Desert tortoise
The desert tortoise is a species of tortoise in the family Testudinidae. The species is native to the Mojave and Sonoran Deserts of the southwestern United States and northwestern Mexico, and to the Sinaloan thornscrub of northwestern Mexico. G. agassizii is distributed in western Arizona, southeastern California, southern Nevada, and southwestern Utah. The specific name agassizii is in honor of Swiss-American zoologist Jean Louis Rodolphe Agassiz. The desert tortoise is the official state reptile in California and Nevada.
The desert tortoise lives about 50 to 80 years; it grows slowly and generally has a low reproductive rate. It spends most of its time in burrows, rock shelters, and pallets to regulate body temperature and reduce water loss. It is most active after seasonal rains and is inactive during most of the year. This inactivity helps reduce water loss during hot periods, whereas winter brumation facilitates survival during freezing temperatures and low food availability. Desert tortoises can tolerate water, salt, and energy imbalances on a daily basis, which increases their lifespans.
Taxonomy
In 2011, on the basis of DNA, geographic, and behavioral differences between desert tortoises east and west of the Colorado River, it was decided that two species of desert tortoises exist: Agassiz's desert tortoise and Morafka's desert tortoise. The new species name is in honor of the late Professor David Joseph Morafka of California State University, Dominguez Hills, in recognition of his many contributions to the study and conservation of Gopherus. G. morafkai occurs east of the Colorado River in Arizona, as well as in the states of Sonora and Sinaloa, Mexico. The acceptance of G. morafkai reduced the range of G. agassizii by about 70% In 2016, based on a large-scale genetic analysis, ecological and morphological data, researchers proposed a split between the Sonoran and Sinaloan populations. This southernmost member of the Gopherus genus was named G. evgoodei, Goode's thornscrub tortoise.Description
These tortoises may attain a length of, with males being slightly larger than females. A male tortoise has a longer gular horn than a female, his plastron 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 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.Habitat
Desert tortoises can live in areas with ground temperatures exceeding 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 in elevation. Tortoises show very strong site fidelity, and have well-established home ranges where they know where their food, water, and mineral resources are.
Desert tortoises inhabit elevations from below mean sea level in Death Valley to in Arizona, though they are most common from around. Estimates of densities vary from less than on sites in southern California to over in the western Mojave Desert, although most estimates are less than. The home range generally consists of. In general, males have larger home ranges than females, and home range size increases with increasing resources and rainfall.
Desert tortoises are sensitive to the soil type, owing to their reliance on burrows for shelter, reduction of water loss, and regulation of body temperature. The soil should crumble easily during digging and be firm enough to resist collapse. Desert tortoises prefer sandy loam soils with varying amounts of gravel and clay, and tend to avoid sands or soils with low water-holding capacity, excess salts, or low resistance to flooding. They may consume soil to maintain adequate calcium levels, and may prefer sites with higher calcium content.
With the creation of off-road vehicles more humans are making their way in and out of the desert tortoises' home environment.
Shelters
Desert tortoises spend most of their lives in burrows, rock shelters, and pallets to regulate body temperature and reduce water loss. Burrows are tunnels dug into soil by desert tortoises or other animals, rock shelters are spaces protected by rocks and/or boulders, and pallets are depressions in the soil. The use of the various shelter types is related to their availability and climate. The number of burrows used, the extent of repetitive use, and the occurrence of burrow sharing are variable. Males tend to occupy deeper burrows than females. Seasonal trends in burrow use are influenced by desert tortoise sex and regional variation. Desert tortoise shelter sites are often associated with plant or rock cover. Desert tortoises often lay their eggs in nests dug in sufficiently deep soil at the entrance of burrows or under shrubs. Nests are typically deep.Shelters are important for controlling body temperature and water regulation, as they allow desert tortoises to slow their rate of heating in summer and provide protection from cold during the winter. The humidity within burrows prevents dehydration. Burrows also provide protection from predators. The availability of adequate burrow sites influences desert tortoise densities.
Each desert tortoise uses about 5 to 25 burrows per year. Some burrows are used repeatedly, sometimes for several consecutive years. Desert tortoises share burrows with various mammals, reptiles, birds, and invertebrates, such as white-tailed antelope squirrels, woodrats, collared peccaries, burrowing owls, Gambel's quail, rattlesnakes, Gila monsters, beetles, spiders, and scorpions. One burrow can host up to 23 desert tortoises – such sharing is more common for desert tortoises of opposite sexes than for desert tortoises of the same sex.
Lifecycle
Reproduction
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 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 resulted in hatching rates exceeding 83%, while incubation at resulted in a 53% hatching rate. Incubation temperatures less than resulted in all-male clutches. Average incubation time decreased from 124.7 days at to 78.2 days at.
The desert tortoise is one of the few known tortoises in existence that has been observed engaging in homosexual intercourse Same-sex intercourse happens in many species, There is no one answer as to why this occurs. One possible explanation for this could be the social component of gaining and establishing dominance.
Maturation
The desert tortoise grows slowly, often taking 16 years or longer to reach about in length. The growth rate varies with age, location, gender and precipitation. It can slow down from 12 mm/year for ages 4–8 years to about 6.0 mm/year for ages 16 to 20 years. Males and females grow at similar rates; females can grow slightly faster when young, but males grow larger than females.Desert tortoises reach their reproductive maturity at ages 15 to 20, when they become longer than 18 cm. However, it is possible for them to mature faster as 10-year-old females that are able to reproduce have been observed.
Activity
Their activity depends on location, peaking in late spring for the Mojave Desert and in late summer to fall in the Sonoran Desert; some populations exhibit two activity peaks during one year. Desert tortoises brumate during winters, roughly from November to February–April. Females begin brumating later and emerge earlier than males; juveniles emerge from brumation earlier than adults.Temperature strongly influences desert tortoise activity level. Although desert tortoises can survive body temperatures from below freezing to over, most activity occurs at temperatures from. The influence of temperature is reflected in daily activity patterns, with desert tortoises often active late in the morning during spring and fall, early in the morning and late in the evening during the summer, and occasionally becoming active during relatively warm winter afternoons. The activity generally increases after rainfall.
Although desert tortoises spend the majority of their time in shelter, movements of up to per day are common. The common, comparatively short-distance movements presumably represent foraging activity, traveling between burrows, and possibly mate-seeking or other social behaviors. Long-distance movements could potentially represent dispersal into new areas and/or use of peripheral portions of the home range.