Mojave Desert Tortoise

These tortoises may attain a length of 25 to 36 cm (10 to 14 in), with males being slightly larger than females. A male tortoise has a longer gular horn than a female, his plastron (lower shell) is concave compared to a female tortoise. Males have larger tails than females do. Their shells are high-domed, and greenish-tan to dark brown in color. The high domes of their shells allow for space for their lungs, which helps them maintain thermoregulation, also known as maintaining internal temperature. Desert tortoises can grow to 10–15 cm (4–6 in) in height. They can range in weight from 8 to 15 pounds, or 3.5 kg to 7 kg. The front limbs have sharp, claw-like scales and are flattened for digging. Back legs are skinnier and very long.

Desert tortoises can live in areas with ground temperatures exceeding 60 °C (140 °F) because of their ability to dig burrows and escape the heat. At least 95% of their lives are spent in burrows. There, they are also protected from freezing winter weather while dormant, from November through February or March. Within their burrows, these tortoises create a subterranean environment that can be beneficial to other reptiles, mammals, birds, and invertebrates. Scientists have divided the desert tortoise into three species: Agassiz's and Morafka's desert tortoises, with a third species, Goode's thornscrub tortoise, in northern Sinaloa and southern Sonora, Mexico. An isolated population of Agassiz's desert tortoise occurs in the Black Mountains of northwestern Arizona. They live in a different type of habitat, from sandy flats to rocky foothills. They have a strong proclivity in the Mojave Desert for alluvial fans, washes, and canyons where more suitable soils for den construction might be found. They range from near sea level to around 1,050 m (3,500 ft) in elevation. …

Tortoises mate in the spring and autumn. Male desert tortoises grow two large white glands around the chin area, called chin glands, that signify mating season. A male circles around the female, biting her shell in the process. He then climbs upon the female and insert his penis (a white organ, usually only seen upon careful inspection during mating, as it is hidden inside the male and can only be coaxed out with sexual implication) into the cloaca of a female, which is located around the tail. The male may make grunting noises once atop a female, and may move his front legs up and down in a constant motion, as if playing a drum.

Months later, the female lays a clutch of four to eight hard-shelled eggs, which have the size and shape of ping-pong balls, usually in June or July. The eggs hatch in August or September. Wild female tortoises produce up to three clutches a year depending on the climate. Their eggs incubate from 90 to 135 days; some eggs may overwinter and hatch the following spring. In a laboratory experiment, temperature influenced hatching rates and hatchling sex. Incubation temperatures from 27 to 31 °C (81 to 88 °F) resulted in hatching rates exceeding 83%, while incubation at 25 °C (77 °F) resulted in a 53% hatching rate. Incubation temperatures less than 31 °C (88 °F) resulted in all-male clutches. …