Sonoran Desert Tortoise

The habitat of the Sonoran desert tortoise includes steep, rocky slopes of mountains (bajadas) and the incised washes that drain these features. The species typically relies on rock-fortified shelters but may also excavate its own burrows. Temporary shelter sites may also include above-ground shelters such as woodrat (Neotoma sp.) middens and pallets under shrubs. The amount of available shelter sites strongly influences population density of Sonoran desert tortoises. The availability of drinking water is critical to the species survival. 

The tortoise eats a variety of plants, including herbs, woody plants, grasses and succulents. Additionally, the species is geophagous, consuming bones and stones for nutrient supplements as well as to help with digestion. Reproductive females regularly eat mineral deposits during the spring and early summer months (April to June) to aid in the development of eggshells. The species selectively eats plants that help with water retention, such as primroses, filaree, legumes, mustards and spurges.

The Sonoran desert tortoise spends a large proportion of its life in shelters (from about mid-November to mid-February), emerging mainly to drink, forage and breed. These shelters provide escape from temperature extremes, protection from predators and nesting opportunities. During this dormancy period, the species may emerge to drink rainwater and bask in the sun for body temperature regulation.

From April to May, egg-bearing (gravid) females typically emerge first, seeking food to support their egg development. Adult males and juveniles of the species are largely active during the summer monsoon (July through September), when new plant growth provides easy access to forage species. The tortoise is active during daylight hours but will also emerge at night to drink in response to rain. Sometimes, juveniles and adults will travel outside their home range for one or more seasons before returning. This type of movement, called sally, may be mistaken for dispersal.

The Sonoran desert tortoise has stocky and elephantine hind limbs. It has flattened forelimb covered with large conical scales, which assist with digging. The species can efficiently drink water from small, shallow pools by using its nares (nostrils) and pulsating its throat to create a vacuum. Males are differentiated from females by having elongated gular (throat) shields, a concave plastron and visible chin glands on each side of the lower jaw, which are most evident during the breeding season. 

The species has a relatively high domed shell, and the adult carapace (upper shell) length ranges in size from 8 to 15 inches (20 to 38 centimeters). 

The Sonoran desert tortoise’s carapace is usually brownish or dark in color with a definite pattern and prominent growth lines. The plastron (bottom shell) is yellowish and does not have a hinge.

The species has three life stages, which are size-dependent, and consists of young juveniles, older juveniles and adults. Tortoises hatch in September and October, following the end of the monsoon season. For approximately five years, they are considered young juveniles. Their behavior and ecology are poorly understood because their small size makes them difficult to detect in the wild. Additionally, their small size and softened shells provide little protection, making young juveniles vulnerable to predation by animals such as coachwhip snakes, red-tailed hawks (Buteo jamaicensis) and American badgers (Taxidea taxus). 

Once their shells are completely ossified, usually by age six and a shell length of 4.3 inches (11 centimeters), they are classified as older juveniles. Survivorship for this age class is higher than that of the young juvenile age class because of their slightly larger size and completely hardened shells. They remain in this size class until they reach sexually maturity. 

Sonoran desert tortoises transition to the adult size class by around age 16, with a shell length of 8.7 inches (22 centimeters) for females, and by age 18, with a shell length of 8.3 inches (21 centimeters) for males. Adult tortoises are relatively protected from natural predation because of their size and hard shells and have a high annual survivorship. Sustaining the reproductive age class populations is important because the mortality rates of juveniles are high, and it takes a long time for the species to reach sexual maturity. 

The long-lived and slow-growing Sonoran desert tortoise can live for more than 50 years in the wild, and many are presumed to live longer. It takes 18 to 20 years for the species to reach sexual maturity. 

The Sonoran desert tortoise’s breeding season occurs from July through October, and environmental conditions influence reproductive success. Immature eggs (follicles) may be reabsorbed if there is not enough food, and drought conditions can reduce or eliminate follicle development in the following reproductive season. Compared to other animals, this species produces few offspring. Adult females can produce as many as 85 eggs in their lifetime, averaging five eggs every other year. Due to their low survival rates, only two or three of those hatchlings may reach reproductive age. Females lay a clutch of eggs around the onset of the summer monsoon and may lay two clutches per year if eggs have been retained from the previous year. 

Similar species to the Sonoran desert tortoise include the desert tortoise (Gopherus agassizii) and the Goode’s thornscrub tortoise’s (Gopherus evgoodei).