Tortoise Conservation No Shell Game

Taylor Edwards holds a desert tortoise on a visit to the Arizona-Sonora Desert Museum in 1977. Credit: Sandra Edwards
November 19, 2012

Taylor Edwards’ research would crawl at a tortoise pace if it weren’t for modern science.

Deciphering DNA is central to his work with desert tortoises, those charismatic reptiles whose lifespans—as long as 80 to 100 years—make simply observing their evolution and heredity all but impossible during a human’s lifetime.

An assistant staff scientist with the University of Arizona Genetics Core (UAGC) and a doctoral candidate in the UA’s School of Natural Resources and Environment, Edwards and his collaborators have assembled a collection of more than 1,400 samples of tortoise DNA by drawing blood from the tortoise’s scaly forearms. He uses the rich genetic reservoir he has compiled to better understand how millions of years of evolution shaped what the desert tortoise is today and how to conserve it for tomorrow.

His efforts will be honored with a Jarchow Conservation Award from the Tucson Herpetological Society. The award “honors individuals or organizations for their service to the conservation of the amphibians and reptiles of the deserts of North America.”

Conservation Genetics

Edwards has a long history with the desert tortoise. He first met a tortoise on a visit to the Arizona-Sonora Desert Museum when he was seven years old.

At that time, visitors to the museum could go into the enclosure, and Edwards has a picture of himself with a tortoise from that visit. When he later worked at the Desert Museum as a keeper, he posted that picture on his locker. He and his family have also two adopted desert tortoises at home.

Since then, his research and conservation work have continued to be intertwined with the tortoise. Edwards has become an expert in DNA testing; over the last seven years he and his team at the University of Arizona Genetics Core conducted the public testing for National Geographic's Genographic Project, which traced human ancestries through DNA samples. As a conservation biologist, he was tapped to join a research team that used DNA sampling to split the tortoise species,Gopherus agassizii, into two distinct species—G. agassizii (Agassiz’s desert tortoise, native to the Mojave Desert) and G. morafkai (Morafka’s desert tortoise, native to the Sonoran Desert). That work was published in 2011.

Edwards has continued his research on the tortoise south into Mexico, working collaboratively with stakeholders on both sides of the Arizona-Sonora border. Here, he is investigating whether there is actually a third species of tortoise.

The split between the Mojave and Sonoran tortoise is an example of allopatric speciation, where the barrier of the Colorado River separated the tortoises geographically, with one species to the north and west of the Colorado, and the other to the south and east. Generally, the Mojave species has a significantly wider shell, while the Sonoran species has a flatter and pear-shaped shell.

Edwards points to a different model of speciation—the process by which new species arise—in Sonora, one he is examining as part of his Ph.D. work under Melanie Culver, an assistant professor in the UA’s School of Natural Resources and the Environment.

“In the south it’s really interesting because there’s no geographic barrier. They are very different from each other genetically, but we don’t know if that was driven through the process of speciation, or they were just temporarily isolated and are currently back together. Perhaps there just hasn’t been enough time for gene flow to integrate them,” he says.

In addition, he is researching the genetics of captive pet tortoises. “Tortoises are bizarre, because they’re probably the only protected and endangered species where people commonly keep them as pets throughout their native range,” he adds. “In some cases we may have more captive tortoises than we will wild ones.”

Implications for Conservation and Definitions of Species

All of Edwards' research on the tortoise has direct implications for conservation. The Mojave desert tortoise (G. agassizii), which is listed as threatened under the Endangered Species Act, now exists on only about 30 percent of the original range it occupied when it was thought to be the same species as the Sonoran desert tortoise (G. morafkai). With the discovery that there is more than one species, each individual population decreases in size and range, and one species is not viable as a conservation back up, or genetic reservoir, for the other.

In addition, tortoise research has also led Edwards into questions of how species are defined. While traditional definitions of species in conservation and management refer to them as static entities, Edwards wonders how a more fluid definition of species, taking evolutionary change into account, could strengthen conservation and management strategies.

“I hope that my research will contribute to how we define species for conservation, so that in a management scenario we can have better definitions that encompass more of what we call the evolutionary potential of a species to continue to grow and maintain its biological diversity. In the context of species conservation, it is not possible for us to determine which individuals contribute most to the evolutionary potential of the species, or more importantly, which adaptive traits will be most critical in the face of environmental change. For a species to persist in a changing environment, genetic diversity provides the foundation for adaptation.” he says.

Edwards’ research is a great example of the curiosity and discovery inherent in science. 

“I think that’s what scientists do. For every question you answer, you have 10 new questions,” he says. “You get excited by having new questions instead of just answering your one question.”