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Sciurus abertiAbert's squirrel

By Amanda Marks

Geographic Range

Abert's squirrels (Sciurus aberti) are concentrated in the mountainous regions of the southwestern United States and in north central Mexico. Four geographically-isolated populations (2 large, 2 small) exist in the United States, and two large but isolated populations exist in Mexico. Each population follows the distribution of ponderosa pines. There are 9 subspecies, none of which overlap in their geographic range. Sciurus aberti aberti is located in northern Arizona, Sciurus aberti kaibabensis is located in the Kaibab Plateau, in northern Arizona, Sciurus aberti chuscensis is located around the Arizona and New Mexico border, Sciurus aberti mimus is located around the border of New Mexico and Colorado, Sciurus aberti ferreus is located in the Rocky Mountains in central Colorado, Sciurus aberti navajo is located in southeastern Utah, Sciurus aberti barberi is located in northwestern Chihuahua, Sciurus aberti durangi is located in Durango, and Sciurus aberti phaeurus is located in Durango and southern Chihuahua. These squirrels were introduced to additional portions of Utah, New Mexico and Arizona to encourage squirrel hunting in these areas. (Keith, 2003; Linzey, 2008; Nash and Seaman, 1977)

Habitat

Abert's squirrels are concentrated in areas with an abundance of ponderosa pine trees, they are not nearly as dependent as was once believed, but they still rely heavily on this preferred tree. Although in New Mexico and Mexico, these squirrels can be found living in mixed forests. Elevations of these pine forests range from 1,830 to 2,590 m, and the typical elevation for these squirrels is 2,160 to 2,380 m. Most of their nests are located in trees that grow within groups of about 200 ponderosa trees with interlocking canopies, which is needed for protective covering as well as mobility. The ponderosa trees the squirrels prefer usually have a diameter at breast height greater than or equal to 30 cm, as these trees produce more cones, an important part of their diet. (Dodd, et al., 2003; Farentinos, 1972; Keith, 2003; Linzey, 2008; Sullivan, 1995)

  • Range elevation
    1830 to 2590 m
    6003.94 to 8497.38 ft
  • Average elevation
    2160 -2380 m
    ft

Physical Description

Abert’s squirrels range from 540 to 971 g, with an average size of about 620 g. At birth, they weigh about 12 g and reach 355 g during the weaning period. On average, they will reach total lengths of 450 to 580 mm. There are 9 subspecies of Abert's squirrels, which may possess several different coat colors. Seven of the subspecies are gray, and the remaining two have either 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, which is common in the squirrels found in eastern Arizona, New Mexico and southern Colorado. Melanistic forms of the species can be common, particularly in northern Colorado. Some of the subspecies also have white eye rings or tails. Their defining characteristic of Abert's squirrels is their long tufted ears, which has gained them the additional common name, 'tassel-eared squirrels'. They have these tufts most of the year; adults lose them from July to September. There is no obvious sexual dimorphism across any of the subspecies. (Dodd, et al., 2003; Keith, 2003; Linzey, 2008; Nash and Seaman, 1977; Ramey and Nash, 1976; Reid, 2006)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    540 to 971 g
    19.03 to 34.22 oz
  • Average mass
    620 g
    21.85 oz
  • Range length
    450 to 580 mm
    17.72 to 22.83 in
  • Average basal metabolic rate
    2.4020 cm3.O2/g/hr
  • Average basal metabolic rate
    2.402 W
    AnAge

Reproduction

Female Abert's squirrels mate seasonally, with multiple male partners of their choosing. The mating process begins with males "chasing" as females play "coy." The dominant male attempts to mount the female and the female tries to prevent copulation. She is more aggressive when trying to discourage the dominant males versus the subordinate males, which may cause the male to express his dominance. The dominant male is the first to copulate with the female; he must guard her from the subordinate males so they cannot interfere. After mating with the dominant male, the female will then mate with the subordinate males. A vaginal plug was found in a preserved female, which is thought to form after successful insemination. These squirrel build two types of nests. First, there are bolus nests, which appear to be pine twigs in a ball shape. The twigs have a diameter of about 1 to 2 cm, with lengths between 30 to 60 cm. They place a mass of twigs against the trunk on a chosen tree branch, while using softer materials such as grass or fabric on the inside of the nest as the liner. Likewise, broom nests are naturally occurring, they are made from the dwarf-mistletoe infections that occur in tree limbs. Minimal work is required to finish such nests; they add twigs where needed, and line the nest in the same fashion as bolus nests. (Farentinos, 1972; Farentinos, 1980)

Abert's squirrels become sexually mature at about 327 days. 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, approximately 70 to 76 days later, they have a body mass of about 355 g. (Tacutu, et al., 2012)

  • Breeding interval
    Abert's squirrels breed once a year
  • Breeding season
    Their mating season lasts from late February to early June.
  • Range number of offspring
    1 to 5
  • Average number of offspring
    3.5
  • Average number of offspring
    3.5
    AnAge
  • Average gestation period
    43 days
  • Average gestation period
    43 days
    AnAge
  • Average weaning age
    70 days
  • Average time to independence
    76 days
  • Average age at sexual or reproductive maturity (female)
    327 days
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    327 days
    AnAge
  • Average age at sexual or reproductive maturity (male)
    327 days

Promiscuous males have little parental investment, besides guarding the female after mating. However, this is less about parental investment 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)

  • Parental Investment
  • altricial
  • female parental care
  • pre-fertilization
    • protecting
      • male
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

Lifespan/Longevity

Not much is known about the lifespan of Abert's squirrels, although one captive squirrel lived to be 7 years old. (Tacutu, et al., 2012; Tacutu, et al., 2012; Weigl, 2005)

  • Average lifespan
    Status: captivity
    7 years

Behavior

Abert's squirrels are diurnal. They are active from just before sunrise until just before sunset. These squirrels are not territorial; multiple squirrels may live in the same nest. In addition to sharing nests, squirrels also seem to use more than one nest. They are found in high densities, 2 to 114 individuals in a square kilometer, within an area of ponderosa trees. During the day, they are fairly solitary, spending most of their time foraging. There is no evidence that this species hibernates or goes into torpor. (Halloran and Bekoff, 1994; Linzey, 2008; Nash and Seaman, 1977)

  • Range territory size
    20000 to 90,000 m^2
  • Average territory size
    73,000 m^2

Home Range

Abert's squirrels have a fairly big home range for their size. They travel between trees, as well as among nests. Because they are non-territorial, they can move from nest to nest easily. Their home range from spring to autumn is between 40.5 to 90 ha, and during winter it is considerably smaller at about 20 ha. (Farentinos, 1979; Halloran and Bekoff, 1994; Nash and Seaman, 1977)

Communication and Perception

Abert’s squirrels show a great deal of social communication during the spring at the start of breeding season. These squirrels are much less social in the summer, fall and winter. They are mostly solitary, and maintain distance among individuals; a somewhat contradictory report observed a majority of squirrels sharing nests frequently. The reasons for the contradicting reports could be due to their population density in the areas, with up to 114 individuals in a square kilometer, and the limited number of nest cavities. Abert's squirrels use several communication behaviors: vocal, visual, touch, and smell or taste. These squirrels make a variety of sounds including clucks, barks, screeches and squeals. Their sounds may be identified from other nearby squirrel species due to its high pitch. When choosing a mate, aggressive communication is typically used and involves a pack of male squirrels. The dominant male leads the inferior males as they follow a female squirrel throughout the forest. One study called it a “mating chase” though that is not necessarily the case. Males follow the females for around 11 hours throughout the forest during the day. (Keith, 2003; Linzey, 2008; Nash and Seaman, 1977; Reid, 2006)

Food Habits

Abert's squirrels rely on ponderosa pines for shelter, protection from predators, as well as for food. Their diets vary by season, but usually always include items from the ponderosa trees such as cones, apical buds, fungi, seeds, and the inner bark. Squirrels introduced into areas without ponderosa trees, such as the Pinaleño Mountains, eat similar plant components from different trees. They have also been observed eating dwarf mistletoe and road dirt. Abert's squirrels usually do not store much food, if any at all, so they must forage constantly for food. The inner bark of the ponderosa twigs is their main food from autumn to the spring. Foraging becomes less frequent during the winter when it snows, due to higher visibility by predators. (Edelman and Koprowski, 2005; Snyder, 1992)

  • Plant Foods
  • wood, bark, or stems
  • seeds, grains, and nuts
  • flowers
  • Other Foods
  • fungus

Predation

The typical predators of this species include northern goshawks, cougars, bobcats and coyotes. Abert's squirrels have no physical anti-predation features, but they exhibit behaviors that discourage predation. They spend much of their time in trees with interlocking canopies, which are convenient for traveling and mobility. This minimizes their time on the ground, making them less susceptible to ground predators. After sunning themselves, they also lay in a prostrate position while clinging to the top of a tree branch. This not only accomplishes a quick heat loss, but also makes them less visible to predators. These squirrels generally do not forage on windy days, this may be because the wind masks the signs of an approaching predator. (Holladay, 2013)

Ecosystem Roles

Abert's squirrels have had an effect on the ponderosa pines they live in because all of their survival needs, such as food and shelter are fulfilled by these trees. Abert's squirrels are also believed to have had an adverse impact on the endangered species Mount Graham red squirrels in the Pinaleño Mountains in Arizona. This impact was caused by resource competition after they were introduced to the red squirrels' native habitat. Abert's squirrels can host a range of internal and external parasites. Of the internal parasites, Abert's squirrels may have various nematode or roundworm parasites, which include Citellinema quadrivittati and Enterobius sciuri, as well as a protozoan, Eimeria tamiasciuri. Abert's squirrels may also have external parasites including fleas such as Ceratophyllus vison, Eumolpianus eumolpi, Opisodasys robustus, Orchopeas caedens caedens and Orchopeas neotomae. (Edelman and Koprowski, 2009; Patrick and Wilson, 1995; Snyder, 1992; Worden and Kleier, 2012)

Commensal/Parasitic Species

Economic Importance for Humans: Positive

One benefit these squirrels provide for humans is as a Management Indicator Species (MIS). Due to their abundant populations, Abert's squirrels were selected for a research project to obtain population data that would be useful in addressing the impact of forest management practices on wildlife. For example, the data would be informative on the best actions to take to prevent endangering species, or to stabilize a species at risk of endangerment. They are also a game animal, as they are used for hunting in Colorado, New Mexico and Arizona. ("Management Indicator Species Assessment", 2011; Holladay, 2013)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

Abert's squirrels offer no negative impacts to humans. (Edelman and Koprowski, 2009; Holladay, 2013)

Conservation Status

All of the subspecies of Abert's squirrels are considered stable, although they are isolated. The biggest threat to their populations is likely clear-cutting operations, which do not take the stability of their populations into account. (Linzey, 2008)

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)

Contributors

Amanda Marks (author), Radford University, Karen Powers (editor), Radford University, Leila Siciliano Martina (editor), Animal Diversity Web Staff.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

diurnal
  1. active during the day, 2. lasting for one day.
endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

induced ovulation

ovulation is stimulated by the act of copulation (does not occur spontaneously)

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

motile

having the capacity to move from one place to another.

mycophage

an animal that mainly eats fungus

native range

the area in which the animal is naturally found, the region in which it is endemic.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

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.

Linzey, A. 2008. "The IUCN Red List of Threatened Species" (On-line). Sciurus aberti. Accessed September 09, 2013 at http://www.iucnredlist.org/details/42461/0.

Nash, D., R. Seaman. 1977. Sciurus aberti. Mammalian Species, 80: 1-5.

Patrick, M., W. Wilson. 1995. Parasites of the Abert's squirrel (Sciurus aberti) and red squirrel (Tamiasciurus hudsonicus) of New Mexico. The Journal of Parasitology, 81/2: 321-324.

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.

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