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Desert Shell Game
by Howard Youth
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The desert tortoise doesn't get anywhere fast. On its rare forays, this football-sized reptile covers a stupefying eight feet per minute. Why hurry? After all, it's already ambled across desert landscapes for ten million years.

Although it's not built for speed, Gopherus agassizii excels at surviving in a land of extremes, where a nutritious meal may not be in the offing for two or more years. A desert tortoise spends about 95 percent of its life in underground burrows, escaping cold or searing heat. But despite the species' low profile, it captures a lot of attention: To those who know and love the Mojave and Sonoran deserts, life without it would be like a Southwest without cacti. It's California's state reptile, and state and federal agencies consider it an indicator species for the health of desert habitats.

But the times are catching up with the desert tortoise. Its shell game—emerging to find bountiful food one year and perhaps none the next—is becoming more treacherous, and its future less certain. Disease, poor nutrition, sprawl, growing traffic, and fires are not only front-page news for humans, they are also factors contributing to desert tortoises becoming scarce in many parts of the arid Southwest over the last 20 to 30 years.

Although the desert tortoise is the best-studied reptile in the Southwest, much remains to be learned about exactly what is driving its declines. Biologists, disease experts, and nutritionists, including some at the Smithsonian's National Zoo, are trying to demystify the tortoise's natural history and conservation requirements. Because the tortoise's fortunes are tied in with those of many other desert creatures and plants, the battle to save it is in many ways a battle to save the desert.

The Tortoise and Its Lair
Desert tortoises live in a wide variety of dry habitats, from flat expanses of the western Mojave Desert sparsely foliated with creosote, white bursage, and Joshua trees to rocky, hilly upland portions of the Sonoran Desert characterized by towering saguaro cacti and feathery paloverde trees. In the United States, desert tortoises live in parts of four states: southern Nevada, southeastern California, extreme southwestern Utah, and western, central, and southern Arizona. In Mexico, they are found as far south as southern Sonora and northern Sinaloa states. In this southern portion of their range, desert tortoises are even forest dwellers. Crawling beneath dense thickets, they may share thorn-scrub forest with military macaws (Ara militaris), parrot snakes (Leptophis diplotropis), and Mexican tree frogs (Smilisca baudinii).

Ongoing lab tests and field studies show that there are genetic, ecological, physiological, and behavioral differences between desert tortoise populations, leading some scientists to conclude that Mojave and Sonoran populations have been isolated from each other for about 5.5 million years. Because of this work, the desert tortoise may in the not-too-distant future be considered desert tortoises—a set of similar-looking subspecies or species.

But taxonomy matters not to a desert tortoise sitting at the mouth of its burrow near Tucson or Las Vegas. This creature may have hatched when Dwight D. Eisenhower was president, more than 50 years ago.

desert tortoise
Desert tortoises spend most of their time in burrows. (Beth Jackson/USFWS)

It used to be that such a ripe old age was common for a tortoise, as long as it survived its first vulnerable decade or two. But many modern dangers now befall adults and young alike, and when populations decline precipitously—as many have in the western part of the tortoise's range—recovery may take centuries, if it comes at all. Tortoises don't reproduce like rabbits. It takes them 15 to 20 years just to reach breeding age. Although Sonoran tortoises rarely lay more than one clutch a year, Mojave females often lay two or, rarely, three in good years; the average clutch of five eggs takes 90 to 120 days to hatch.

Throughout its range, the desert tortoise is the only land turtle, and finding shelter is a key to its survival. Using its strong claws, a tortoise gouges out burrows that extend up to 30 feet long. Desert tortoise burrows host not only their makers, but other creatures large and small seeking escape from the sun, cold, or predators. Within the wide-mouthed, shady tunnels, Andrew Walde, a Helendale, California-based wildlife biologist, has found, or found evidence of, seven lizard species including desert iguana (Dipsosaurus dorsalis), six snake species, burrowing owl (Athene cunicularia), horned lark (Eremophila alpestris), a whip-poor-will relative called the common poor-will (Phalaenoptilus nuttallii), six rodents, desert cottontail (Sylvilagus audubonii), black-tailed jackrabbit (Lepus californicus), and kit fox (Vulpes macrotis). Walde has heard that American badgers (Taxidea taxus) and coyotes (Canis latrans) may also modify the burrows for their use. "My research into the desert tortoise burrows is in its infancy," says Walde. "We do know that virtually every desert vertebrate will go into a desert tortoise burrow. Some snakes and lizards seem to use them more frequently than other desert vertebrates." Walde has also found more than 100 invertebrate species in the burrows, including beetles, moths, ticks, mites, spiders, flies, silverfish, and cockroaches.

Although one tortoise may shuttle between seven to 12 burrows, most of desert tortoises' time is taken up doing nothing at all. "They spend a lot of their life in shut-down mode," says National Zoo research nutritionist Olav Oftedal. "Their strategy basically is to get down in a hole and wait it out. Their area of expertise is ‘if you don't like it, just wait. It's going to get better, hopefully.'"

As head of nutrition at the Zoo's Department of Conservation Biology, Oftedal has studied the nutritional requirements of a variety of animals and how they may be threatened by changes in the amount and quality of food available to them in their habitats.

Since 1991, Oftedal has often found himself in the Mojave Desert, keeping his eyes on the tortoise and its puzzling diet of desert greens, as well as watching captive tortoises and lab-testing food samples. His goal: To figure out if the tortoise's nutritional needs underlie their perilous state.

Fast Times for a Desert Ancient
While an adult tortoise's hard carapace and thick, tough-scaled legs protect it from many predators, they do little to ward off bulldozer treads, car tires, bullets, cattle hooves, fire, or disease. Humans play direct or indirect roles in all of these dangers, and conservationists have tried to bolster the tortoise's chances. In 1990, following sharp declines in Mojave populations and concerns over a rapid spread in disease and the expanding scope of development and other disturbances, the U.S. Fish & Wildlife Service (USFWS) included about 30 percent of desert tortoise populations on the Endangered Species List, including those in California, Nevada, Utah, and far northwest Arizona. These populations are now listed as threatened with extinction, and despite their presence on the List for the last 16 years, many declining populations continue to fall in numbers.

In developed areas like Tucson and Las Vegas, it's easy to see why. Once small human enclaves surrounded by broad desert expanses, these southwestern cities are spreading farther into the desert, bringing new roads, garbage dumps, subdivisions, shopping malls, and other barriers that stifle tortoise movement. While such construction is tough on tortoises, it benefits one of their main predators, common ravens (Corvus corax). This species, the largest of the crow family in North America, nests on power towers, raids trash cans and landfills, eats road kill, drinks fountain water, and is doing better than ever before.

American badger
Badgers and many other animals use desert tortoise burrows.

Over the last 37 years, raven numbers grew by some 800 to 1,400 percent in the Mojave and Sonoran deserts, according to a recent analysis of data from the North American Breeding Bird Survey, an annual bird count coordinated by the U.S. Geological Survey and the Canadian Wildlife Service. "Threats [to prey species including desert tortoises] from ravens may be particularly severe because predators are more likely to cause extinctions in prey when food subsidies [i.e. road kill or meat scraps at the dump] allow their populations to remain high as prey populations decline," wrote biologists William B. Kristan III, William I. Boarman, and John J. Crayon in the Wildlife Society Bulletin in 2004.

At hatching, young tortoises are about the size of a quarter. For their first five to ten years, the small reptiles have soft shells and are what Oftedal calls "walking morsels." Many fall prey to ravens, and others are killed by coyotes, kit foxes, western spotted skunks (Spilogale gracilis), golden eagles (Aquila chrysaetos), some snakes, southern fire ants (Solenopsis xyloni), and white-tailed antelope squirrels (Ammospermophilus leucurus).

Even more far-flung places can turn into tortoise danger zones. Off-road vehicles rumbling over open desert crack shells and flatten burrows. Military maneuvers at some desert bases necessitate the removal and relocation of desert tortoises that would likely be killed by tanks and other machinery or munitions. Despite legal protections that prohibit harming them, tortoises sometimes prove irresistible targets for rifle-toting pot-shotters. And sheep and cattle, which methodically chew down the scant vegetation tortoises need to survive, also sometimes trample the reptiles.

Although the landscape has become increasingly complicated for tortoises to navigate, under the USFWS species recovery plan for the Mojave desert tortoises, thousands of acres are now protected from many of these hazards. So why are tortoises still disappearing, even from the best-protected areas?

Fire is one of the less-predictable factors. In the summer of 2005, for example, 14,741 acres of the Red Cliffs Desert Reserve near St. George, Utah, were scorched; 10,446 acres, or 71 percent, of these were tortoise habitat at the northeastern extreme of the species' range. An estimated 38 percent of the reserve's adult tortoises died in the fire or were killed by smoke.

Many fires in the Mojave Desert, set off by lightning or by discarded cigarettes, hot tailpipes, or arsonists, spread fast and far, fueled by dense mats of non-native annual grasses that fill once-open gaps between native plants. Oftedal and his colleagues have also noted that these introduced plants, including Mediterranean grass (Schismus barbatus) and red brome (Bromus rubens), are not optimal tortoise foods and crowd out more nutritious native plants.

But invisible threats—pathogens—may do far more damage than fire or invasive exotic plants. As males battle over mates, pairs copulate, and individuals share burrows, many are becoming infected with the highly contagious upper respiratory tract disease (URTD). This ailment is spread via body fluids and caused by recently described bacteria Mycoplasma agassizii and M. testudinium.

Scientists can test wild tortoises for antibodies to Mycoplasma and know that some wild populations of desert tortoises and their relatives in the southeastern U.S., gopher tortoises (Gopherus polyphemus), have been exposed to the disease. Visible symptoms of active infection include swollen eyes, runny nose, and labored breathing. While these outward signs might not sound alarming, URTD causes lesions in respiratory passages in the head and is associated with large tortoise die-offs. As far as is known, infected tortoises never recover, but they may enter a long, dormant phase in which they do not exhibit symptoms. URTD is widespread in the western Mojave and some other areas, but its occurrence varies.

In 1988, URTD was first linked with a major wild tortoise die-off, in southern California's 24,700-acre Desert Tortoise Research Natural Area, a protected haven for a particularly large population of the reptiles. Between 1979 and 1992, the total tortoise population density within the reserve dipped by 76 percent, while a 90 percent loss of adults was noted. "The deaths of [these] tortoises and the population declines are attributable to several causes. ...However, between 1988 and 1992 the declines of adults are clearly attributable to URTD caused by M. agassizii," wrote U.S. Geological Survey tortoise biologist Kristin H. Berry in the 1997 proceedings of an international conference on tortoise and turtle conservation.

Desert tortoises may be able to stave off sickness through proper nutrition. Oftedal, along with bacteriologist Mary Brown of the University of Florida in Gainesville and immunologist Jerry Simecka of the University of North Texas in Denton, plans to study the possible link between nutrition and the tortoise's immune response to URTD. "It makes sense that a downhill slide in tortoise nutrition would lead to both greater expression of disease in individual tortoises and to increased transmission of the disease throughout the population," says Oftedal. "That's what we plan to study in more detail."

"We now suspect that it was not just a 'Typhoid Mary' situation—that the Mycoplasma is probably a long-term resident in the population but when animals become stressed by, for example, prolonged drought, inadequate nutrition, habitat disturbance, or other environmental factors, they catch the disease and spread it to their weakened neighbors," says Oftedal. He adds, "Disease outbreaks probably arise due to a combination of human-induced and natural causes." URTD's epidemiology remains poorly understood. No one knows, for example, how long desert tortoise populations have harbored Mycoplasma, what conditions are required for rapid transmission, or why some outbreaks are so deadly, killing 70 to 80 percent of a population in a few years.

Another illness, cutaneous dyskeratosis, turned up in wild tortoises starting in the 1980s. It creates lesions on limbs and shell plates, or scutes, but likely indicates more serious illness. Cutaneous dyskeratosis may have contributed to some localized population declines, such as a 54 percent population drop and 61 percent adult tortoise decline noted between 1982 and 1992 in another California tortoise reserve, the Chuckwalla Bench Area of Critical Environmental Concern. Pathologists aren't yet sure of its cause, but speculate that nutritional deficiencies or naturally occurring or human-wrought toxic chemicals may play a role. Herpesvirus also poses potential threat to captive and wild tortoises.

Keeping in Step with PEP
Oftedal and his Zoo nutrition lab colleagues have analyzed thousands of plant samples from about 400 species, and observed tortoises take tens of thousands of bites out of plants in the Mojave and Sonoran deserts.

Thanks to this painstaking work, they now know that tortoises count on plants that seldom appear. In areas such as the western Mojave and northwestern Sonoran deserts, if rainfall arrives, it usually comes during winter, when desert tortoises are dormant in their burrows and their metabolic rates are low. When tortoises emerge in spring and become active, there may be little rainfall and, consequently, few plants. "They basically have to get all of their water from their food," says Oftedal, "and in dry years they may not get any food for more than a year."

Unlike many other reptiles, including desert iguanas, desert tortoises do not have specialized salt glands to excrete excess potassium—an element that is abundant in many desert plants because it helps draw moisture from dry soils—but frequently consume plants with higher concentrations of potassium than they can void in fluid urine. Oftedal says that this situation is akin to humans drinking seawater.

evening primrose
In wet years, desert tortoises seek out evening primroses and other high-PEP plants. (Charles T. Bryson/USDA ARS)

Desert tortoises cannot afford to accumulate potassium in their bodies because it is potentially lethal. (Potassium salts, for example, are used for euthanasia.) So, to get rid of potassium, Oftedal says they "rob Peter to pay Paul": They divert the protein in their food to produce a waste product, uric acid, that precipitates in their bladders in the form of potassium urates. On high-potassium diets, tortoises must use their protein for potassium excretion rather than for growth of tissue or development of eggs. Under these circumstances, potassium robs them of nutrients they would otherwise use to maintain body condition, which is particularly detrimental for females striving to produce eggs.

In dry years, tortoises may find no food or just some shriveled cacti or dried grasses. In moderate-rainfall years, they find a limited array of foods, including some non-native grasses, and just get by. But in wet years, when winter rains bring an inch or more of water to nourish abundant spring vegetation, Oftedal and his colleagues have found that tortoises become extremely selective in what they eat. In these years, tortoises seek out plants with high levels of what Oftedal calls PEP—Potassium Excretion Potential. High-PEP plants are high in protein and water relative to potassium, so there is plenty of surplus for the tortoises to store in their tissues and developing eggs. "With high-PEP plants," says Oftedal, "tortoises can make up this difference and forge ahead. This is very important in terms of reproduction, juvenile survival, and disease."

High-PEP plants include various legumes and evening primroses; some plants, such as desert dandelion (Malacothrix glabrata), produce high-PEP leaves that small tortoises seek. Most of these plants are annuals that survive from year to year as seeds in the soil but only germinate in response to heavy winter rains, showing in the spring. "Tortoises really chow down when they get a big rainfall year like an El Niño year, when flowers carpet the deserts," says Oftedal. This might happen only every four or five years, between which a drought or two may hit and modest rainfall years fill the gaps. Like many other aspects of the tortoise's biology, the situation varies from place to place and year to year due a combination of time, temperature, and rainfall.

Oftedal and others worry that stressors such as invasive plants and overgrazing further push tortoises to the brink by cutting supplies of already sporadic high-PEP plants. A 1998 U.S. Geological Survey study found that in at least part of the Mojave Desert, cattle went after some of the same high-PEP foods as tortoises, consuming them before tortoises could.

Some of Oftedal's work has helped influence land-management decisions in cattle country leased out by the Bureau of Land Management, or BLM. The BLM, part of the U.S. Department of the Interior, oversees about 75 percent of remaining high-quality tortoise habitat, but in many parts of the West, environmentalists accuse it of favoring cattle concerns over those of wildlife.

This was not the case in southern Nevada, where hard-scrabble pastures are now bathed in the glow of booming Las Vegas. There, The Nature Conservancy helped mediate the buyout of ranchers' allotments on BLM land and local, state, and federal agencies worked with developers to forge the Clark County Habitat Conservation Plan. The plan ensures conservation of tortoise and other wildlife habitat while allowing development in some desert tortoise areas, including the Las Vegas metropolitan area. Developers pay mitigation fees that fund research and the Desert Tortoise Conservation Center southwest of Las Vegas, where many tortoises displaced by development are relocated.

habitat near St. George, Utah
Desert tortoise habitat near St. George, Utah. (Beth Jackson/USFWS)

On some former BLM grazing areas, Oftedal and his colleagues have documented the return of high-PEP plants after cattle were removed in the early 1990s. Their survey work, funded in part by Washington, D.C.-based Defenders of Wildlife and the California-based Redlands Institute, also documented a high abundance of high-PEP plants in the northeastern and western Mojave Desert and in the Sonoran Desert. "But the vast bulk of the Mojave had low densities and we suspect livestock have something to do with this," says Oftedal.

Oftedal hopes conservationists can move along the recovery of high-quality tortoise habitat. He is working with the U.S. Department of Agriculture and the University of Arizona in Tucson to propagate high-PEP plants for desert restoration programs that reclaim fire-scorched, overgrazed, or bulldozed areas. Right now few, if any, restoration projects use legumes or other plants beneficial to desert tortoises.

Big changes are afoot in tortoise country, but Oftedal and many other conservationists and concerned citizens have the plodding tortoise's welfare in mind. "It's the large vertebrate that people identify with deserts out there," says Oftedal. To many, the tortoise is the desert and it is hard to imagine hiking in the arid open and knowing there are no tortoises down there, burrowed under the ground with countless other creatures, waiting for the green flush that follows a good winter rain.

—Contributing editor Howard Youth writes on a variety of conservation-related issues.

ZooGoer 35(4) 2006. Copyright 2006 Friends of the National Zoo. All rights reserved.

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