Abert's squirrel or the tassel-eared squirrel (Sciurus aberti) is a tree squirrel in the genus Sciurus native to the southern Rocky Mountains from the United States to the northern Sierra Madre Occidental of Mexico, with concentrations found in Arizona, New Mexico, and southwestern Colorado. It is closely associated with, and largely confined to, mature ponderosa pine forests. It is named in honor of the American naturalist John James Abert; nine subspecies are recognised. It is recognizable by its tufted ears, gray color, pale underparts and rufous patch on the lower back. The squirrel feeds on the seeds and cones of the Mexican pinyon and the ponderosa pine when they are available, but will also take fungi, buds, bark, and carrion. Breeding normally occurs in summer, with a spherical nest being built high in the canopy.
Etymology Abert's squirrel is named after Colonel John James Abert, an American naturalist and military officer who headed the Corps of Topographical Engineers and organized the effort to map the American West in the 19th century.
Taxonomy The currently accepted scientific name for Abert's squirrel is Sciurus aberti Woodhouse, 1853.Hall, E. Raymond. 1981. The mammals of North America. 2nd ed. Vol. 2. New York: John Wiley and Sons Woodhouse had initially described the species as Sciurus dorsalis in 1852, but this name turned out to be preoccupied by Sciurus dorsalis Gray, 1849 (now a subspecies of variegated squirrel S. variegatoides), and thus the present species was renamed.
There are nine recognized subspecies, including the Kaibab squirrel (S. a. kaibabensis), formerly recognized as a separate species (S. kaibabensis). The nine subspecies are listed in the Distribution section.
Physical characteristics
Abert's squirrels are 46–58 cm long with a tail of 19–25 cm. They are easily recognized by their long ear tufts, which extend up from each ear 2–3 cm. This gives this species a striking similarity to the Eurasian red squirrel, aside from its differing dark coloration.
Abert's squirrels vary in color across their range. Most populations have a gray coat with a reddish stripe down the back, a dark stripe along the side, and a white underbelly. Some populations lack the reddish back stripe, instead having fully gray upperparts. A fully black color morph is present in the foothills of the Rocky Mountains in Colorado. Abert's squirrels on Arizona's Kaibab Plateau, commonly known as Kaibab squirrels, have black underparts and a striking white tail.
Distribution Abert's squirrel is confined to the Colorado Plateau and the southern Rocky Mountains of Colorado, Utah, Arizona, and New Mexico; its range extends south in the Sierra Madre Occidental to Chihuahua and Durango in Mexico. Abert's squirrel also extends into Wyoming where ponderosa pine (Pinus ponderosa) is present.
The range of Abert's squirrel has been extended through deliberate introductions in Arizona and New Mexico. The populations established in the Hualapai, Pinaleño, and Santa Catalina Mountains in Arizona consist of the nominate aberti subspecies. Abert's squirrels transplanted to the Graham and Santa Catalina Mountains of Arizona have established stable populations. Mellott and Choate reported Abert's squirrels present in the Spanish Peaks State Wildlife Area, southeast of the previously known Abert's squirrel range.
The distribution of Abert's squirrel subspecies in the Southwest is coincident with the disjunct ponderosa pine forests. Subspecies distributions are as follows:
S. a. aberti Woodhouse, 1853 – Arizona south of the Grand Canyon, southeastern Utah, southwestern Colorado, western New Mexico. S. a. barberi Allen, 1904 – western Chihuahua. S. a. chuscensis Goldman, 1931 – New Mexico-Arizona border area. S. a. durangi Thomas, 1893 – western Chihuahua and Durango. S. a. ferreus True, 1900 – south-central Colorado and Wyoming, New Mexico east of the Rio Grande. S. a. kaibabensis Merriam, 1904 – Kaibab Plateau, north of the Grand Canyon. S. a. mimus Merriam, 1904 – New Mexico-Colorado border area S. a. navajo Durrant and Kelson, 1947 – southeastern Utah S. a. phaeurus Allen, 1904 – northwestern and southwestern Durango and southwesternmost Chihuahua
Habitat Abert's squirrels in the U.S. make almost exclusive use of ponderosa pine for cover, nesting, and food. In Mexico where ponderosa pines are absent, the species is found in stands of the closely related Pinus arizonica. Abert's squirrels favor areas of forest with more closed canopies and denser growth, which provide better foraging and shelter from predators. Forests with heterogeneous distribution of trees and uneven tree aging are particularly favored. Optimum habitat has some ponderosa pine over d.b.h., which are the best cone producers.Clary, Warren P. 1987. Overview of ponderosa pine bunchgrass ecology and wildlife habitat enhancement with emphasis on southwestern United States. In: Fisser, Herbert G., ed. Wyoming shrublands: Proceedings, 16th Wyoming shrub ecology workshop; 1987 May 26–27; Sundance, Wyoming. Laramie, Wyoming: University of Wyoming, Department of Range Management, Wyoming Shrub Ecology Workshop: 11–21 Larson and Schubert report that ponderosa pine d.b.h. produced an average of 446 cones per tree per crop. Trees less than d.b.h. produced fewer than 100 cones per crop.Larson, M. M.; Schubert, Gilbert H. (1970). Cone crops of ponderosa pine in central Arizona, including the influence of Abert squirrels. Res. Pap. RM-58. Fort Collins, Colorado: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station
In central Arizona, Abert's squirrel summer home ranges averaged and ranged from . Ranges were somewhat smaller in winter. Ramey reports that the mean Abert's squirrel home range for spring and summer was in Black Forest, Colorado. Subadult males had spring home ranges of about , and adult females had somewhat larger summer home ranges than adult males.Ramey, Craig Anthony. (1973). The movement patterns and coat color polymorphism of Abert's squirrel, Sciurus aberti ferreus. Fort Collins, Colorado: Colorado State University. Dissertation Patton reported the ranges of three squirrels as 10, 30, and 60 acres (4.0, 12.2, and 24.4 ha) in Arizona.Patton, David R. (1975). Nest use and home range of three Abert squirrels as determined by radio tracking. Res. Note RM-281. Fort Collins, Colorado: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station Hall reported the home range of an adult female as .Hall, Joseph G. (1973). "The Kiabab squirrel". In: Symposium on rare and endangered wildlife of the southwestern United States: Proceedings; 1972 September 22–23; Albuquerque, New Mexico. Santa Fe, New Mexico: New Mexico Department of Game and Fish: 18–21
In Colorado, Ramey found a density of 83 squirrels per square mile (30/km2) in spring 1970 but only 33 squirrels per square mile (12/km2) in spring 1971. In another Colorado study, Farentinos estimated 227 squirrels per square mile (82/km2) in fall 1970 and 317 per square mile (114/km2) in fall 1971.
Plant communities
Abert's squirrel is closely associated with, and nearly confined to cool, dry interior ponderosa pine forests. In Arizona, ponderosa pine forests are most extensive between elevation.Patton, David R.; Green, Win. (1970). Abert's squirrels prefer mature ponderosa pine. Res. Note RM-169. Fort Collins, Colorado: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station Abert's squirrels occur in pure ponderosa pine stands or stands with associated Gambel oak (Quercus gambelii), Colorado pinyon (Pinus edulis), junipers (Juniperus spp.), quaking aspen (Populus tremuloides), and Douglas fir (Pseudotsuga menziesii). Findley and others mention that Abert's squirrels are common in mixed conifer canyons in New Mexico.Findley, James S.; Harris, Arthur H.; Wilson, Don E.; Jones, Clyde. (1975). Mammals of New Mexico. Albuquerque, New Mexico: University of New Mexico Press. .
In Durango and Chihuahua the squirrels are said to inhabit high altitude ponderosa pine stands. However, the ponderosa pines of Mexico have relatively recently (since at least 1997) been reclassified as a separate species Pinus arizonica (syn. P. ponderosa var. arizonica), and some regard much of that population as yet another species Pinus cooperi (P. arizonica var. cooperi).
Conservation This is a common species with a wide range and the International Union for Conservation of Nature has rated its conservation status as being of "least concern". Nonetheless, in Mexico, where much of its habitat has been logged, this species is considered 'vulnerable' by the Mexican environmental protection agency SEMARNAT and is protected.
Ecology and behavior Abert's squirrels are diurnal. They are often active for a short time before sunrise and active for periods throughout the day, and they usually return to shelter before sunset. Abert's squirrel does not store food, as other North American squirrels do.Edelman, A.J., and J.L. Koprowski. 2009. Introduced Abert's squirrels in the Pinaleño Mountains: a review of their natural history and potential impacts on the red squirrel. The Last Refuge of the Mt. Graham Red Squirrel: Ecology of Endangerment (H.R. Sanderson and J.L. Koprowski, eds.). University of Arizona Press, Tucson. Pp 370-388.
The most apparent causes of Abert's squirrel mortality are food shortage and injuries (such as broken teeth) that lead to mortality.
Reproduction The reproductive patterns of Abert's squirrels are strongly seasonal. Males possess external testes between winter and summer, which retract into the body cavity when the breeding season ends. Females are in estrus for a single day, during which males compete for access to them. Females typically breed with multiple males. Paternal care does not occur.
In central Arizona, breeding occurs from May 1 to June 1 and there are young in the nest from June 10 to July 27. Farantinos reported a 46-day gestation period. Eight litters were composed of two to five young each. Three or four young per litter is typical.Hall, E. Raymond; Kelson, Keith R. (1959). The mammals of North America, Volume II. New York: The Ronald Press Company Young Abert's squirrels are born naked, with ears and eyes closed. At 2 weeks thin short hair is noticeable and the ears are slightly open. By 6 weeks the pelage has developed and the eyes are open. By 7 weeks the tail has broadened and is held over the back, ears are held erect. Mushrooms and bark have been added to the diet at this time. Captive young first venture from the nest at about 7 weeks, but do not venture to the ground until about 9 weeks. By 10 weeks Abert's squirrels are weaned. Mature size is reached by 15 to 16 weeks. Female Abert's squirrels usually bear only one litter per year. Hall and Kelson, however, report that two litters are often borne per year in the southern parts of Abert's squirrel range.
Nesting Nests are built by the female Abert's squirrel out of pine twigs or less in diameter and long. Nests are lined with a variety of materials. Summer nests are built by Abert's squirrels on ponderosa pine branches, in Gambel oak cavities, and sometimes in cottonwood (Populus spp.) branches. Ponderosa pine seldom have cavities big enough for Abert's squirrels. Nests are typically built within denser stands of trees than the surrounding forest. In central Arizona nest trees ranged from 12 to 41 inches d.b.h. and were tall.