Abert’s squirrels can weigh between 540 to 971 g, but they average about 620 g. At birth, they weigh about 12 g and reach 355 g while they are being weaned. On average, they are 450 to 580 mm long. There are 9 subspecies of Abert's squirrels, which may have several different coat colors. Seven of the subspecies are gray, and the other two have black or brown coats. Many of the subspecies have a red stripe that runs down their back, it can be well-defined in populations north of the Grand Canyon or faded or not present at all, in squirrels in eastern Arizona, New Mexico and southern Colorado. Completely black forms of the species can be common, especially in northern Colorado. Some of the subspecies also have white eye rings or tails. Abert's squirrels can be easy to identify due to their long tufted ears, which have gained them another common name, 'tassel-eared squirrels'. They have these tufts most of the year, and adults lose them from July to September. Both males and females of this species look similar. (Dodd, et al., 2003; Keith, 2003; Linzey, 2008; Nash and Seaman, 1977; Ramey and Nash, 1976; Reid, 2006)
Abert's squirrels (Sciurus aberti) are mostly found in the mountainous areas of the southwestern United States and north central Mexico. Populations of this species are often separated, but are usually found near ponderosa pines. There is no overlap in the geographic range of the 9 subspecies. They are found in the United States in northern Arizona, the Kaibab Plateau in Arizona, the Arizona and New Mexico border, the border of New Mexico and Colorado, in the Rocky Mountains in central Colorado and in southeastern Utah, likewise, they are found in Mexico in northwestern Chihuahua, Durango and in Durango and southern Chihuahua. In Utah, New Mexico and Arizona, extra populations of these squirrels were introduced to encourage squirrel hunting in these areas. (Keith, 2003; Linzey, 2008; Nash and Seaman, 1977)
Abert's squirrels are usually found in areas with many ponderosa pine trees, although in New Mexico and Mexico, they can be found living in mixed forests. The elevation of these pine forests ranges from 1,830 to 2,590 m, and the squirrels are usually found at an elevation of 2,160 to 2,380 m. Most of their nests are found in large groups of about 200 ponderosa trees that have interlocking canopies, this is needed so they can hide their nests and move freely from tree to tree. Abert's squirrels usually prefer larger ponderosa trees that produce many cones, an important part of their diet. (Dodd, et al., 2003; Farentinos, 1972; Keith, 2003; Linzey, 2008; Sullivan, 1995)
Female Abert's squirrels mate seasonally. Males and females select multiple partners, a mating system known as polygynandry. The mating process begins when males chase females, in return, the females usually act coy. Dominant males are the first to mate with the females, after which females may mate with several other males. These squirrels use two types of nests. First, bolus nests, which look like pine twigs in a ball shape. To build these nests, they place twigs on a branch against the tree trunk and use softer materials such as grass or fabric on the inside of the nest as the liner. Likewise, broom nests occur naturally, they are made from the dwarf-mistletoe infections that occur in tree limbs. These nests require very little work, when needed; they add twigs and line the nest with soft materials. (Farentinos, 1972; Farentinos, 1980)
Abert's squirrels become sexually mature when they are about 327 days old. Their mating season lasts from February until June. They have a 43-day gestation period, with an average litter size of 3.5 individuals, ranging from 1 to 5 young. Their offspring weigh about 12 g at birth, but by the time they are weaned, about 70 to 76 days later, they weight about 355 g. (Tacutu, et al., 2012)
Since males mate with so many females, they cannot offer parental care to all of their offspring. They do guard the female after mating, but that is less about parental care and more about ensuring his genes pass on to the next generation. Females care for their young until they are independent, at about 10 weeks old. (Farentinos, 1980; Tacutu, et al., 2012)
Abert's squirrels are active during the day, from just before sunrise until just before sunset. These squirrels are not territorial, and multiple squirrels may live in the same nest. In addition to sharing nests, they often use more than one nest. As many as 2 to 114 Abert's squirrels may be found in a square kilometer, within an area of ponderosa trees. During the day, they are mostly solitary, spending most of their time foraging. There is no evidence of these squirrels hibernating or going into torpor. (Halloran and Bekoff, 1994; Linzey, 2008; Nash and Seaman, 1977)
Abert's squirrels have a fairly big home range for their size. They travel between trees, as well as among nests. These squirrel are not territorial, so they often more from nest to nest. Their home range from spring to autumn is between 40.5 to 90 ha, and during winter it is much smaller at about 20 ha. (Farentinos, 1979; Halloran and Bekoff, 1994; Nash and Seaman, 1977)
Abert’s squirrels communication often during the spring, when breeding season starts. They are much less social in the summer, fall and winter. They are mostly solitary, and maintain their distance, although they sometimes share nests. This might be mostly due to the limited number of nest cavities. Abert's squirrels communicate vocally and visually, as well as by touch, smell and taste. These squirrels make several sounds including clucks, barks, screeches and squeals. Their sounds may be identified from other nearby squirrel species due to its high pitch. Packs of male Abert's squirrels are mostly aggressive when choosing a mate. The dominant males lead the others as they follow a female squirrel throughout the forest. Males follow the females for around 11 hours throughout the forest during the day. (Keith, 2003; Linzey, 2008; Nash and Seaman, 1977; Reid, 2006)
Abert's squirrels use ponderosa pines for shelter, protection from predators and food. Their diets change seasonally, but usually include items from the ponderosa trees such as cones, buds, fungi, seeds and the inner bark. Squirrels introduced into the Pinaleño Mountains live on different conifer trees, but still eat the same basic items. They have also been seen eating dwarf mistletoe and road dirt. Abert's squirrels usually do not store food, so they must forage constantly. The inner bark of ponderosa twigs are their main food item from autumn to the spring. They forage less often during the winter when it snows, probably because they are more easily spotted by predators. (Edelman and Koprowski, 2005; Snyder, 1992)
Predators of Abert's squirrels include northern goshawks, cougars, bobcats and coyotes. Physically, Abert's squirrels have no anti-predation adaptations, but they perform behaviors that discourage predation. They spend much of their time in trees with interlocking canopies, which are convenient for traveling. This reduces their time on the ground, making them less available to ground predators. After sunning themselves, they also lay down and cling to the top of a tree branch. This helps them lose heat and makes them less visible. These squirrels usually do not forage on windy days, this may be because the wind hides the signs of an approaching predator. (Holladay, 2013)
Abert's squirrels have had an effect on the ponderosa pines they live in, as all of their survival needs, such as food and shelter are fulfilled by these trees. Abert's squirrels may also have had a negative impact on the endangered species Mount Graham red squirrels in the Pinaleño Mountains in Arizona, due to competition for resources after they were introduced to the red squirrels' habitat. Abert's squirrels host several parasites. They may have various nematode or roundworm parasites, protozoa and fleas. (Edelman and Koprowski, 2009; Patrick and Wilson, 1995; Snyder, 1992; Worden and Kleier, 2012)
Abert's squirrels are used as a Management Indicator Species (MIS), which means they are researched to help determine how to best take care of natural areas. These squirrels are also hunted in Colorado, New Mexico and Arizona. ("Management Indicator Species Assessment", 2011; Holladay, 2013)
Currently, all of the subspecies of Abert's squirrels are stable, although they are isolated. The biggest threat to these squirrels is probably destroying their habitats. (Linzey, 2008)
Amanda Marks (author), Radford University, Karen Powers (editor), Radford University, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
USDA Forest Service - Southwestern Region. Management Indicator Species Assessment. Taos, NM: USDA. 2011. Accessed December 19, 2013 at http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5336081.pdf.
Dodd, N., J. States, S. Rosenstock. 2003. Tassel-eared squirrel population, habitat condition, and dietary relationships in north-central Arizona. Journal of Wildlife Management, 67/3: 622-633.
Edelman, A., J. Koprowski. 2005. Diet and tree use of Abert's squirrels (Sciurus aberti) in a mixed-conifer forest. The Southwestern Naturalist, 50/4: 461-465.
Edelman, A., J. Koprowski. 2009. Introduced Abert's squirrels in the Pinaleno Mountains: a review of their natural history and potential impacts on the red squirrel. Pp. 370-388 in H Sanderson, ed. The Last Refuge of the Mt. Graham Red Squirrel: Ecology of Endangerment. Tucson: University of Arizona Press. Accessed September 04, 2013 at http://www.academia.edu/1531226/Introduced_Aberts_squirrels_in_the_Pinaleno_Mountains_a_review_of_their_natural_history_and_potential_impacts_on_the_red_squirrel.
Farentinos, R. 1972. Nests of the tassel-eared squirrel. Journal of Mammalogy, 53/4: 900-903.
Farentinos, R. 1979. Seasonal changes in home range size of tassel-eared squirrels (Sciurus aberti). The Southwestern Naturalist, 24/1: 49-61.
Farentinos, R. 1980. Sexual solicitation of subordinate males by female tassel-eared squirrels (Sciurus aberti). Journal of Mammalogy, 61/2: 337-341.
Halloran, M., M. Bekoff. 1994. Nesting-behavior of Abert squirrels (Sciurus aberti). Ethology, 97/3: 236-248.
Holladay, K. 2013. "Abert's squirrel (Sciurus aberti)" (On-line pdf). Wildlife Notes. Accessed October 08, 2013 at http://www.wildlife.state.nm.us/education/wildlife_notes/documents/Abertssquirrel.pdf.
Keith, J. 2003. "The Abert’s squirrel (Sciurus aberti): a technical conservation assessment" (On-line pdf). www.fs.fed.us. Accessed September 19, 2013 at http://www.fs.fed.us/r2/projects/scp/assessments/abertsquirrel.pdf.
Nash, D., R. Seaman. 1977. Sciurus aberti. Mammalian Species, 80: 1-5.
Pogany, G., S. Allred. 1995. Abert's squirrels of the Colorado Plateau: their reproductive cycle. Reproduction in Abert squirrels, 1: 293-305.
Ramey, C., D. Nash. 1976. Coat color polymorphism of abert's squirrel, Sciurus aberti, in Colorado. The Southwestern Naturalist, 21/2: 209-217.
Reid, F. 2006. A Field Guide to Mammals of North America. New York: Houghton Mifflin Company.
Snyder, M. 1992. Selective herbivory by Abert's squirrel mediated by chemical variability in Ponderosa pine. Ecology, 73/5: 1730-1741.
Sullivan, J. 1995. "Wildlife species: Sciurus aberti" (On-line). www.fs.fed.us. Accessed September 11, 2013 at http://www.fs.fed.us/database/feis/animals/mammal/scab/all.html.
Tacutu, R., T. Craig, A. Budovsky, D. Wuttke, G. Lehmann, D. Taranukha, J. Costa, V. Fraifeld, J. de Magalhaes. 2012. "Human Ageing Genomic Resources: integrated databases and tools for the biology and genetics of ageing" (On-line). AnAge: The animal ageing and longevity database: Sciurus aberti. Accessed September 05, 2013 at http://genomics.senescence.info/species/entry.php?species=Sciurus_aberti.
Weigl, R. 2005. Longevity of mammals in captivity : from the living collections of the world : a list of mammalian longevity in captivity. Germany: Stuttgart : Schweizerbart.
Wood, D., J. Koprowski, P. Lurz. 2007. Tree squirrel introduction: a theoretical approach with population viability analysis. Journal of Mammalogy, 88/5: 1271-1279.
Worden, K., C. Kleier. 2012. Impact of thinning Ponderosa pines (Pinus ponderosa) on populations of Abert's squirrels (Sciurus aberti). The Southwestern Naturalist, 57/4: 380-384.