National Zoo Nutritionist Studies Complex Eating Habits of Wild Desert Tortoises
Observing desert tortoises (Gopherus agassizii) hunt for food must be a little like watching paint dry. Cruising speed for the animals is only about eight feet per minute—although who knows what a tortoise can do with the wind at its back.
But learning which foods a hungry tortoise shuns and which it stops to eat is an important part of efforts to keep the reptiles from disappearing from their range in the Mojave Desert of the western United States. Work to save the tortoise, by understanding what it puts in its stomach, is being led by researchers from the Nutrition Laboratory of the Smithsonian’s National Zoological Park.
Tortoises in the Mojave, which includes parts of Nevada, California, Utah, and Arizona, have been listed by the federal government as a threatened species since 1990. Once abundant, desert tortoises have suffered local population crashes of as much as 90 percent.
Loss of wild living space due to development is only partly to blame for tortoise declines. In many parts of their range, tortoises are having to share their limited food with livestock. On top of that, the food supply itself is changing. Additionally, scientists are concerned that these changes may have left tortoises prone to widespread disease.
Over the course of an 80 or even 100 year lifespan, a desert tortoises eats almost nothing but plants—but not just any plants. In a desert where rainfall is scarce to nonexistent, the tortoise's vegetarian diet must provide enough water and enough protein in the form of nitrogen for survival. At the same time, tortoises must avoid accumulating too much dietary potassium, as potentialy lethal levels occur in many desert plants.
"They're doing a balancing act between getting enough water and protein in their food and avoiding getting too much potassium," says research nutritionist Olav Oftedal, of the National Zoo. "As a consequence, we've predicted they'll be very selective in what they eat."
Head of the nutrition lab, Oftedal has studied the dietary requirements of animals from bears to bats. For the last decade, he has been researching the nutritional ecology of desert tortoises in two different parts of the Mojave.
At the Desert Tortoise Conservation Center, a 240-acre site near Las Vegas, Nevada, Oftedal and other scientists work with temporarily captive tortoises—animals that have been displaced by local development projects and are awaiting relocation to appropriate wild habitat.
"We do controlled feeding studies," says Oftedal, in which tortoises in pens are fed carefully formulated pellets that reflect varying nutritional characteristics. "The composition of these pellets is based on our research on the composition of the plants we collect in the wild," says Oftedal. So far, about 400 species of Mojave Desert plants have been analyzed at the Zoo's nutrition lab in Washington, D.C.
By looking at how dietary variations affect tortoise growth, reproduction, and overall health, the researchers are providing wildlife managers with information that can help in evaluating the quality of habitat that may be used for tortoise reserves.
How tortoises feed themselves is being studied in California, at the U.S. Army's Fort Irwin Training Center. Juvenile desert tortoises, hatched in fenced outdoor enclosures, are observed while they forage for food. "We watch them and keep a tally of which plants they pass, which they stop and bite, how many bites they take, and what parts they're eating," explains Oftedal.
In spring 2001, a study of 15 juveniles recorded more than 33,000 bites for leaves, flowers, fruits, or the occasional piece of gravel or scat. That kind of detailed observation is necessary because the nutritional composition of plants varies widely according to the life stage at which a plant is eaten and the plant part that a tortoise selects.
Watching two-to-three-inch-long juvenile tortoises munch on evening primrose, desert dandelion, and other plants allows researchers to evaluate what they call the PEP index (for Potassium Excretion Potential) of the reptile's diet. A measure of the relative amounts of potassium, nitrogen, and water in foods, the PEP index provides one scale for rating the nutritional value of desert tortoise food plants. For example, a diet with a high PEP index has a surplus of nitrogen and water and relatively low amounts of potassium.
In a paper presented in March 2002 at a meeting of desert tortoise researchers, Oftedal discussed study results suggesting that juvenile tortoises seek out a high PEP diet when it is available, such as when winter rains encourage the growth of a broad range of annual plants in the desert. Because such a diet provides protein and water beyond what is needed to excrete excess potassium, it favors growth of the young tortoise's shell, muscles, and internal organs.
A bigger, stronger shell provides protection from being eaten by ravens, one of the chief predators of juvenile tortoises. Muscle and internal organs can store nutrients that enable tortoises to survive lean times when foods are scarce. Thus a high PEP index diet should enhance survival of juvenile tortoises.
Over the years, the bulk of funding for the Zoo's tortoise nutrition research has come from the U.S. Bureau of Land Management. That's because vast stretches of desert tortoise habitat are on federal land that ranchers use for grazing livestock. "One of the political issues in the conservation of desert tortoises has to do with restrictions on livestock grazing," says Oftedal. "Based in part on our study results, the Bureau of Land Management has restricted grazing in southern Nevada."
Annual wildflowers and grasses that are choice tortoise foods are also heavily consumed by cattle and sheep. Cattle also trample the slow-moving tortoises and the burrows in which the reptiles spend more than 90 percent of their life.
For desert tortoises, modern life has meant a host of troubles. Off-road vehicles racing across the desert flatten tortoises and their burrows. Over the course of the 20th century, hundreds of thousands of desert tortoises were plucked from the wild to become pets. "There was quite a fad of keeping them in the twenties.... We used to call them Hollywood Bedbugs," wrote Wallace Stegner in his novel Crossing to Safety.
One of the most serious current threats to tortoises is the spread of nonnative grasses and other alien plants that "are outcompeting our native wildflowers that the tortoises eat," says Kristin Berry, a U.S. Geological Survey biologist who has long studied tortoise population declines. "We're seeing more and more tortoises that are ill," says Berry, who worries that changes in what plants are available to tortoises may be partly to blame.
"I think what Oftedal has been doing for the last ten years is very significant," she says of the National Zoo researcher's investigations of desert tortoise nutritional ecology.
In the long run, whether the desert tortoise maintains a foothold in the Mojave depends in part on what scientists like Oftedal learn about what the reptile eats and about what it will take to keep the tortoise's desert garden growing.
Results of Oftedal's work appear in a chapter in the 2002 book, The Sonoran Desert Tortoise. Natural History, Biology and Conservation. more
A shorter version of this article by Michael Lipske first appeared in The Torch, the Smithsonian Institution’s employee newsletter.
—by Michael Lipske
