Desert Tortoise Ecology
An important difference between the 2 populations is the absence of tortoises in valleys in the Sonoran Desert. Sonoran tortoises generally live on steep desert-mountain slopes in rocky burrows, while their counterparts in the Mojave Desert live primarily in valley floors and low mountain bajadas.
Studies in the Mojave Desert suggest that local populations need 20,000 to 60,000 tortoises to ensure survival of future generations. Local populations in the Sonoran Desert occur in isolated mountain ranges, most probably containing fewer than 20,000 tortoises. Their habitat has been fragmented and isolated by roads, canals, and agricultural and urban development, possibly severing corridors of dispersal. Currently, we do not know how many tortoises each local population in the Sonoran Desert needs to survive.
The Arizona Game and Fish Department’s Desert Tortoise Project is working to determine how the relatively small tortoise populations in Arizona persist, and to predict the likelihood of their persistence into the future, by asking the following questions: At what rate do desert tortoises reproduce? How does reproduction vary by year and by individual? What factors affect reproductive rate? Also, the study of tortoise movements and home range may help identify other factors limiting local population sizes, such as specific habitat features necessary for shelter (e.g. boulder cover sites).
Desert tortoises can live to at least 30 years of age (some may live to 100), so information collected over many years is important to determine how reproductive rate, movements and home range, and survival vary during tortoises’ long life spans. Obtaining this information will help ensure that tortoises in the Sonoran Desert do not become endangered in the future.
The Desert Tortoise Project relies primarily on the use of radio telemetry to study tortoises in the wild. We attach small radio transmitters, each with a unique frequency, to individual tortoises. By tuning a receiver to a particular tortoise’s frequency, project biologists can locate the same individual repeatedly over time. By tracking a number of individuals in this manner, we can determine activity patterns, rates of movement, home range size, and patterns of burrow use. Radio telemetry does not harm the tortoises and has allowed us to find individual tortoises in concealed or cryptic shelters (e.g. boulder piles, rock crevices filled with packrat nest debris).
Radio telemetry also allows us to monitor reproduction. During the reproductive season, project biologists locate female tortoises and transport them to a base camp where each tortoise is radiographed with a portable X-ray machine. Eggs appear on the developed radiographs as ping pong ball-shaped circles (see illustration below). This technique allows us to determine how many tortoises lay eggs each year and how many eggs they lay.
The Sponsor-a-Tortoise program provides an opportunity for interested individuals or organizations to support conservation and research efforts for desert tortoises. You can sponsor a tortoise by making a tax-deductible donation to the Desert Tortoise Project. The following suggested levels of contribution provide support for varying types of research supplies (examples in parentheses), but most contributions will be applied toward radio transmitters: $25, Egg (misc. field supplies); $50, Hatchling (X-ray film & supplies); $100, Juvenile (refurbish 1 transmitter); $200, Adult (1 new transmitter).
All sponsors will receive a color photograph of “their” tortoise, contributors of $50 or more will receive a project summary at the end of the year, and contributors of $100 or more will also receive a more specific account of their sponsored tortoise for that year.
If you would like to sponsor a desert tortoise, please send us your name and address, the level of sponsorship you would like (egg-$25, hatchling-$50, juvenile-$100, or adult-$200) along with a check, made payable to “Nongame Donations Fund.” Our address is The Desert Tortoise Project, Arizona Game and Fish Dept., Nongame Branch, 2221 W. Greenway Rd, Phoenix, AZ 85023-4399.