Sciurus aberti
Abert's squirrel

Sciurus aberti is found in ponderosa or yellow pine (Pinus ponderosa) communities of the Southwest, usually between 1800 and 3000 m, in portions of Wyoming, Colorado, New Mexico, Arizona, and Utah in the United States and in the Sierra Madre Occidental from Northern Sonora and Chihuahua to southern Durango in Mexico (Nash and Seaman, 1977).

Sciurus aberti is found in coniferous forest habitats. All subspecies live in close association with ponderosa pine, which provides both shelter and food. Although the species is usually confined to ponderosa forests, S. aberti is common in mixed conifer forests in many New Mexican canyons (Nowak and Paradiso, 1983).

Abert's squirrel has long and broad ears that bear pronounced tufts or tassels in the winter pelage. The tail is short and unusually broad. The upper parts, including the tail, are mainly gray and the underparts are white. The lateral line is usually black and distinct (Encyclopedia Britannica, 1997). The skull is short and broad and the frontal area is flattened. The rostrum is narrow and laterally compressed. There are two upper pairs and one lower pair of premolars. Head and body length ranges from 463 to 584 mm and tail length from 195 to 255 mm. No major difference in size between males and females has been noted (Nash and Seaman, 1977).

Breeding season of Abert's squirrel is in April or May. Gestation period usually lasts 40 days. Three or four young are typically born to each female and there is often more than one litter each year, especially in the southern parts of the range. The young are altricial; they are born naked and their eyes and ears are covered by membranes. Vibrissae are present on the face and the toes bear well-developed claws. Young normally weigh 12 g and measure 60 mm at birth. The exact age of independence in the wild is not known. In captivity, the young first venture from the nests at about seven weeks, but it is not until nine weeks of age that they climb to the ground. Young are weaned at about ten weeks of age and mature size is not reached at until 15 or 16 weeks (Nash and Seaman, 1977). Juvenile males do not possess definitive scrota. In adult males, the testes are abdominal during early to late autumn. The testes begin to descend by February and are fully descended by mid-March. They remain large until August then begin to regress again (Nash and Seaman, 1977).

Abert's squirrels are strictly diurnal, becoming active shortly before sunrise and returning to their nests before sunset. These arboreal squirrels have been observed to travel frequently from tree to tree, jumping distances up to 2.4 m between trees. Like other tree squirrels, mating chases have been observed for S. aberti. Mating chases or bouts typically involve several males and a female in estrus. Chases usually begin early in the morning and last until dusk with a mean duration of 11 hours. Aggressive encounters between dominant males and females are common at this time. Little behavior associated specifically with courtship has been observed. Although the dominant male usually succeeds in copulating first, subordinate males also frequently copulate.

Nests are typically built in the branches of ponderosa pine. They consist of small twigs placed within pathogenic growths caused by dwarf mistletoe infection. Frequently, more than one nest is utilized (Nash and Seaman, 1977).

Abert's squirrels are herbivorous. They utilize ponderosa pine extensively as a source of food during the entire year. The inner bark, seeds, terminal buds, and staminate flowers of ponderosa pines are eaten. These squirrels also feed on fleshy fungi, carrion, bones, and antlers. They do not store food in large caches but have been reported to bury single pine cones in shallow pits. During the winter, the inner bark of twigs comprise the staple diet (Keith, 1965).

Abert's squirrel is a favorite game of hunters and its flesh is often eaten. The fur is not particularly valuable yet has been used for pelts (Nowak and Paradiso, 1983). This species may be involved in the dispersal of mycorrhizal fungi.

S. aberti causes damage to trees; it has been known to reduce ponderosa pine cone production by one-fifth in some areas (Nash and Seaman, 1977).

Population numbers of S. aberti appear to fluctuate widely over time and space but there is in no danger of extinction. Population cycles may be related to cyclic variation in the biomass of the pine seed crops. Eight subspecies of S. aberti are listed in CITES-Appendix III (Hall and Kelson, 1959).

Many populations of Abert's squirrel are relatively isolated as a result of the discontinuous distribution of ponderosa pine. This is one of the reasons for subspecific recognition of S. aberti (Nash and Seaman, 1977). Fossils of Abert's squirrel from the Middle Pliocene have been excavated from Nebraska and Oregon (Keith, 1965).