ASU Research E-Magazine: Speeding Tortoises

For many species of desert tortoises, survival 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 survival as a species. The competition is keener and much more brutal. The competition is us.

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

Desert tortoises (Gopherus agassizii) once roamed freely throughout southwestern California, southern Nevada, and western Arizona. Today they are in trouble. The Mojave Desert population is officially listed as “threatened” by the federal government. Arizona’s Sonoran Desert tortoise population may not be far behind.

The sad truth is that tortoises do not stand a chance against cars. Hundreds of the creatures are crushed every year by cars or off-road vehicles that encroach upon their range. Escaped or released pet tortoises often take back to the wild a highly contagious, fatal respiratory disease.

That story’s 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 perfect head-starting techniques for tortoise reproduction efforts. A goal of their Desert Tortoise Project is to accelerate the growth and maturation process of tortoise hatchlings through dietary change.

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

That task is tougher than it sounds. In the wild, desert tortoises do not reach sexual maturity 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, department chair of Life Sciences at ASU West. “The mortality 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. In that study, desert tortoises grew significantly faster when fed high-protein diets instead of greens. They grew to full size in just five years instead of the typical 20.

The National Zoo study did have a weakness. Many of the subject tortoises were captured in the wild. As a result, their ages could not be confirmed. The ASU/Phoenix Zoo study controls that variable 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 received high-protein pelleted food. Tortoises from each clutch of eggs, or family, were distributed equally between the groups. Such distribution allowed Pough’s group to factor in differences between families that might otherwise skew results.

Desert tortoise hatchlings designated for the high-protein diet were then gradually switched from salad to the experimental food. Interestingly enough, those tortoises had to be taught to eat pellets. Protein pellets were initially ground into a powder and sprinkled on their greens. The tortoises were gradually weaned off the salad onto straight pellets. That process took until March.

By July 1996, size differences were already apparent. The high-protein group had gained an average of 50 grams in weight (an increase of 114 percent), compared to a 38 gram (88 percent) average for the salad group.

Growth differences among family groups were also quite apparent. Members of the largest family had gained an average of 65 grams, while members of the smallest averaged only a 23-gram gain. Controlling for the effects of family groups, therefore, appears to be a wise choice in this project.

Pough and his colleagues then added a second variable to their experiment. Since prior studies had suggested that hibernation plays a role in stimulating reproduction by reptiles, Pough’s team sub-divided members of each diet group. Half of each group was allowed to hibernate, the others were kept warm and allowed to continue eating throughout the winter.

The study now included four different treatment groups: salad diet with hibernation, salad diet without hibernation, pelleted diet with hibernation, and pelleted diet without hibernation.

By April 1997, the high-protein group of tortoises weighed an average of 146 grams (about 5.1 ounces) compared to an average of 121 grams (4.3 ounces) for the salad group.

Family still appeared to be a factor. Members of the largest family averaged 200 grams in weight (just under half a pound). Members of the smallest family averaged less than half of that, only 88 grams.

“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 feasible,” Pough says. “We hope this method can be used to head-start other tortoise species that are more endangered than desert tortoises.”

“Our goal is to develop sound husbandry techniques and protocols that will allow us to take tortoise hatchlings and rapidly raise them to high-quality adults,” explains Mike Demlong of The Phoenix Zoo. “If successful, we can apply similar techniques to, say, endangered Galapagos tortoises.”

Demlong is the zoo’s curator of ectotherms, or cold-blooded animals. He also is an ASU graduate and master’s degree candidate.

According to Demlong, this is the first time The Phoenix Zoo has actively participated in a collaborative, scientifically-based research project—one that involves testing hypotheses, collecting data, peer review, and eventual publication.

“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 they’re becoming more and more distant from.

“For us, the desert tortoise project is an all out education process that encompasses everything from behind-the-scenes tours to reaching people via the Internet,” he says. “It’s 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 when they’re out 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 but has not yet appeared in any of The Phoenix Zoo subjects.

Demlong defines high-quality tortoises as being able to forage, dig burrows, avoid predators, and function behaviorally like their wild tortoise counterparts. That is critical, 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.

If the answer is young adults, captive-breeding programs will have real benefit. If not, head-starting may yield little more than a more cost-effective way for zoos to raise display tortoises.

This is the point where Ellen Smith enters the story. An academic professional 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 seven and 11 years old. Smith’s work focuses on social behavior and endocrinology. She’s using behavior observations and hormonal analysis to find answers.

“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 with an opposite sex adult then watching what occurred.

Normal adult tortoise behaviors include extensive nose-to-nose sniffing. Males “bob” their heads during courtship and bite at the front legs of females. Females signal their willingness to mate by staying still. Otherwise, they just walk away.

The research group in Las Vegas documented those behaviors—and the lack thereof. They collected blood samples to compare hormone levels of the superjuvies to levels in adults. They also collected and artificially incubated all eggs that were laid to find out whether those eggs were fertile.

While Smith has not yet finished analyses of the behavior and hormonal data, 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 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 observing the female superjuvies from the National Zoo to determine 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.

Unfortunately, answers from the Phoenix tortoises are still at least three years away since its subjects are not yet two years of age.

“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 desert tortoises play a unique role in their habitat. 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 largely at the mercy of the desert’s extreme temperature swings.

While the Desert Tortoise Project’s scientific merits stand alone, the project was purposely designed to include both community and school outreach facets. Members of the community have been instrumental in providing hatchlings; project members strive to provide solutions and education in return. One specific part of the Desert Tortoise Project’s educational emphasis targets Phoenix-area school students.

“Tortoises are inherently interesting to kids,” Pough says, “They are durable creatures. With the help of a local science teacher named Tracy Diaz, we’re developing interactive lesson plans that will get students involved directly in this project. Those plans are based on the biology and conservation of tortoises and are written for classes ranging from mathematics and science to art and history.”

The Phoenix Zoo and ASU West have also teamed up to offer a zoo biology course for college students, complete with on-site work and internships. Both programs bring students to weigh the tortoises, and learn how to design experiments and analyze data. Students also go behind the scenes to pat the giant Galapagos and Aldabra tortoises and feed them cactus pads.—Danielle Brooks