This species lives in dens made from crevices, other rock formations and even dens that other small mammals have left (Best et al. 1990). They generally burrow around and under bushes, boulders, or cacti and leave nothing on the surface to indicate that they have burrowed there (Best et al. 1990). Species in the genus Ammospermophilus are generally less territorial than other species of ground squirrel in the genus Spermophilus (Yensen and Sherman, 2003). They are thought to be active for the whole year without hibernating, or at least only hibernating a short time during November and December (Davis and Robertson, 1944; Best et al. 1990). Most of their activity occurs during the day, especially early in the afternoon when the temperature is at its peak (Best et al. 1990).
The Texas Antelope squirrel is thought to hide from predators and nest in the deep folds and crevices of lava beds in New Mexico (Bradt, 1932). This species also roams around in the patches of desert soil that had accumulated on the lava beds (Bradt, 1932). This could potentially be due to foraging behaviors as this is where the vegetation in the area (cacti, grasses, shrubs, mesquite, and juniper) is found (Bradt, 1932). They eat primarily seeds, berries, and insects (Best et al. 1990).
They normally start their breeding season in February and have litters ranging in size from 5 to 14 individuals (Best et al. 1990). They leave their nests when they are about 25% matured(Best et al. 1990). Males and females in this genus show reproductive behaviors by the time they are a year old, which is relatively young for ground squirrels (Yensen and Sherman, 2003).
They are quick to be startled and dart away with great speed towards their burrow or nearby rocks (Davis and Robertson, 1944; Best et al. 1990). Individuals can usually be seen darting from one bush to another, resting on a rock, climbing cacti or bushes, or running over rocks (Best et al. 1990). They have also been described as fidgety, wary, secretive, and nervous (Best et al. 1990). When they are nervous they flick their tails back and forth but they usually keep their tails up over their backs (Best et al. 1990).
The Texas Antelope squirrel has alarm calls similar to those in the genus Ammospermophilus, but the alarm calls are specific to the species (Bolles, 1988). Their calls tend to vary more than other Ammospermophilus species such as A. harrisii, A. leucurus, A. insularis, and A. nelsoni (Best et al. 1990). This species has calls that are relatively short, start with wide band of frequencies, and have trills of a lower pitch than other species (Bolles, 1988). The calls are a minimum of 0.6 kHz with a mean length of 1.07 seconds from start to end (Best et al. 1990; Garcia-Navas and Blumstein, 2016). Males and females in the genus Ammospermophilus do not seem to have differences in alarm calls (Bolles, 1988; Best et al. 1990). Other factors such as developmental stage, temperature, and season also do not appear to affect aspects of these alarm calls (Best et al. 1990).
Ammospermophilus interpres is found traditionally in rocky habitats within the vicinity of and inside of desert mountain ranges. It can also be found grassland and woodland habitats, but is most commonly seen in places where boulders or large rocks are in close propinquity to juniper and other large shrubs (Timm et al., 2008). Specifically, the Texas antelope squirrel resides in the high intermontane of the Chihuahuan Plateau from Jaral, Coahuila and Ciudad Lerda, Durango through northern and western Texas and into central New Mexico (Best et al., 1990).It has also been found to reside in the Mexican Plateau, which spans Chihuahua, Zacatecas, Nuevo Leon and San Luis Potos (Escalante et al., 2007).
In establishing its current range, it appears that this species went along with the expanding desert influences north into New Mexico (Best et al., 1990). The Texas Antelope squirrel was among the handful of lava-dwelling small mammals of New Mexico lava beds (Bradt, 1932). Ammospermophilus is widely distributed throughout the arid lands of western North America and presents an assemblage of geographically isolated species and populations (Mantooth et al., 2010). A.interpres represents a lineage that may have been isolated during the late Miocene to the early Pliocene (ca. 11 mya) when the secondary uplifting of the Sierra Madre Occidental and Central Mexican Plateau occurred concurrently with global climate change and the development of deserts (Mantooth et al., 2013). A series of filter-barriers drove the divergence and maintains genetic separations between Ammospermophilus taxa that share a Sonoran-Chihuahuan dispersal history (Mantooth et al., 2010).
It resides at 1,800 m in the Chisos Mountains in Western Texas (Best et al., 1990). It is widely distributed from the gorge bottom at Big Bend, Texas up to 1,650 m elevation into the mountains (Bryant, 1942). In the Trans-Pecos region in Texas, A. interpres occurs from 540 m to 1,830 m, but is most commonly discovered at elevations between 1,000 and 1,650 m (Timm et al., 2008). The Sierra del Carmen-Sierra Madre Oriental escarpment acts as a geographical barrier for the known eastern range of this species. The southern barrier for the distributional range lays near latitude 25°N along the southern Coahuila-Rio Nazas area (Best et al., 1990). A. interpres has been captured from Crockett, Reagan, and Val Verde counties and those specimens represent the easternmost records and the only records of the species caught from the Edwards Plateau (Schwertner et al., 2011). The species has also been seen northward to the southern edge of the High Plains in Gaines County, Texas (Schmidly, 2004). The species lies north of 25°N in the Central Plateau (Baker, 1963).
The Texas Antelope squirrel (Ammospermophilus interpres) can be found in a variety of locations and habitats. It appears to seek out generally rocky desert areas associated with mountain ranges, canyons, and cliff faces (Best et al. 1990).
This species' Chihuahuan Desert habitats usually have vegetation such as tarbush (Flourensia), creosotebush (Larrea), lechuguilla (Agave), and sotol (Dasylirion) (Best et al. 1990).In New Mexico, their habitats are associated with juniper (Juniperus) woodlands (Best et al. 1990). In their Texan habitats the main vegetation is creosotebush, pinion (Pinus), juniper and oak (Quercus) (Best et al. 1990).
An earlier survey showed that they, and a handful of other small mammals, could be found in lava bed areas (or “Malpais”) in Lincoln Country west of Carrizozo, New Mexico (Bradt, 1932). The rock present there is black and difficult to walk on (for people and larger animals such as horses and cattle) due to its sharp and uneven surfaces (Bradt, 1932). The lava bed is 44 miles long and as much as 5.75 miles across, making the full extent of the area around 120 square miles along the Tularosa Basin (Bradt, 1932). The slope of the lava bed is around 30 feet/mile (Bradt, 1932). This change of elevation allows for this habitat to include differences in vegetation from one end to the other, such as juniper trees being present at the northern (higher elevation) end and not at the southern (lower elevation) end (Bradt, 1932). Sandy desert soil can be found in some small areas that the lava flow seems to have missed as well as in some of the lava folds or crevices where it has accumulated over time (Bradt, 1932). These soil areas are where vegetation grows (Bradt, 1932). At higher elevations juniper trees are dominant but at lower elevations mesquite is the dominant vegetation. Other types of vegetation include cacti, shrubs and grasses (Bradt, 1932). Within this habitat, the Texas Antelope squirrel has been found in shallow to deep lava bed crevices, under bushes on top of the lava beds, under juniper on top of the lava beds, and on the desert soil areas (Bradt, 1932). Other small mammals that were trapped in these locations and share habitat with the Texas Antelope squirrel include the dark Malpais woodrat (Neotoma albigula melas), the rock squirrel (Otospermophilus grammur), the black woodrat, the cactus mouse (Peromyscus eremicus), the pinyon mouse (Peromyscus truei), the long-nosed deer mouse (Peromyscus nasutus), the rock pocket mouse (Peromyscus intermedius intermedius), the Malpais pocket mouse (Peromyscus intermedius ater), the Baird pocket mouse (Perognathus flavus), and the Ord kangaroo rat (Dipodomys ordii ord) (Bradt, 1932).
The cheekpouches are quite large and extend below the level at which the manubrium sterni sits. The ears do not project outwardly against the skull, but rather sit mostly flattened against the head. Each pinna is round and the width of the ear is at least equal to or greater than the height of the ear, which are overall short and broad. There is also no intertragal notch. The tail length ranges from 25% to 33% of the total body length and the legs are relatively long. The soles of the feet are significantly covered with thick hair (Bryant 1945).
Average measurements, taken from eight males and two females, are as follows: total length, 226 mm; length of tail vertebrae, 74.2 mm; length of hind foot, 37.8 mm; and length of ear, 9.8 mm (Baker, 1956). Average cranial measurements, taken from eight males and six females, are as follows: greatest length, 39.3 mm; palatilar length, 17.3 mm; zygomatic breadth, 22.7 mm; cranial breadth, 18.9 mm; interorbital breadth, 9.9 mm; post-orbital constriction, 14.5 mm; length of nasals, 12.6 mm; and length of maxillary toothrow, 6.6 mm. Average weight, taken from four adult, nonpregnant females, was 110.2 g (Baker, 1956). The dental formula for A. interpres is I 1/1, c 0/0, p 2/1, m 3/3, totaling twenty-two teeth in all (Ingles, 1965).
A. interpres has a large white stripe bordered by black fur located dorsolaterally on both sides of the body. There are no distinct stripes on the head. The fur on the dorsal portion of the body is a fine gray color, faintly tinted red towards the posterior and tawny on the nose. When displaying its winter coat, the upper part of A. interpres appears a pale gray color from afar, when really the tips of the hairs are white with a sub-terminal band of dark. The front of the face, including the nose, are tinted a slight pinkish cinnamon color, and the ring around the eye is white. The front and hind legs, including the feet, and also hips are a pinkish cinnamon color. The top side of the tail is a mixed black and white with a patch of a pale pink of the proximal end (Howell, 1938). The bottom side of the tail is white with two complete black bands that have a white border. Each individual hair on the lateral portion of the tail is basally black, with two more black zones that alternate with three white zones (Best et al., 1990).
A. interpres males possess a baculum that is fan-shaped and less than 2 mm in length, with lateral proximal and distal expansions. The only apparent function of the baculum is to facilitate insertion of the penis during mating. The functional attributes of the fan-shape, asymmetry, and tooth-like projections of the baculum are unknown. The mammae of females are arranged in five pairs, suggesting that typical litters are of at least five or more young (Best et al., 1990).
