White-tailed Antelope Squirrel, Ammospermophilus leucurus

White-tailed Antelope Squirrel, Ammospermophilus leucurus. Photograph taken in the greater Bahía de los Ángeles, Baja Calfornia Sur, November 2019. Photograph courtesy of George Flicker, Bahía de los Ángeles, Baja California Identification courtesy of Mary & George Flicker, Bahía de los Ángeles.

The White-tailed Antelope Squirrel, Ammospermophilus leucurus, is a member of the Sciuridae family of Flying Squirrels, Ground Squirrels and Tree Squirrels. The Sciuridae Family consists of two-hundred and eighty-six members placed into fifty-eight genera. There are five species in the Ammospermophilus genus. The White-tailed Antelope Squirrel are known in Mexico as Ardilla-antelope Cola Blanca.

White-tailed Antelope Squirrels appear quite similar to other Antelope Ground Squirrels with short, rounded ears and relatively long legs. However, this species can be distinguished by their brown to gray dorsal pelage that is marked with two white stripes from their shoulder to their rump. Their ventral pelage is creamy white and the outer surfaces of all their legs are somewhat reddish in color. Their tail is dark dorsally, white ventrally, and has one subterminal black band. The hair on their tail molts once in the fall, while the rest of their fur molts twice per year. The timing of their molt ranges from April to July in spring and August to November in autumn, varying according to geographic location and altitude. Their summer pelage is shorter and coarser than their winter pelage. White-tailed Antelope Squirrels have internal cheek pouches that they use to carry food. Females have ten mammae, which is slightly more than other ground squirrels who typically have four to seven. Sexual dimorphism is displayed with males growing slightly larger and weighing more than the females. They range in total length from 18.8 cm (7.4 inches) to 23.9 cm (9.4 inches), with a tail length of 4.2 cm (1.7 inches) to 8.7 cm (3.4 inches). They weigh between 96 g (3.4 ounces) and 117 g (4.1 ounces).

Unlike other ground rodents, the White-tailed Antelope Squirrel cannot survive indefinitely without free water. Death from water deprivation occurs after eight to thirty-five days. They can withstand dehydration of 20 to 25% of their field weight, and they regain weight quickly when water becomes available. Under extreme heat stress, they salivate profusely to avoid heat exhaustion. This behavior, which is commonly observed in animals confined in traps, is likely rare under normal conditions. The White-tailed Antelope Squirrel has effective kidneys that can produce urine with 12.2 times greater osmolarity than plasma, comparing favorably with other desert rodents. They can maintain body weight with a daily water ration as little as 2 to 5% of their body weight. This minimum water requirement fluctuates seasonally in accordance with available water in the environment.

White-tailed Antelope Squirrels have activity patterns that are strongly influenced by thermoregulatory constraints. During summer, daily activity peaks mid-morning and late afternoon, with a lull during midday when temperatures are extreme. White-tailed Antelope Squirrels maintain an active body temperature of 38℃ (100℉), which can fluctuate depending upon the environmental temperature. They tolerate hyperthermia well, allowing their body temperature to reach up to 44℃ (111℉) with no ill effects. This allows them to store heat over a longer period while they are active above ground. When critical temperatures are reached, individuals retreat to a burrow and sprawl flat on the ground with their ventral surface in contact with the substrate, quickly releasing excess heat via convection. During high temperatures they move rapidly while above ground, pausing to forage in the shade of shrubs and rocks. Individuals may spend up to one-fourth of their active time moving. During winter, they minimize energy costs by decreasing their body temperature at night to 31℃ (88℉) to 33℃ (91℉). White-tailed Antelope Squirrels tolerate cold poorly and will sometimes enter torpor if exposed to low temperatures. However, they cannot arouse from torpor until the ambient temperature rises above 20℃ (68℉). In winter, daily activity begins approximately one hour after sunrise with constant activity throughout the day, and ends approximately one hour before sunset. They favor open areas in winter, basking in the sun during much of the day to absorb solar radiation. During inclement weather, activity is greatly reduced.

White-tailed Antelope Squirrels form stable, linear hierarchies of dominance rather than defending exclusive territories. This dominance system is maintained by visual and tactile cues. Greetings usually consist of naso-nasal or naso-oral touching with variations depending upon the rank of individuals. When dominance is contested, displays can increase from lunging and side displays, to boxing and roll fights. Teeth chattering, low growls, and chirps accompany agonistic displays. Changes in dominance can occur gradually over the course of several days or may result from a singular agonistic encounter. White-tailed Antelope Squirrels live in burrows and females den with their current young. In winter, several individuals will live in a burrow and huddle together at night in order to conserve heat. Entrances to nest burrows are often close to shrubs and their tunnels descend approximately 45 cm in depth, with a nest chamber that is 13 to 20 cm in diameter. White-tailed Antelope Squirrels breed in spring from February to June. Both males and females first breed at one year of age, and a single litter of five to fourteen (average of eight) young is produced each year. Occasionally, a female may produce two litters in one year. Gestation lasts approximately thirty to thirty-five days and the young are altricial, being born with their eyes and ears closed and lacking adult fur. At birth, they weigh 3 to 4 g. Development is slow and the young display adult pelage about thirty-five days after birth. They do not acquire complete thermoregulatory capabilities until they are forty-five days old. Females build nests for their young made from dry plant materials and animal fur. Juveniles emerge from their burrow approximately one to two weeks before weaning, at about two months of age. Initially, juveniles do not respond to social signals by adults, but learn these signals gradually over the course of two to three weeks. While their average lifespan is not clearly documented, individuals in captivity have lived over five years. The average lifespan for other members of this genus is typically one year in the wild.

White-tailed Antelope Squirrels are omnivores and consume green vegetation, seeds, and insects depending upon seasonal availability and water content. They forage in trees, shrubs, or on the ground. They use their cheek pouches to store food until they return to their burrows, where they store their food in a cache. White-tailed Antelope Squirrels are preyed upon by diurnal desert predators including Raptors, Foxes, Coyotes, Badgers, and Snakes. If a predator is nearby, they create an alarm vocalization characterized by rapid chitters or trills. These trills can be heard at other times, even if predators are not present, for no obvious reason. White-tailed Antelope Squirrels are quick to flee from predators, and they very seldom look back when running away.

The White-tailed Antelope Squirrel may be confused with Harris’s Antelope Squirrel, Ammospermophilus harrisii, due to their similar appearances and overlapping ranges. These species can be differentiated visually and geographically. Harris’s Antelope Squirrel has a pelage with dark-brown roots, white in the middle, and brownish-white tips. Their ventral pelage is a mix of white and black. Additionally, Harris’s Antelope Squirrel is only found throughout southern Arizona, southwestern New Mexico, and northwestern Sonora, Mexico. Their range does not extend north of the Grand Canyon.

From a conservation perspective, the White-tailed Antelope Squirrel is currently considered to be of Least Concern due to their wide distribution and presumed large population size. No major threats have been identified for this species. Local populations on the two islands in the Gulf of California may be threatened by human activities and predation from feral cats.