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Mammals 2


Order Artiodactyla

The artiodactyls have the axis of the foot pass between the third and fourth toes (the other major group of hoofed mammals [ungulates], the Perissodactyla, consisting of horses, tapirs, and rhinoceroses, have the axis passing through the third toe). There are four families among the artiodactyls that are native to the Chihuahuan Desert. The Tayassuidae consists of the peccaries, of which the Collared Peccary (Pecari tajacu) is the species in our area. The Cervidae is the deer family, with White-tailed Deer (Odocoileus virginianus) and Mule Deer (Odocoileus hemionus) associated with the desert. Elk (Wapiti, Cervus elaphus) were native in some of the northern desert mountains such as the Guadalupes; extirpated by overhunting, they now have been reintroduced from more northern populations. The Pronghorn (Antilocapridae, Antilocapra americana—often called antelope, but true antelope are Old World creatures). The Bovidae includes, marginally in our area, the American Bison (Bos bison) and not-marginally, the Bighorn (Mountain) Sheep, Ovis canadensis. Populations of the latter still occur in the San Andres Mountains of southern New Mexico and in the Big Hatchets (and into Mexico) in the extreme southwest of New Mexico.

There are a number of artiodactyls that now occur in the Chihuahuan Desert but are not native. Feral pigs (Sus scrofa, family Suidae) are widespread. Barbary Sheep, or Aoudad (Ammotragus lervia), has spread from areas where it was introduced as a game animal into at least the margins of the desert. Domestic stock that have had severe effects on the desert include cattle (Bos taurus), sheep (Ovis aries), and goats (Capra hircus). Overgrazing, in combination with drought, has degraded much of the desert grassland into scrub desert.


Rodents make up the largest order of mammals. They are distributed worldwide with the exception of Antarctica, and except for a few weird forms, the Australian region. Oceanic islands, originally rodent free, have almost entirely be colonized by introduced rodents, and continental areas have been extensively colonized by exotics.

Among the various features common to rodents, dentition is easiest to see. Rodents all have a total of two upper and two lower incisors (one on each side of each jaw), and none has any canine teeth. The incisors have enamel only on the front surface, and they are evergrowing. Much of their success apparently is due to the nature of the jaws and teeth: with the lower jaw pulled backward, the cheekteeth can be used for chewing; with the lower jaw advanced forward, the incisors work together for gnawing.

The most primitive family of rodents in our desert is the Sciuridae: Squirrels. Pretty much lacking trees, the desert habitat lacks tree squirrels except in the marginal higher forested mountains, but has ground squirrels (Spermophilus—literally, seed lovers), Black-tailed Prairie Dog (Cynomys ludovicianus), and antelope squirrels (Ammospermophilus).

Ground squirrels use burrows for protection, but feed on the surface. Like most sciurids, they are diurnal, retreating to their burrows at night (most other desert rodents are crepuscular—active at dusk and dawn—to nocturnal). A variety of foods are eaten, including green fodder, seeds, nuts, insects, and even meat. The Texas Antelope Ground Squirrel (Ammospermophilus interpres) was named from a specimen from El Paso. These are small squirrels of rocky terrain. On the other hand, the Black-tailed Prairie Dog is a moderately large squirrel living in colonies where there is deep, mostly rockless, soil. Limited pretty much to grasslands at the higher elevations in the desert, they feed extensively on leafy material. They have been reduced to a small proportion of their original numbers by intentional poisoning.

The genus Spermophilus is represented by three species: the Spotted Ground Squirrel (S. spilosoma, Mexican Ground Squirrel (S. mexicanus), and Rock Squirrel (S. variegatus.

Pocket gophers, family Geomyidae ("earth mice", literally) are fossorial animals, meaning they spend most of their lives underground in burrow systems. Usually the only indications of their presence are mounds of dirt pushed out onto the surface from their construction or remodeling of the underground tunnels. All have externally opening, fur-lined cheek pouches into which food material may be temporarily stored for transportation to storage or eating areas. Feeding is primarily on the underground parts of plants, though they may open a tunnel to the surface and, keeping next to the opening, gather plant food from the immediate vicinity. They tend to be very antisocial, with one animal per tunnel system except during the reproductive season. Where different species meet geographically, the ranges may interdigitate, but not overlap.

Four species of pocket gophers occur in the Chihuahuan Desert. Generally speaking, the Yellow-faced Pocket Gopher (Cratogeomys castanops) lives in relatively deep, tight soils, whereas the Desert Pocket Gopher (Geomys arenarius) prefers sandy soils. In the regions of overlap with either of these species, Botta's Pocket Gopher (Thomomys bottae) is limited to the shallow, rocky soils of the desert mountains (where the other species are absent, however, it may descend into the deeper soils). The Southern Pocket Gopher inhabits the shallow rocky soils of the higher mountain ranges from southwestern New Mexico and adjacent Arizona south.

Closely related to the Geomyidae (the pocket gophers and this family are both members of the superfamily Geomyoidea), but having adapted to a different way of life, the Heteromyidae includes the pocket mice and kangaroo rats. Like the pocket gophers, these animals have external, fur-lined cheek pouches. The family tends to reach its center of diversity in the Southwest and Mexico. Eleven species are known from the Chihuahuan Desert; these can be divided into the spiny-rumped pocket mice (Chaetodipus), silky pocket mice (Perognathus), and kangaroo rats (Dipodomys). All feed primarily on seeds, though small amounts of other food may be taken. All are relatively lightly built compared to the pocket gophers and have some degree of specialization for jumping (saltation). Seeds are gathered and stored temporarily in the cheek pouches; this allows relatively rapid gathering of food without long exposure to predators (mostly owls, but also such terrestrial predators as foxes). The kangaroo rats tend to forage into open areas where they are particularly vulnerable to owl predation. Their middle ear chambers are greatly enlarged, allowing them to pick up the very faint, low frequency sound made by an attacking owl's wings when it brakes while reaching its talons for its prey; a split-second panic jump on the part of the rodent may (or may not) save it.

When multicellular organisms burn food for energy, the waste products are CO2 and H2O. The latter, known as metabolic water because it is formed during the metabolizing of food, is usable by the organism, but usually forms only a small part of the water requirements. Some of the kangaroo rats are so adapted for arid conditions that they are able to survive solely on metabolic water while feeding on dry seeds (that doesn't mean they like it—only that they can do it when necessary).

Beavers (family Castoridae, Castor canadensis) are limited to the water courses of the larger streams and rivers. With the shallow, fluctuating waters of Southwestern rivers, the classic building of dams and lodges is not practical; instead, dens are excavated into river banks, the entrance opening under water. Feeding is on the bark of willows and other riparian growth.

The Muridae has been variously defined in recent years, from including the bulk of North American non-geomyoid rodents to including only the introduced Old World House Mouse (Mus musculus), Roof Rat (Rattus rattus), and Norway Rat (Rattus norvegicus). Currently we are recognizing the latter classification, with the House Mouse and Roof Rat occurring around human habitations in the desert.

Previously recognized as subfamilies within the Muridae, the native American mice and rats along with the voles are now recognized as members of the family Cricetidae (which takes them back to a yet earlier classification). Within the North American Cricetidae, there are three subfamilies recognized: Neotominae, Sigmodontinae, and Arvicolinae.

The Neotominae includes the bulk of the Chihuahuan Desert cricetids. Most of these fall into groups known commonly as woodrats (or packrats), white-footed mice, grasshopper mice, and harvest mice. The woodrats belong to the genus Neotoma. These are rat sized but generally considered more attractive than the Old World rats. Nests are protected by accumulations of materials, usually sticks along with anything else transportable, such as cow patties, scavenged bones, and soft-drink cans. The names packrat and trade rat comes from the propensity of dropping whatever they are carrying if something more attractive catches their eyes. Thus a diamond ring may be carried off with a discarded stick left in its place—a fair trade from the viewpoint of the rodent.

In protected rocky areas (such as rock shelters, cave entrances, or protected fissures), discarded food items mixed with fecal pellets and urine may be preserved for long periods of time (to at least something over 40,000 years). Much of the information about past vegetation in what now is Chihuahuan Desert has come from investigation of such packrat middens. Preservation of plant parts often is exquisite, allowing identification to species in many cases.

Neotoma is common in most of the West and Mexico, with some occurrence in the eastern U.S. The White-throated Woodrat (N. albigula) is widespread west of the Rio Grande and south in Mexico west of the Rio Conchos. A cryptic species (one not obviously different to the eye, but discernable as different from genetic data) is present to the east; this is the Eastern White-throated Woodrat, N. leucodon. In areas where these overlap with the Southern Plains Woodrat (N. micropus), they tend to remain in desert mountains and outcrops of rocky terrain; in areas without N. micropus, which is most of the desert except the northern part and the extreme eastern margin, they will inhabit both habitats. Both species usually build houses of sticks, etc., around the base of bushes when building out in the basins.

The range and characteristics of the relatively small Goldman's Woodrat (N. goldmani) are poorly known, apparently occurring south of the border in Chihuahua and Coahuila. The Mexican Woodrat (N. mexicana) occurs throughout the region, but usually in higher elevations (pinyon-juniper and higher); under special circumstances (such as deep rock flows), it may descend to lower elevations.

Grasshopper mice (Onychomys) are rodents that in some ways act more like carnivores than the average mouse. They rely heavily on insects in the seasons when those are common and will eat mammalian flesh more often than most other rodents. Mammalogists using snap traps to collect small rodents are used to finding some trapped mice partially eaten by grasshopper mice. The tails of these mice are shorter and fatter than those of other mice with which they might be confused. The Northern Grasshopper Mouse (O. leucogaster) enters the northern part of the Chihuahuan Desert and also occurs along the eastern edge; its range extends far to the north in the U.S. Mearns Grasshopper Mouse (O. arenicola) is primarily Chihuahuan Desert, though extending somewhat beyond its western and eastern boundaries and far beyond to the south.

White-footed mice (Peromyscus) consist of a large number of species of mice generally similar in appearance. Differences tend to be the relative length of the tail, hind foot size, ear size, and sometimes details of coloration. Usually several species occur in an area with habitat differences. For instance, around El Paso, P. eremicus tends to occur in rocky hillside habitats or sand-dune habitats; P. leucopus in rather heavily vegetated arroyos; P. boylii in the vicinity of oaks in the mountains; and P. nasutus in the highest elevations of the Franklins. Peromyscus maniculatus seems to slip in wherever a space is left open by the other species.

The harvest mice (Reithrodontomys) are small mice with grooved upper incisors, setting them off from the white-footed mice and House Mice with which they might be confused. The Western Harvest Mouse (R. megalotis) is the common species, with R. montanus rare in grasslands in the northern Chihuahuan Desert.

Although there are numerous mice in the subfamily Sigmodontinae, the Chihuahuan Desert forms are all cotton rats. The lower elevations are inhabited by the Hispid Cotton Rat (Sigmodon hispidus), while the Tawny-bellied Cotton Rat occurs in the western portions, but not east of the Rio Grande. In highland areas from southwestern New Mexico and southeastern Arizona and from the Guadalupe and Davis mountains south, the Yellow-nosed Cotton Rat S. ochrognathus occurs. Cotton rats are, in some ways, the low elevation version (ecologically) of voles. Like the voles, they tend to live in grassy habitats, construct tunnels through the grass, and often are active during the day as well as during the night.

The voles belong to the subfamily Arvicolinae and are mostly highland forms on the periphery of the Chihuahuan Desert. However, the Meadow Vole has a relictual population in northern Chihuahua and there is a distinct possibility that populations may have survived into historic times in the desert proper where there has been continuous water available since the end of the Pleistocene ice age. There also is some indication that this mouse may have survived into recent times in the Bootheel of New Mexico. One member of the subfamily is widespread in southern New Mexico and adjacent Trans-Pecos Texas: the Muskrat (Ondatra zibethicus) essentially is an overgrown semi-aquatic vole. It inhabits drainage ditches and quiet areas of the Rio Grande. There don't appear to be records farther south in Mexico.

The Beaver is the largest of the North American rodents; the Porcupine (Erethizon dorsatum, family Erethizontidae) is the second largest. Porcupines are best known for their quills, which are modified hairs. These have small barbs pointed toward the root end of the quill so that when the quill penetrates, it is difficult to dislodge. Porcupines trace back their ancestry into South American, moving northward after the connection was formed between South and North America in the Pliocene. This species is widespread in northern North America from Alaska to Labrador and south in the western U.S. into the northern Chihuahuan Desert. Although most common in the sky islands of the arid lands, it does get down into creosotebush desert.

ventral view of the skull of Sylvilagus nuttallii get down into creosotebush desert.

ventral view of the skull of Sylvilagus nuttallii

The order Lagomorpha consists of distinctive mammals seldom confusable with any other group, but showing relatively little variation within the order. Lagomorphs have two upper incisors on each side but differ from other mammals in that the second incisor is behind the first rather than to the side; looking a rabbit in the mouth, only an upper pair is visible (though the first pair has a groove running down the center which might confuse the unwary into thinking they were seeing four). Characteristically, numerous small opening are present within the skull.

A ventral view of a Nuttall's Cottontail skull shows the grooves of the first incisors and the position of the peg-like second incisors behind the first.

Within the Chihuahuan Desert region, only one of the two families is represented (the Leporidae; the Ochotonidae are the pikas and, in the Southwest, animals of timberline regions) and only two genera: Lepus and Sylvilagus.

The two jackrabbits of our area are Lepus californianus, the Black-tailed Jackrabbit, and L. callotis, the White-sided Jackrabbit. The latter is basically marginal to the western desert, generally being elevationally higher than true desert. The Black-tailed Jackrabbit, however, is common throughout the desert region. The jackrabbits are hares, characterized by the young being precocial (born in an advanced state, eyes and ears open, etc.). Morphologically, a bone at the posterior of the skull known as the interparietal is fused and so not visible as a separate bone in the adult.

The genus Sylvilagus is represented by a couple of species of cottontails. They are altricial (born helpless with eyes and ears sealed; altricial animals require a period of time to develop into the state that precocial animals are born at). The Desert Cottontail (S. audubonii) is widespread throughout the desert lowlands. The second species may actually be several species and are marginal, occurring at generally higher elevations. These have been considered to belong to the Eastern Cottontail (S. floridanus), but there is no consensus as to what species or how many species are involved.


Last Update: 27 Jun 2006