Old Turtle, New Tortoise: The Colorado River Splits Heirs

U.S. Geological Survey/photo by Ken Nussear

A new species of desert tortoise? So says U.S. Geological Survey biologists in a paper published late last month. Reporting in the journal ZooKeys, co-authors Robert W. Murphy (Centre for Bioldiversity and Conservation Biology) and Kristin H. Berry (USGS) published the findings of their work investigating desert tortoise biologists’ growing suspicions that the desert-burrowing species known as Agassiz’s land tortoise (Gopherus agassizii; aka the desert tortoise) in fact represents two morphologically and genetically unique species geographically isolated by the Colorado River. The range of the new old turtle – Agassiz’s land tortoise (G. agassizii) – is restricted now to the Mojave Desert and the parched environs of California, Nevada, and Utah north and west of the Colorado River. And the old new turtle? That would be Morafka’s desert tortoise (G. morafkai), which inhabits the Sonoran Desert and its surroundings in Arizona and Mexico south and east of the Colorado River.

This rift in tortoise taxonomy doesn’t come as a surprise. For more than 20 years, biologists have observed differences between these now-distinct species. The Agassiz’s desert tortoise has a domed, boxy carapace (upper shell), digs its burrows in valley bottoms characterized by desert and saltbrush/creosote scrub-lands and yucca woodlands, and nests between April and May. The newly-recognized Morafka’s desert tortoise instead has a flatter, pear-shaped carapace, digs or hides beneath rock crevices on slopes and rocky hillsides in thornscrub and grasslands, and nests later in the year, between June and August. More species may be forthcoming as scientists take a second look at Morafka’s desert tortoise to see if the tortoises inhabiting the southern portions of the states of Sonora and Sinaloa, Mexico, warrant further taxonomic tinkering and, just maybe, a new forest-dwelling species.

For now, this schism in species leaves the pared-down, federally Threatened Agassiz’s desert tortoise at a distinct disadvantage: a drop in distribution amounting to 30% of its former range, and the harsh reality for resource managers that these two distinct species can not be used interchangeably as genetic reservoirs or population sources. As it is, desert tortoises throughout their range face a mounting list of threats: predators ranging from ants and Gila monsters to ground squirrels, coyotes, golden eagles, and ravens; habitat and forage loss from invasive, non-native grasses and forbs; and the devastating effects of wildfires, environmental contaminants, and bacterial/viral diseases.

Let’s hope that this newfound species ignites a newfound interest in protecting our desert tortoises, old and new alike.

Learn more about…

The USGS Western Ecological Research Center’s work with desert tortoises

The USGS’s Press Release:  “Genetic Analysis Splits Desert Tortoise into Two Species”

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