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Scientists at the USGS Western Ecological Research Center study the many ecosystems of the Pacific Southwest. Follow our expeditions and projects through this outreach page, and learn more about your local landscape with our library of Outreach Factsheets and photos. Thanks for joining us!

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Desert Tortoise --Photographer: Ken Nussear, USGS
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AskWERC: The Desert Tortoise

On Friday, December 10, in Twentynine Palms, California, WERC lead scientist Kristin Berry will give a public lecture titled “Desert Tortoises: Yesterday, Today and Tomorrow”. The event is hosted by the Joshua Tree National Park Association, the nonprofit educational support group of its namesake park.

See event announcement for times and details

Desert tortoises (Gopherus agassizii) living in the western Sonoran Desert (also known as Colorado Desert) and the Mojave Desert are listed as threatened under the Endangered Species Act. These animals are managed by the U.S. Fish and Wildlife Service and other government agencies. Populations of this reptile are decreasing due to habitat loss from human development, deaths from increasing wildfires and a variety of other causes.

Berry has been studying desert tortoises in California for the last 40 years. I decided to ask Dr. Berry to shed a little light on the biology of this secretive shell-bearer:

Male desert tortoise combat --Photographer: Steve Wessells, USGS
Desert tortoises are more than meets the eye. Male tortoises can engage in fast-paced combat through biting and flipping. Image credit: Steve Wessells/USGS.

Q: How old can desert tortoises get?
  Wild, free ranging and captive tortoises have the potential to live for decades. Tortoises in captivity have been documented to live for more than 80 years. In the wild, we have limited, relatively short-term data from a few sites. I estimate that adults may reach 80 to 100 years or more.   

Q: Are tortoises always so slow?
  Tortoises may appear to be slow -- and certainly in comparison to large mammals and birds, they may appear to be slow. However, adult tortoises can walk almost a mile in a day. If you turn your back on them for ten minutes in the desert, you may not be able to easily find them again because they can move quickly.   

Q: What can actually eat a tortoise and bite through its thick shell?
  Many different species of predators eat tortoises. The type of predator depends on the size of the tortoise. Hatchling and juvenile tortoises are more susceptible to predation than adults because their shells are fragile and thin. Ants, Gila monsters, ravens, antelope ground squirrels, kit foxes, badgers and coyotes attack and eat the small tortoises. Snakes may also eat the small tortoises.

Adults can be killed and eaten by golden eagles, ravens, kit foxes, badgers, bobcats and coyotes. Golden eagles can carry adults to their nests (called “eyries”) and drop them on rocks, breaking them open. Ravens can flip an adult tortoise on its back and then peck open the soft parts near the tail and hind legs. Similarly, coyotes and other mammals can bite the legs, pull them out and gain access to the interior of the tortoise. Mammalian predators often use their teeth to puncture the shell and break it apart.

Q: What’s the coolest fact that most people don’t know about desert tortoises?
  Adult tortoises have a social system that includes a dominance hierarchy among the males. In undisturbed populations, adults appear to have preferences for specific mates and may have long-term relationships with each other. Female tortoises can choose their mates using a variety of postures and behaviors.

WERC embedded video should be here. You may not have a browser with enabled video embedding.
CLICK TO PLAY VIDEO: Watch Kristin Berry explain her California desert tortoise research in this USGS documentary, The Heat is On: Desert Tortoises & Survival. (Berry appears at the 4:27 mark.)

Q: The public hears a lot about desert tortoises being found on land planned for solar energy projects. What do ecologists still need to learn to help resource managers best plan for both tortoises and alternative energy?
  Some solar energy projects are being developed in desert tortoise habitat. Solar and other renewable energy projects have far reaching impacts to tortoises because of habitat loss, increased human use of the area and fragmentation of the landscape. One mitigation or minimization procedure is “translocation” or moving of tortoises to a new site. But translocation is not a proven technique that is recommended by the Independent Science Advisors for the California Desert Renewable Energy Plan. Translocation raises numerous issues such as disease, high mortality rates and the lack of appropriate new home sites.

For example, we need to avoid transmitting infectious diseases from tortoises in one area to another and to screen tortoises for infectious diseases with a high degree of accuracy. In the last 20 years, we have identified a few new pathogens that are serious threats to tortoises. Substantial research will be necessary to identify new and emerging diseases and to develop methods to test for these new diseases. We need to know how to conduct a successful translocation which results in long-term, high survival rates for the tortoises.

We also need to be more effective at reducing impacts from dirt and paved roads; from human-subsidized predators such as ravens, coyotes and domestic dogs; and from invasive plants that thrive in disturbed areas. These are but a short list of topics that need attention.

-- Ben Young Landis

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