By Mike Giusti
Photos by Rebecca Jones and Magdelena Rodriguez
The Desert Tortoise (Gopherus agassizii), also known as the Agassiz’s desert tortoise, is the California state reptile and listed as a threatened species under the State and Federal Endangered Species Acts. Tortoises are found in the Sonoran, Mojave and Colorado Desert ecoregions. The Colorado River serves as a natural genetic barrier between the Agassiz’s desert tortoise and the Morafka’s desert tortoise which is found in Arizona and Nevada.
Tortoises are a long lived species with a life expectancy of between 30 to 50 years or more. Tortoises grow slowly and can take 16 years or longer to reach 8 inches in length. The females reach sexual maturity at 15 to 20 years of age. Mating occurs in the spring and autumn, with the female laying a clutch of 4 to 8 eggs, usually in the summer months of June and July. The eggs will usually hatch in August or September. Female tortoise may lay 0 to 3 clutches of eggs in a year and the incubation period of the eggs is 90 to 135 days depending on temperature.
Desert tortoises are adapted to living in areas in which ground temperatures can exceed 140oF (60oC). Tortoise’s ability to dig underground burrows, where they spend as much as 95% of their time, allows them to escape the high desert temperatures. Small tortoises will often modify other animal burrows so they can reduce their energy expenditure in digging a new burrow. Their inactivity while they are in burrows helps reduce water loss and regulate their body temperature.
Tortoises are most active during rainy seasons. During the winter, tortoises go into a state of hibernation which helps them survive near freezing temperatures and survive during condition of low food availability. Tortoises will emerge from their burrows in early spring as the temperatures begin to warm and when new spring plant growth begin to emerge. While tortoise will drink water from puddles, most of their fluid intake comes from grasses and spring flows.
Tortoise habitat is generally considered to be alluvial fans and washes where creosote desert scrub is often the dominant plant community. Tortoises are known to exhibit strong site fidelity and can have well established home ranges of 10 to 100 acres in size. Territories are selected based on their ability to provide food, water, and needed minerals. Tortoise populations have decreased by 90% since the 1950’s. In the 1950’s, tortoise densities averaged 200 per square mile; currently they are estimated at 5 to 60 per square mile.
The desert tortoise is considered by most conservation experts to be an indicator species for desert habitats. With its ability to survive in this harsh environment and long life span, tortoises often reflect how environmental changes are affecting the desert as well as other species.
There are many threats to the tortoise and their habitats. They face predatory threats from ravens, kit foxes, badgers, roadrunners and coyotes. Ravens are known to prey heavily on hatchling and juvenile tortoises. The small tortoises have soft shells and are therefore easy prey for ravens and other species. Raven predation is particularly high near landfills and other areas with high human presence. Also, ravens use power transmission poles as perch and nesting sites from which they hunt tortoises.
Human disturbance is also a direct threat to the desert tortoise and its habitat. Off-road vehicles (ORV) can cause burrows to collapse and frequently tortoise are hit and killed by ORV’s. The major threat to tortoise is the proposed renewable energy projects. While efforts to avoid much of the known tortoise habitat are being made, there are projects being developed in close proximity of high quality habitat. These projects attract additional predators and will likely disrupt movement patterns of resident tortoise. Another threat is the potential for the spread of an upper respiratory disease from the release of captive tortoises into the wild.
Through the State Wildlife Action Plan, we are developing strategies to conserve existing tortoise habitat and will look to partner with other conservation agencies and organizations to minimize the impacts to the desert tortoise from renewable energy projects and urban development. We are also looking forward to partnering with these organizations to advocate for siting of projects in locations where impacts to tortoises and other species will be avoided or in areas which are already converted or disturbed by other uses.