perhaps most intrigu-ing addition to a short list of species whose presence patterns a whole ecosystem. It is what biologists call a keystone species. Most other examples are in watery systems. Sea otters eat sea urchins that would otherwise prevent the growth of kelp forests. Beaver dams create ponds. Mussel-eating starfish prevent the in-exorable shellfish from creating an intertidal monoculture. But the effects of kangaroo rats' activities are far more subtle and surprising - although no less crucial, based on a 12-year study of a series of fenced plots of desert near Tucson, conducted by James Brown, currently professor of biology at the University of New Mexico. Brown fenced off plots about the size of a residential lot. In some, he used poison to kill all the ants which are determined seed gatherers themselves. In some he used wire mesh with holes cut big enough to admit small mice but not kangaroo rats. In others he left no holes, excluding both small mice and kangaroo ratsbut not big rodent climbers like the pack rat. He expected subtle changes in the types of flowers and grasses that came and went with the rains. But to his astonishment, the plots from which the kangaroo rats had been excluded turned from desert into grassland. He doesn't know why. It couldn't have been just the seeds the kangaroo rats would have eaten, but didn't, because the seeds are many, but the ro-dents are small and live in densities of perhaps six per acre. Possibly the answer lies in the kangaroo rat's remarkably indus-trious life-style.

They are the preeminent rodents of the Southwestern deserts and a strong presence in California chaparral. They're also mar-vels of evolutionary engineering: they never drink water - al-though they can be taught to drink by a patient scientist. They get all their moisture from seeds and an occasional beetle. And (BELOW AND OPPOSITE PAGE) Bannertail kangaroo rats live in huge mounds with underground tunnels and chambers where they store the seeds they eat.

Text by Peter Aleshire Photographs by Marty Cordano

They hoard every drop. Their kidneys recycle moisture until the urine is a concentrated white paste. They never sweat. And their intricately chambered nasal cavities capture the moisture in their breath before it leaves their bodies.

They boast an exquisite sense of smell, useful for locating buried seed caches and finding true love in the scattered dark when the moon is full and the hormones are hot.

The rats also are proud owners of an intricately chambered inner ear tuned to low-frequency sound, such as the stealthy slither of snake scales across sand or the whisper of owl wings in the cloaking dark. The owls have feather baffles along the leading edge of their wings to cut sound, but this doesn't fool the kangaroo rat's middle ear. On a split-second reflex, the rodents leap from danger. They leap backward from a snake, which cannot change the length of its strike once launched. They leap to the side when an owl swoops, because an owl can easily adjust the length of its glide but can't veer to the right or left. Mostly the little hoppers escape.

But it's in their seed-gathering behavior that kangaroo rats probably have the greatest impact on their world. Here again, the little rodents have worked out remarkable adaptations to a harsh environment.

Some species build underground castles bulging with bushels of seeds. The mounds may be 10 feet in diameter, stick up a foot or two above the surface, and contain huge storage chambers three feet below the surface. Usually the mound is occupied by a single fiercely territorial individual, but females may let their children, especially daughters, hang around for six months or so. The mounds generally are passed from generation to generation, although sometimes they're conquered by upstart families.

Other smaller species of kangaroo rats live in modest burrows and bury small stashes of seeds throughout their loosely defined territory. They simply scratch out a shallow pit with their undersized front paws and then bury the several hundred seeds they had stuffed into their cheek pouches. Somehow they can remember the location of hundreds of these caches in their home territory. They constantly revisit the hideouts, checking for mildew, moving the seeds from one spot to another, fussing with them like a proud gourmet. Somewhere in all of this scratching and storing lies the difference between grassland and desert, at least in those transitional areas where there's enough rainfall to sustain year-round grasses. Perhaps the kangaroo rats so disturb the soil that grass seeds can't germinate. Perhaps the constantly turned soil dries out. Perhaps they expose seeds to birds. Perhaps they tip some competitive balance between grasses by removing and storing the larger seeds.

Perhaps they have some other completely unanticipated indirect effect, like the lost horseshoe nail that cost a kingdom.