The Desert Tortoise (Gopherus agassizii)
A Natural History

Produced by the 亚利桑那-索诺拉网上买球十大正规平台's 乌龟收养计划

一般的陆龟

The tortoises comprise the turtle family Testudinidae consisting of thirty-nine living species in ten genera. Today, tortoises are found in Asia, Europe, Africa, oceanic islands and the Americas. 其中包括两个巨大的形体, the well-known Galapagos tortoise (Geochelone elephantopus) of the Galapagos Islands in the Pacific Ocean and the Aldabran tortoise (G. 印度洋上塞舌尔岛上的一种巨蜥. Given the ling evolutionary history of tortoises in North America and the dynamic environment of the last thirty million years, it is indeed remarkable that the form of the four living North American tortoises have changed so little from their ancestors. One of these, the desert tortoise, (Gopherus (xerobates) agassizii Cooper) is found throughout much of the Sonoran Desert region and is therefore of special interest.

Tortoises may be distinguished from other turtle families by the following characteristics: the hindlegs are cylindrical and elephantine in shape: the feet are short, 棒状的宽的和棒状的. In some genera, the forelimbs are flattened and adapted for digging and burrowing and the toes are completely unwebbed. The carapace (upper shell) is usually high and domed with the ribs modified in alternating triangular wedges. 它们都是陆生的，基本上都是食草动物, 虽然它们偶尔会吃无脊椎动物和腐肉.

北美陆龟的分类

North American tortoises are grouped in the genus Gopherus although two distinctive groups, 被一些权威认为代表亚属的, are now recognized. The primitive gopher tortoises, including the desert tortoise (G. (旱生陆龟)和德州陆龟(G. (x.)，被归为旱螺亚属. 地鼠龟(G. (g.波吕斐摩斯)和伯尔森龟(G. (g.) flavomarginatus) are more advanced tortoises in the subgenus Gopherus in the genus Gopherus. This arrangement clarifies the evolutionary relationships among the living North American tortoises as presently understood.

沙漠龟的自然历史

The desert tortoise, 北美西部的古代居民, occurs today in the Mohave and Sonoran deserts in southwestern Utah, southern Nevada, southeastern California and western Arizona in the United States. In Mexico,"la tortuga de tierra"发生在索诺拉州的大部分地区, including Isla Tiburón in the Gulf of California and south into northwestern Sinaloa.

它的生态有相当大的变化, behavior, morphology, DNA已经在其范围的不同部分被发现. Three distinctive subspecific populations have been defined in the Mohave Desert, Sonoran Desert, 和热带落叶森林. The desert tortoise lives in a variety of habitats form sandy flats to rocky foothills, with a strong proclivity in the Mohave Desert for alluvial fans, washes and canyons where more suitable soils for den construction might be found. It is found from near sea level to around 3,500 feet in elevation.

沙漠龟的平均长度为6到14.6英寸，雄性比雌性长得更大. A gigantic specimen, 据称来自墨西哥, 在圣地亚哥自然历史博物馆展出, has a shell of 15.9 inches long. Other large individuals have been found in the Mohave Desert in California.

The desert tortoise occurs in a number of plant communities ranging from sparse creosote bush desertscrub in the winter rainfall Mohave Desert to palo verde-saguaro desertscrub in the bi-seasonal Sonoran Desert and eventually to summer rainfall tropical thornscrub and deciduous forest in Sonora and Sinaloa. 在索诺兰沙漠, tortoise density seems to be related to the density of perennial plants and plant species composition which are controlled by the amount of rainfall and winter freeze frequency. 在50年代早期之前, many populations reached densities of several hundred tortoises per square mile. Today, most populations contain no more than five to fifty tortoises per square mile.

Native and introduced grasses comprise the bulk of the desert tortoise diet. Otherwise, they eat any available edible plants including spring and summer annual wildflowers, 仙人掌和仙人掌的果实. Tortoises forage selectively, often sniffing or sampling various plants before consumption. 岩石和土壤也被吸收, perhaps as a means of maintaining intestinal digestive bacteria and as a source of supplementary calcium or other minerals. Stones may function as gastroliths enabling more efficient digestion of plant material in the stomach.

The extent of the home range (total habitat area used to fulfill life functions) of the desert tortoise depends upon various factors such as the densities of food plants, and the age, size, sex of the tortoise. These factors and presumably the size of the home range vary throughout the species' range. There is some evidence that tortoises utilize their feces in making home ranges, dens and burrows. 他们可能是在检测泄殖腔腺的分泌物. It has been suggested that tortoises rarely move more than two miles from their natal nest in their entire lives.

Well-adapted physiologically and behaviorally to live in dry desert environments, desert tortoises derive almost all their water intake from the plants they eat. A large urinary bladder can store over forty percent of the tortoise's body weight in water, urea, 尿酸和含氮废物. Water conservation is further aided by an ability to precipitate solid urates in the bladder, allowing water and ions to be reabsorbed while uric acid is eliminated in semi-solid form. During periods of sufficient rainfall tortoises drink copiously from temporary rainpools and eliminate solid urates. A common defensive behavior when molested or handled is to empty the bladder, leaving the tortoise at a considerable disadvantage in drier conditions. For this reason, desert tortoises should not be handled when encountered in the wild. Other avenues of water loss include respiration, defecation, and evaporation.

Activity patterns of the desert tortoise help in water conservation. It is chiefly active in the day (diurnal) or morning and evening (crepuscular), 视温度和季节而定. 夏末最热的时候, driest periods of the year conserves water already stored in the body. This is especially important in the hot, dry Mohave Desert summers. Burrow humidity is often as high as forty percent or more, thus reducing the rate of evaporation. 冬季冬眠也有助于减少水分流失. Burrows and dens are also used by the desert tortoise as an aid in regulating body temperature.

The flattened forelimbs of the desert tortoise an other gopher tortoises are capable tools for burrow construction. They dig with the front legs, stopping intermittently to sniff the soil. 随着土壤的迁移, the tortoise will frequently exit the burrow and kick the excavated soil still further from the burrow entrance. The entrance is half-moon shaped and high enough for the tortoise to comfortably enter without fully extending the legs.

The location, extent and type of burrow or den varies geographically. Tortoises in the Mohave Desert in California and the northern limits of the range in Nevada and Utah seem more inclined to construct extensive burrows, 长度可达35英尺. Such burrows stabilize temperature and humidity providing protection form intense winter freezes. 它们可能被一只或多只乌龟年复一年地使用. As many as twenty-five hibernating tortoises have been found in one den, although a more typical aggregation would contain no more than five individuals. 犹他州南部的一些洞穴估计有5000年的历史. 洞穴通常位于岩石或灌木丛下, 最好沿着斜坡走, and along washes, 在底部或从底部升起的.

在索诺兰沙漠 in southern Arizona and presumably south into Sonora, 沙漠龟几乎不打洞. Refuges merely cover the carapace and are often modified from mammal burrows or natural refuges in rocky terrain. Sonoran desert tortoise retreats are often on rocky slopes in mountains, avoiding the deep soiled valley situations favored by Mohave desert tortoises. Pallets are shallow depressions constructed under low shrubs at various points within the tortoises' home range, 提供临时休憩处. They are especially prominent in southern desert tortoise populations where mild winters mitigate the need for extensive burrows and desert tortoises may not hibernate. Here, burrows are often dug into the base of packrat houses rather than in the gravelly soil.

像大多数其他穴居动物一样, the desert tortoise creates a subterranean environment beneficial to other reptiles, mammals, 鸟类和无脊椎动物. Animals which share tortoise burrows benefit from permanent or temporary shelters afforded by the tortoise dens and burrows, 尽管它们给乌龟提供的很少或根本没有.

Many behavioral attributes of the desert tortoise are well documented. 当面对捕食者时, 乌龟通常会缩回头, feet, and tail, 将前膝折叠在头部前方, thus exposing only the shell and heavy scales of the armored forelimbs. This is an effective defense against most predators except people. 如果有人试图将乌龟从洞穴中移走, 它会后退到内部或延伸腿, 将甲壳楔入洞穴顶部. The defensive behavior of adult tortoises is usually passive while juveniles can be surprisingly pugnacious.

Social behavior consists of a series of head bobs for species and gender recognition, 求爱和威胁. Head bobbing normally precedes agonistic (combative) behavior between males, 尽管雌性也可能具有攻击性. Prominent chin glands in male tortoises produce a secretion which aids in sex recognition and often evokes combative behavior. Male combat is most intensive in spring and late summer in the Sonoran Desert. 在这些遭遇中, 每只雄企鹅站得越高越好, making short rushes toward his adversary while attempting to use the gular horn at the front of the plastron (undershell) to overturn the other or drive him away. An overturned tortoise can usually right itself using its head and a forelimb; if not, 乌龟可能会过热而死在沙漠的阳光下. 沙漠龟发出各种各样的声音(嘶嘶声), grunts, pops, whoops, huhs, echs, bips, etc.) which seem to be the most important when vocalized to an unfamiliar tortoise.

A loose male dominance hierarchy is apparently established by aggression. Dominant males court and mate with females more often than other males. Courtship involves extensive head bobbing as the male attempts to nip and bite at the edges of the female's carapace and legs while circling her. If the female is receptive, she will allow the male to mount her from behind. At this point the female will remain still as the male probes with his tail while grunting and enthusiastically stamping his hind feet. 婚礼的拥抱一直持续到雌企鹅离开为止. The mating posture is facilitated by a strong depression in the male's plastron that fits neatly onto the convex carapace of the female. The males's longer tail enables the penis to penetrate the cloaca. The neatly upright copulatory position of the male is further aided by the inward curve at the rear of the male's carapace. 成熟雌性的不同之处在于有扁平的板, a shorter tail, and an outward curve at the rear of the carapace which probably provides a wider space for egg laying. The gular horn of the male is longer and more curved, the claws more massive.

While sexual maturity in the wild is estimated to take twelve to twenty years, 这是一个成长和大小的因素，而不是年龄. 圈养的乌龟成熟得更快. Mating has been observed from early spring to fall with the highest frequency in late summer in the Sonoran Desert. Viable sperm retained in the cloaca of the female has resulted in fertilization a year and a half after copulation. Other turtle species have laid fertile eggs as long as four years after mating. Sperm retention is an excellent survival adaption in non-colonial animals that wander and whose numbers can decline in fluctuating climates of deserts.

筑巢地点通常选择在洞穴或托盘内或附近. 雌性用后腿挖窝洞. She urinates before, during and after the nest hole is dug as well as after covering the eggs, possibly to deter predators by camouflaging the nest and to prevent egg desiccation. In one observation, a female fought a Gila monster (Heloderma suspectum) attempting to eat newly laid eggs suggesting that nest sites may be defended for a period of time following egg deposition. Desert tortoises normally construct nests and lay eggs in May or June. In the southern portions of the range, a second clutch may be produced in late summer. These late summer clutches may undergo extended incubation periods hatching in the fall or next spring. The normal incubation period is ninety to one hundred thirty-five days depending upon incubation temperature. The clutch size varies from two to fourteen eggs with an average of three to five although some eggs may not be fertile. The eggs are hard-shelled, moisture proof, white and nearly spherical to ellipsoid in shape. Mortality of both eggs and juveniles is extremely high due to predation and environmental conditions. Probably no more than one hatchling from every fifteen to twenty nests will reach sexual maturity in the wild resulting in very low recruitment to the population. The fifty to eighty year life span estimated for desert tortoises suggests population turnover is not only low but should be very episodic following fluctuating climates. The desert tortoise could reach a "point of no return" as more reproducing adults fall victim to humanity's expanding impact in fragile desert environments.

The desert tortoise has long been utilized by southwestern peoples. The tortoise was relished as food by the Piman, Paijute and Seri Indians. 贝壳被用作烹饪器皿和贸易物品. The Seris of the coast of the Gulf of California in Sonora used tortoise parts for medicine and shell rattles as musical instruments and toys. 根据理查德·菲尔格和丽贝卡·莫泽的说法, 他们广泛地研究了这个系列的民族生物学, "如果一个女人只生了一个女孩, she is said to have eaten the reproductive organs of a female desert tortoise. 如果她的后代都是男性, it is said that as a child she had been hit in the small of her back with the reproductive organ of a male desert tortoise playfully thrown at her by a girlfriend.“塞里的民间传说以沙漠龟为特色, 叫做ziix heheet cquiij，意思是“坐在灌木丛里的东西”.后来，乌龟被白人定居者和勘探者吃掉. Mexican traders carried them alive as a source of fresh meat and water. 今天在索诺拉，人们偶尔会吃乌龟.

The desert tortoise is protected as a Threatened species under the U.S. Endangered Species Act north and west of the Colorado River in California, Nevada, and Utah where an upper respiratory tract disease has decimated many populations. Numerous other factors have contributed to this decline including residential development, road construction, 农业矿产开发, 使用越野车, overgrazing, malicious vandalism, 作为宠物收藏.

The status of the populations in Arizona is apparently less serious but warrants continued monitoring and research. 而偏远地区的种群似乎很稳定, those near urban or recreational centers have declined significantly. The desert tortoise is fully protected in Arizona and collection from the wild is strictly prohibited without a permit issued by the Arizona Game and Fish Department.

Many Arizonans are interested in having a desert tortoise for a number of reasons. This interest can be met through regulatory mechanisms and captive tortoise recycling programs like the Desert Museum's 乌龟收养计划 (TAP). 模仿加州的类似项目, applicants are carefully screened for intent and responsibility before being assigned a tortoise, 还是亚利桑那州的财产. All tortoises placed are urban foundlings, unwanted captives, or their progeny. 程序的目的, 由亚利桑那州渔猎部批准, are to provide appropriate care and custody for tortoises already in captivity while vigorously discouraging the taking of tortoises from the wild. 在亚利桑那州，成千上万只乌龟被圈养. It is ironic that people's attraction to the tortoise has become a significant threat to its future. Unfortunately, release of captive tortoises is considered a high risk to existing populations because of the potential to introduce disease, 破坏人口结构, 并混合来自不同地区的遗传资源. Management of the captive population separately from those in the wild may actually aid conservation of wild tortoises. Under Arizona law, one tortoise per family member may be possessed if the tortoises are obtained from a captive source and properly documented.