California Department of Fish and Game
R005 Desert Tortoise Gopherus agassizi
Family: Testudinidae Order: Testudines Class: Reptilia
Written by: R. Marlow
DISTRIBUTION, ABUNDANCE, AND SEASONALITY
The desert tortoise is widely distributed throughout the Mojave and Colorado deserts from below sea level to 1250 m (4130 ft) or higher. It is most common in desert scrub, desert wash, and Joshua tree habitats, but occurs in almost every desert habitat except on the most precipitous slopes. Densities range from 3.5 per km 2 (9.2 per mi 2) in the eastern Mojave to more than 1000 per km 2 (2600 per mi 2) in the western Mojave near California City (Stebbins 1954, Marlow 1979).
SPECIFIC HABITAT REQUIREMENTS
Feeding: Tortoises are herbivorous, eating
annual forbs and grasses; many species are taken, but forbs are preferred
over grasses and green vegetation is preferred over dry.
Cover: This species normally excavates a burrow under bushes, overhanging soil or rock formations, or digs into the soil in the open. Burrows are most extensive in the northern part of the range where winter temperatures are coldest. On occasion, a tortoise will take cover under a bush or any natural shelter. The burrows are often crucial to survival, especially in hot weather when the direct rays of the sun can kill a tortoise in an hour or less (Marlow 1979).
Reproduction: This species requires friable, sandy, well-drained soil for excavation of nests.
Water: Not required, but tortoises will drink if water is available.
Pattern: Desert tortoises occur in a wide variety of habitats in arid and semiarid regions. They require friable soil for burrow and nest construction. Highest densities are achieved in creosote bush communities with extensive annual wildflower blooms, such as occur in the western Mojave. However, tortoises can be found in areas of extensive lava formations, alkali flats and most other desert habitats.
SPECIES LIFE HISTORY
Activity Patterns: Tortoises may be active at any time of year, but most activity takes place between March and June, and to a lesser extent in late summer in areas with summer rains (eastern Mojave). In early spring, tortoises may be active all day but by late spring, activity is reduced to less than an hour in early morning, 1 out of 4 days (Marlow 1979).
Seasonal Movements/Migration: In Utah, tortoises migrate from winter dens to summer feeding areas but this is not known to happen in California (Woodbury and Hardy 1948).
Home Range: Home range size seems to depend upon the quality of the habitat. In the western Mojave, home ranges are 2-15 ha (5-38 ac), but in the eastern Mojave, home ranges may be 10 times as large.
Territory: Tortoises are aggressive but there is no evidence that they defend territories.
Reproduction: Copulation begins shortly after the tortoises become active in late March or early April. Eggs are laid in early summer (late May to July). Clutches average 5 (range 2-9) eggs and take 3-4 months to hatch (Miller 1932, Woodbury and Hardy 1948, Stebbins 1954). Nests are often constructed at the entrance to burrows. Failure of rainfall and consequent scarcity of annual plants may result in reproductive failure.
Niche: When tortoises are young and vulnerable, they are eaten by many predators (eagles, coyotes, foxes, etc.). When mature, they have few enemies (very hungry coyotes may attack an adult tortoise). Captive tortoises are subject to respiratory infections but this has not been observed in wild populations. Nothing is known about serious parasite problems. Rabbits and a few rodents may compete with tortoises for food but this has not been established (Marlow 1979).
General Comments: See Bramble (1982) for a proposed revision of this genus.
Auffenberg, W., and R. Franz. 1978. Gopherus
Cat. Amer. Amphibians and Reptiles 212.