Sonoran Desert

Speeding Tortoises

by Danielle Brooks

For many desert tortoises, survival of the species is now a numbers game. To improve the odds, scientists at ASU West and The Phoenix Zoo are trying to give young tortoises a head start toward sexual maturity.

Slow and steady, the fabled tortoise eventually out-raced the hare. Times have changed. Today, the desert tortoise is in a race for its very life. The competition is smarter and much more brutal. Human beings are the new competition.

Once home to the few and hearty, the southwestern United States has experienced a population explosion in recent years. For example, Phoenix, Ariz., is home to more than 1 million people, making it the nation's seventh largest city. Surrounding cities, from Scottsdale to Las Vegas, have also traded cactus blooms for population booms. That trend may spell the doom of desert tortoises.

Desert tortoises once roamed freely throughout southwestern California, southern Nevada, and western Arizona. Today they are in trouble. The sad truth is that tortoises do not stand a chance against cars. Hundreds of the animals are crushed every year by cars or off-road vehicles. Pet tortoises released into the wild provide a new danger. These animals often take back deadly diseases to the wild.

That story is the same throughout the world. Tortoise populations in Africa, Asia, and South America are shrinking as human populations grow.

A group of researchers from The Phoenix Zoo and Arizona State University is working to give tortoises a head start on their reproduction. A goal of their Desert Tortoise Project is to help tortoise hatchlings grow up faster by changing their diets. This will let the animals lay more eggs over the course of their lifetimes.

"For many tortoises, survival of the species is now a numbers game," explains ASU biologist Harvey Pough. "Each female must produce at least two offspring who survive to produce just to keep the population size stable."

That job is tougher than it sounds. In the wild, desert tortoises do not reach sexual maturity—when they can produce babies—until age 20. They then lay, on average, three to seven eggs per year—which often get eaten by coyotes, snakes, and Gila monsters. Very few tortoise hatchlings survive their first year or two.

"A baby turtle is just an Oreo cookie to a predator," says Pough, a professor of Life Sciences at ASU West. "The death rate of juveniles is about 95 percent before age five."

Pough and his team are testing a diet developed by scientists at the National Zoo in Washington, D.C. Those scientists found that desert tortoises grew faster when they ate high-protein diets instead of greens. They grew to full size in just five years instead of the typical 20 years.

The National Zoo study did have a weakness. Many of the subject tortoises were captured in the wild. As a result, no one knew their exact ages. The ASU/Phoenix Zoo study fixes that problem by using only animals whose age is known.

Researchers started the Phoenix Desert Tortoise Project in August 1995 with 24 hatchlings. The scientists tagged each hatchling then placed it in quarantine for a month to ensure a disease-free start.

Tortoises were divided into two treatment groups. One group was fed a traditional zoo salad diet. The second group got high-protein pellets as food. Tortoises from each family were divided equally between the groups. This allowed Pough's group to factor in family differences that might otherwise affect the results. Like human families, some tortoise families are naturally bigger than others no matter what they eat.

Desert tortoise hatchlings in the high-protein group were then switched from salad to the experimental food. Interestingly enough, those tortoises had to be taught how to eat pellets. At first, researchers ground the pellets into a powder and sprinkled it on their greens. The tortoises were slowly weaned off the salad onto straight pellets. That process took until March 1996.

By November 1999, tortoises in the protein group weighed about twice as much as tortoises that ate salad. The average pellet-eater weighed in at a hefty 800 grams, compared to 360 grams for the salad group.

"If we really can head-start tortoises so that they reach adult size in five years or less, captive breeding and release programs for endangered species may be possible," Pough says. "We hope this method can be used to head-start other tortoise species that are more endangered than desert tortoises."

This is the first time The Phoenix Zoo has participated in a collaborative, scientifically based research project, according to Mike Demlong, the zoo's curator of ectotherms, or cold-blooded animals.

"This study is helping the zoo staff become better scientists," Demlong says. "That is important, because zoos aren't just about fun and recreation anymore. We're centers of conservation and research. As such, we're about building connections between an increasingly urban population and the natural environment that people are becoming more and more distant from."

He says that the Desert Tortoise Project "is about inspiring people to live in ways that promote the well-being of the natural world–even if it's as basic as helping people understand why they should try to avoid running over tortoises with their all-terrain vehicles."

Demlong says The Phoenix Zoo has dedicated special summer and winter habitats to the Desert Tortoise Project, as well as a total of about $300 per tortoise per year in food, facilities, and veterinary care.

"That's a big investment for the zoo, given that we receive no government funding. But it will be money well spent it if we can gain clear, conclusive data concerning the proper protein and fiber diet ratios for head-starting healthy, high-quality tortoises," he adds.

Demlong defines healthy as being free from all diseases–especially respiratory disease and a common shell abnormality called "pyramiding." Pyramiding has been linked to other high-protein diets. As yet, it has not affected The Phoenix Zoo tortoises.

Demlong defines high-quality tortoises as being able to forage, dig burrows, avoid predators, and generally behave like tortoises in the wild. That is important, he says, or else head-started tortoises will never be released.

ASU's Pough agrees. "The real question is, even if the project's ‘superjuveniles' reach adult size within five years, will they be physiologically and behaviorally mature? Will we have 'young adults' or 'big babies'?" he asks.

This is the point where Ellen Smith enters the story. A scientist at ASU West, Smith studies members of the original National Zoo study group at the Desert Tortoise Conservation Center in Las Vegas. Those tortoises are now between nine and 13 years old. Smith's work focuses on social behavior and hormones.

"If superjuvies could reproduce at age five, that would be four times faster than in the wild," Smith says. "That would be a clear head-start."

Smith and her Las Vegas colleagues began by pairing up each superjuvie—or head-started tortoise—with an opposite sex adult. Then they watched what happened.

Tortoise "dating" behavior is a pretty strange sight. Normal adult tortoises do a lot of nose-to-nose sniffing. Males "bob" their heads during courtship and bite at the front legs of females. Females show their willingness to mate by staying still. Otherwise, they just walk away.

The research group in Las Vegas watched to see if the superjuvies showed these adult behaviors. They collected blood samples to compare hormone levels of the superjuvies to levels in adults.

Adults that are ready to reproduce have higher levels of sex hormones in their blood than do immature tortoises. The scientists also studied all eggs that were laid to find out whether those eggs were fertile.

All the eggs from 1996 did hatch. Most female superjuvies did not lay any eggs; the few eggs they did produce never hatched. Male superjuvies, on the other hand, were able to successfully fertilize their adult partner's eggs.

"Right now, I'd say superjuvie males appear mature while the females seem unable to reproduce," Smith says. "But not knowing the exact age of the National Zoo tortoises makes interpretation difficult."

Smith and the Las Vegas group will continue watching the female superjuvies from the National Zoo to see when they begin to reproduce successfully. However, researchers expect data from the Phoenix study to answer more questions because age, family relationships, and hibernation history will all be known.

"Obviously, our ultimate hope is that superjuvies reach sexual maturity at the age of five or six, and that the females continue producing eggs until age 60 so they can help restore the population," Pough says. "The desert tortoise is a very important species in the desert habitat."

The ASU scientist says that tortoises play a unique role in the desert. They are big and strong enough to dig burrows in the tightly compacted desert sand. Coyotes, snakes, lizards, toads, insects, and mice all use abandoned tortoise dens for their own retreats. Tortoises dig burrows throughout their 40 to 50 acre ranges. Without them, other creatures would be at the mercy of the desert's extreme temperature swings.

The Desert Tortoise Project is a collaboration between ASU and The Phoenix Zoo. For more information, contact Harvey Pough, Ph.D., ASU West, (602) 543-6048. E-mail at FHPough@asu.edu. Or contact Mike Demlong at the Phoenix Zoo, (602) 914-4373. E-mail at Mdemlong@phoenix-zoo.org