New Species of Desert Tortoise, Genetic Study Says

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The desert tortoise -- that slow-footed shelled sentinel of America's desert southwest -- is actually comprised of two distinct species, according to a new study coauthored by USGS. The newly recognized species has been named Morafka's desert tortoise (Gopherus morafkai). Most tortoises found in Arizona and Mexico will now be assigned to this species.

It took some sophisticated genetic analysis to sort the two species out -- not too surprising for a drab, secretive animal adapted to blend perfectly into the desert environment and keep out of sight.

Desert tortoises were first known to science 150 years ago, says Kristin Berry, a USGS Western Ecological Research Center biologist who was a coauthor on this study, which was published online today. Described in 1861 by Army doctor and scientist James Graham Cooper, the desert tortoise eventually received the scientific name Gopherus agassizii.

Effects of fire on desert tortoise habitat

Male desert tortoise (Gopherus agassizii). (Credit: Todd C. Esque, USGS WERC. Public domain.)

All tortoise populations within its known range -- California, Nevada, Utah, Arizona down to Mexico -- were thought to belong to that single species, Gopherus agassizii. But in recent decades, scientists and wildlife managers began to suspect some differences between populations of this basketball-sized burrower.

“Populations on opposite sides of the Colorado River had different habitat preferences,” says Berry, who has studied desert tortoise biology for more than 40 years. “Tortoises south and east of the Colorado prefer to hide and burrow under rock crevices on steep, rocky hillsides, while tortoises north and west of the Colorado prefers to dig burrows in valleys.”

There were other minute differences as well, such as egg-laying seasons and litte details in the tortoises' shell -- differences you couldn't tell from a casual glance at these leathery clawed crawlers. So Berry and colleagues Bob Murphy, Taylor Edwards, Alan LevitonAmy Lathrop and Daren Riedle organized a little detective hunt.

The hunt involved sorting through and digging up hundred-year-old pickled specimens of desert tortoises from the vaults of the Smithsonian and California Academy of Sciences -- then using sophisticated DNA analysis to compare tortoise genes from these ancient, preserved tissue with samples from living tortoises from throughout the desert southwest. 

It's an intriguing story and history lesson of its own, involving the 1906 San Francisco earthquake, misplaced labels, and lost specimens.

-- Ben Young Landis