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Overview

Brief Summary

Description

"Abert's Squirrels have a complicated relationship with ponderosa pine trees. These squirrels mostly live in pine forests and use the trees for shelter, nesting sites, and food. Where they exploit the pines extensively, the trees produce extra terpenes—chemicals that give pines their scent—to discourage the squirrels' appetites. These trees grow more slowly than pines in areas where Abert's Squirrel is absent and the trees produce less of these chemicals. The pines vary in the amount of toxins produced, and the squirrels select trees that are less toxic. A pine growing in squirrel range may suffer reduced vitality as a consequence of having its stems and seeds eaten by squirrels, or have its growth rate reduced because it is producing more toxins. However, the squirrels provide an important benefit to the pines by distributing fungal spores (through their feces), which as mature fungi are essential to the pines' health, so the relationship is a fascinating one."

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  • Original description: Woodhouse, S.W., 1853.  Description of a new species of SciurusProceedings of the Academy of Natural Sciences of Philadelphia, 6:110.
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Distribution

Geographic Range

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).

Biogeographic Regions: palearctic (Native )

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Introduced Populations

As the result of intentional introductions, Abert’s squirrels now occur in several montane areas outside of their native range in both Arizona and New Mexico. The purpose of these introductions was to provide hunting opportunities for local residents (Davis and Brown 1988, 1989). Unfortunately, several of these areas already contained native tree squirrel species prior to introductions (Hoffmeister 1986). Competition from introduced Abert’s squirrels is believed to have contributed to the decline of the Arizona gray squirrel (Sciurus arizonensis catalinae) in the Santa Catalina Mountains and the Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) in the Pinaleño Mountains of Arizona (Lange 1960; Minckley 1968; Edelman et al. 2009).

  • Davis, R., and D.E. Brown. 1988. Documentation of the transplanting of Abert’s squirrels. Southwestern Naturalist 33:490–492.
  • Davis, R., and D.E. Brown. 1989. Role of post-Pleistocene dispersal in determining the modern distribution of Abert’s squirrel. Great Basin Naturalist 49:425–434.
  • Edelman, A.J., J.L. Koprowski, and S.R. Bertelsen. 2009. Potential for nest site competition between native and exotic tree squirrels. Journal of Mammalogy 90:167-174.
  • Hoffmeister, D.F. 1956. Mammals of the Graham (Pinaleno) Mountains, Arizona. American Midland Naturalist 55:257–288.
  • Lange, K.I. 1960. Mammals of the Santa Catalina Mountains, Arizona. American Midland Naturalist 64:436–458.
  • Minckley, W.L. 1968. Possible extirpation of the spruce squirrel from the Pinaleno (Graham) Mountains, south-central Arizona. Journal of the Arizona Academy of Science 5:110.
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Range Description

This species is found in the U.S. and Mexico. In the U.S. it is found in south-eastern Utah, south and west Colorado, extreme south-eastern Wyoming, west and central New Mexico, and Arizona (USA); in Mexico, it occurs in Chihuahua, Durango, and Sonora (Mexico). There are also introduced populations in many other locations in Arizona, New Mexico, and Utah.
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occurs (regularly, as a native taxon) in multiple nations

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National Distribution

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

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Global Range: Southwestern U.S. (southern Wyoming, southeastern Utah, central Colorado, New Mexico, and Arizona) and northern Mexico. S. A. KAIBABENSIS is found only in the Kaibab Plateau of north-central Arizona. Many populations are relatively isolated. See Davis and Brown (1988) for records of introductions in Arizona, New Mexico, and Utah. See Davis and Bissell (1989) for distribution in Colorado. See Davis and Brown (1989) for discussion of factors involved in explaining modern distribution.

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The 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 [17]. Abert's squirrel also extends a short distance
into Wyoming where ponderosa pine (Pinus ponderosa) is present. Abert's
squirrels transplanted to the Graham and Santa Catalina mountains of
Arizona have established stable populations. Mellott and Choate [22]
reported Abert's squirrels present in the Spanish Peaks State Wildlife
Area, 43 miles (72 km) 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 [24]. Subspecies
distributions are as follows [17]:

S. a. aberti: northern Arizona
S. a. barberi: northwestern Chihuahua
S. a. chuscensis: New Mexico-Arizona border area
S. a. durangi: Durango
S. a. ferreus: Rocky Mountains, central Colorado
S. a. kaibabensis: Kaibab Plateau, northern Arizona
S. a. mimus: New Mexico-Colorado border area
S. a. navajo: southeastern Utah
S. a. phaeurus: Durango and extreme southern Chihuahua
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 22. Mellott, Ron S.; Choate, Jerry R. 1984. Sciurus aberti and Microtus montanus on foothills of the Culebra Range in southern Colorado. The Southwestern Naturalist. 29(1): 135-137. [25365]
  • 24. Patton, David R. 1975. Abert squirrel cover requirements in Southwestern ponderosa pine. Res. Pap. RM-145. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 12 p. [25366]

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Regional Distribution in the Western United States

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This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):

7 Lower Basin and Range
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont

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Occurrence in North America

AZ CO NM UT WY



MEXICO

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Physical Description

Morphology

Abert’s squirrels typically have a white venter, slate-gray dorsum with a russet patch on the back, white eye ring, and a gray bushy tail fringed with white. Partial or complete melanic forms (black or brown) commonly occur in populations from the Mogollon Plateau of Arizona and New Mexico to the northern extent of the species (Brown 1984).

  • Brown, D.E. 1984. Arizona’s tree squirrels. Arizona Game and Fish Department, Phoenix.
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Physical Description

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).

Average mass: 702.5 g.

Average basal metabolic rate: 2.402 W.

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Size

Size in North America

Sexual Dimorphism: None

Length:
Range: 463-584 mm

Weight:
Average: 620 g
Range: 540-971 g
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Length: 58 cm

Weight: 908 grams

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Ecology

Habitat

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).

Terrestrial Biomes: forest

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Habitat and Ecology

Habitat and Ecology
This species is strongly dependent on ponderosa and yellow pine forest habitat, with some use of adjoining pinyon woodlands and mixed coniferous forests.

Systems
  • Terrestrial
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Comments: Mainly ponderosa pine forests at elevations of 1800-3000 m. On Kaibab Plateau, optimum habitat consists of relatively open stands of pure pine where trees larger than 15 cm DBH predominate (Hall 1981). May also extend into mixed conifer and upper pinyon-juniper woodland. When active, spends more time on ground than in trees (Arizona, Hall 1981). Builds bulky nests high in pines (nests infrequently in cavities in oak or cottonwood tree. Appears to use several alternate nests (Hall 1981). Nests usually are close to the trunk and more than 10 m above ground (Hall 1981). In northern Arizona, nests were in the crowns of large ponderosa pines, most often on the east to south side of the bole, mostly 10-15 m above ground; tree chemistry appeared to play a role in nest-site selection (Snyder and Linhart 1994).

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Cover Requirements

More info for the terms: formation, tree

Nesting: Summer nests are built by female 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. In central Arizona nest trees ranged
from 12 to 41 inches d.b.h. and were 20 to 110 feet (33.5 m) tall [17].
In another Arizona study nest trees ranged from 11.6 to 36.6 inches
(29.4-93 cm) d.b.h. Most nests are placed in the upper third of the
tree crown [37]. Nests were placed from 16 to 90 feet (4.9-27) above
the ground, usually on a large limb against the bole, or in the forks of
smaller branches. Nests were most often built on the southern to
southeastern side of the tree [17]. Patton [24] reported that nest
trees in Arizona had crowns that were 35 to 55 percent of the total tree
height, and most often were 14 to 16 inches (36-41 cm) d.b.h. Nests are
built in trees occurring as part of a grouping of trees with
interlocking crowns [3,24]. Dwarf mistletoe (Arceuthobium vaginatum)
infestations that cause the formation of "witches brooms" are often
incorporated into or support Abert's squirrel nests [7].

In winter pairs of Abert's squirrels, usually an adult female and one
subadult (presumed) offspring, use the same nest for shelter [17].
  • 3. 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, WY. Laramie, WY: University of Wyoming, Department of Range Management, Wyoming Shrub Ecology Workshop: 11-21. [13913]
  • 7. Farentinos, R. C. 1972. Nests of the tassel-eared squirrel. Journal of Mammalogy. 53(4): 900-903. [25360]
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 24. Patton, David R. 1975. Abert squirrel cover requirements in Southwestern ponderosa pine. Res. Pap. RM-145. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 12 p. [25366]
  • 37. Snyder, Marc A.; Linhart, Yan B. 1994. Nest-site selection by Abert's squirrel: chemical characteristics of nest trees. Journal of Mammalogy. 75(1): 136-141. [23150]

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Preferred Habitat

More info for the terms: basal area, cover, density, tree

Abert's squirrels make almost exclusive use of ponderosa pine for cover,
nesting, and food [17]. Optimum Abert's squirrel habitat is composed of
all-aged ponderosa pine stands with trees in even-aged groups, densities
of 168 to 250 trees per acre (496-618/ha), and 150 to 200 square feet
per acre (34.4-45.3 sq m/ha) basal area. In optimum habitat average
diameter of ponderosa pines is 11 to 13 inches (28-33 cm), with Gambel
oaks in the 11.8- to 14-inch (30-36 cm) d.b.h. range [11]. Optimum
habitat has some ponderosa pine over 20 inches (51 cm) d.b.h., which are
the best cone producers [3]. Larson and Schubert [20] reported that
ponderosa pine 36 to 40 inches (91-102 cm) d.b.h. produced an average of
446 cones per tree per crop. Trees less than 24 inches (61 cm) d.b.h.
produced less than 100 cones per crop.

Home Range: In central Arizona Abert's squirrel summer home ranges
averaged 18 acres (7.3 ha) and ranged from 10 to 24 acres (24.7-59.3
ha). Ranges were somewhat smaller in winter [17]. Ramey [30] reported
that the mean Abert's squirrel home range for spring and summer was 20
acres (8.13 ha) in Black Forest County, Colorado. Subadult males had
spring home ranges of about 27 acres (11 ha), and adult females had
somewhat larger summer home ranges than adult males [30]. Patton [25]
reported the ranges of three squirrels as 10, 30, and 60 acres (4.0,
12.2, and 24.4 ha) in Arizona. Hall [13] reported the home range of an
adult female as 29 acres (11.8 ha).

Population Density: In Colorado, Ramey [30] found a density of 83
squirrels per square mile (30/sq km) in spring 1970 but only 33
squirrels per square mile (12/sq km) in spring 1971. In another
Colorado study, Farentinos [5] estimated 227 squirrels per square mile
(82/sq km) in fall 1970 and 317 per square mile (114/sq km) in fall
1971.
  • 3. 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, WY. Laramie, WY: University of Wyoming, Department of Range Management, Wyoming Shrub Ecology Workshop: 11-21. [13913]
  • 5. Farentinos, Robert C. 1972. Social dominance and mating activity in the tassel-eared squirrel (Sciurus aberti ferreus). Animal Behaviour. 20: 316-326. [25472]
  • 11. Flyger, Vagn; Gates, J. Edward. 1982. Pine squirrels: Tamiasciurus hudsonicus and T. douglasii. In: Chapman, Joseph A.; Feldhamer, George A., eds. Wild mammals of North America: Biology, management, and economics. Baltimore, MD: The Johns Hopkins University Press: 230-238. [25232]
  • 13. 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, NM. Santa Fe, NM: New Mexico Department of Game and Fish: 18-21. [25568]
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 20. 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, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 15 p. [4714]
  • 25. Patton, David R. 1975. Nest use and home range of three Abert squirrels as determined by radio tracking. Res. Note RM-281. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 3 p. [25470]
  • 30. Ramey, Craig Anthony. 1973. The movement patterns and coat color polymorphism of Abert's squirrel, Sciurus aberti ferreus. Fort Collins, CO: Colorado State University. 179 p. Dissertation. [25552]

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Associated Plant Communities

The Abert's squirrel is closely associated with, and nearly confined to,
cool, dry interior ponderosa pine forests [17]. In Arizona ponderosa
pine forests are most extensive between 5,500 and 8,500 feet
(1,676-2,590 m) elevation [27]. Abert's squirrels occur in pure
ponderosa pine stands or stands with associated Gambel oak (Quercus
gambelii), true pinyon (P. edulis), junipers (Juniperus spp.), quaking
aspen (Populus tremuloides), and Douglas-fir (Pseudotsuga menziesii)
[17]. Findley and others [9] mention that Abert's squirrels are common
in mixed conifer canyons in New Mexico.

REFERENCES :
NO-ENTRY
  • 9. Findley, James S.; Harris, Arthur H.; Wilson, Don E.; Jones, Clyde. 1975. Mammals of New Mexico. Albuquerque, NM: Univeristy of New Mexico Press. 360 p. [25205]
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 27. Patton, David R.; Green, Win. 1970. Abert's squirrels prefer mature ponderosa pine. Res. Note RM-169. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 3 p. [13582]

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Habitat: Ecosystem

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This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):

FRES21 Ponderosa pine
FRES35 Pinyon-juniper

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Habitat: Rangeland Cover Types

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This species is known to occur in association with the following Rangeland Cover Types (as classified by the Society for Range Management, SRM):

412 Juniper-pinyon woodland

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Habitat: Cover Types

More info on this topic.

This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):

237 Interior ponderosa pine
239 Pinyon-juniper

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Habitat: Plant Associations

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This species is known to occur in association with the following plant community types (as classified by Küchler 1964):

K019 Arizona pine forest
K023 Juniper-pinyon woodland

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There are a number of anecdotal observations of Abert’s squirrels in non-ponderosa pine habitat throughout their native range. Individuals have been observed in oak-dominated woodland (Baker and Greer 1962), piñon pine woodland (Reynolds 1966), mixed-conifer forest (Findley et al. 1975, Hall 1981, Polechla 2005), spruce-fir forest (Allen 1895, Pedersen et al. 1976), and alpine tundra (Cooper 1987). Introduced Abert’s squirrels in the Pinaleño Mountains of Arizona extensively inhabit mixed-conifer and spruce-fir forests that contain almost no ponderosa pine and primarily nest in and feed on other conifer species such as Douglas-fir, southwestern white pine, and Engelmann spruce (Hutton et al. 2003, Edelman and Koprowski 2005a, b).

  • Allen, J.A. 1895. On a collection of mammals from Arizona and Mexico, made by Mr. W.W. Price, with field notes by the collector. Bulletin of the American Museum of Natural History 7:193–258.
  • Baker, R.H., and J.K. Greer. 1962. Mammals of the Mexican State of Durango. Publications of the Museum, Michigan State University, Biological Series 2:25–154.
  • Cooper, D.J. 1987. Abert’s squirrel above treeline on the San Francisco Peaks, Arizona. Southwestern Naturalist 32:507.
  • Findley, J.S., A.H. Harris, D.E. Wilson, and C. Jones. 1975. Mammals of New Mexico. University of New Mexico Press, Albuquerque, New Mexico.
  • Hall, J.G. 1981. A field study of the Kaibab squirrel in Grand Canyon National Park. Wildlife Monographs 75:1–54.
  • Hutton, K.A., J.L. Koprowski, V.L. Greer, M.I. Alanen, C.A. Schauffert, and P.J. Young. 2003. Use of mixed-conifer and spruce-fir forests by an introduced population of Abert’s squirrels (Sciurus aberti). Southwestern Naturalist 48:257–260.
  • Edelman, A.J., and J.L. Koprowski. 2005a. Selection of drey sites by Abert’s squirrels in an introduced population. Journal of Mammalogy 86:1220-1226.
  • Edelman, A.J., and J.L. Koprowski. 2005b. Diet and tree use of Abert’s squirrels (Sciurus aberti) in a mixed-conifer forest. Southwestern Naturalist 50:461-465.
  • Pederson, J.C., R.N. Hasenyager, and A.W. Heggen. 1976. Habitat requirements of the Abert squirrel (Sciurus aberti navajo) on the Monticello District, Manti-La Sal National Forest of Utah. Utah State Division of Wildlife Resources Publication 76-9, Salt Lake City.
  • Polechla, P.J., Jr. 2005. Mammals. Pages 169-195 in R. Julyan and M. Stuever, editors. Field guide to the Sandia Mountains. University of New Mexico Press, Albuquerque.
  • Reynolds, H.G. 1966. Abert’s squirrels feeding on pinyon pine. Journal of Mammalogy 47:550–551.
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Migration

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

A female followed for one year had a home range of 14 hectares (Hall 1981). Male sciurids typically have substantially larger home ranges than females.

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Trophic Strategy

Comments: Feeds primarily on Ponderosa pine: seeds, inner bark (when seeds are scarce), terminal buds, and staminate flowers. Also feeds on fungi (may be important in summer diet), carrion, bones and antlers. May bury single pine cones in shallow pits; apparently does not store food in large caches

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Food Habits

Abert's squirrels consume ponderosa pine year-round. Parts eaten
include seeds, which are the most highly preferred item, inner bark
(particularly of young twigs), terminal buds, staminate buds, and pollen
cones. Other foods include fleshy fungi (particularly hypogeous fungi),
carrion, bones, and antlers. Severe weather is not always a deterrent
to feeding activity [17]. Where Mexican pinyon (Pinus cembroides) seeds
are available, Abert's squirrels consume them in preference to ponderosa
pine seeds [13]. Gambel oak acorns may also provide substantial food
for Abert's squirrels [32].

Ponderosa pines produce large cone crops every 3 to 4 years; cones are
virtually absent about 1 year out of 4. Abert's squirrels begin eating
immature seed shortly after cone development begins in late May. Seeds
are eaten through the summer as the cones mature. Seeds from up to 75
cones may be eaten per day per squirrel during the months when seeds
form the squirrels' major food. Seeds are disseminated from cones in
October and November. Abert's squirrels continue to consume seed from
late maturing cones and collect single seeds from the ground. The
succulent inner bark of twigs is eaten all year, but most heavily in
winter. Needle clusters are clipped from the twigs, the outer bark is
removed, the inner bark is consumed, and then the twig is discarded. In
winter a single squirrel consumes about 45 twigs per day [17]. Most
feed trees range from 11 to 30 inches (30-76 cm) d.b.h. [27]. After
seeds have been disseminated Abert's squirrels are dependent on inner
bark, which forms the bulk of the diet from November to April. The soft
inner tissue of small apical buds is also a preferred item. In May
staminate buds and cones and immature ovules are consumed as available.
New staminate cones are entirely consumed; only the pollen is eaten from
dried cones. The bark of areas infected with dwarf mistletoe also
appears to be preferred [17].

Fleshy fungi consumed include members of the following genera:
Agaricus, Amanita, Boletus, Hypholoma, Lepiota, Lycopedon, Russula,
Tuber. Mushrooms poisonous to humans are consumed by Abert's squirrels
without difficulty, including destroying angels (A. muscaria and A.
vaginata) and a species of Russula [17].

Water is obtained mostly from food, but Abert's squirrels sometimes
drink at stock ponds or other standing water (i.e., rain puddles) [17].
  • 13. 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, NM. Santa Fe, NM: New Mexico Department of Game and Fish: 18-21. [25568]
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 27. Patton, David R.; Green, Win. 1970. Abert's squirrels prefer mature ponderosa pine. Res. Note RM-169. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 3 p. [13582]
  • 32. Reynolds, Hudson G.; Clary, Warren P.; Ffolliott, Peter F. 1970. Gambel oak for Southwestern wildlife. Journal of Forestry. 68(9): 545-547. [1960]

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Food Habits

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).

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Associations

Predators

Reynolds [31] suggested that northern goshawks (Accipiter gentilis) may
take enough Abert's squirrels to regulate Abert's squirrel populations.
Hawks (Buteonidae and Falconidae) prey on Abert's squirrels in central
Arizona, but even though other potential predators are present (i.e.,
gray fox [Urocyon cinereoargenteus], bobcat [Lynx rufus], coyote [Canis
latrans]) there is no evidence that they prey on Abert's squirrels [17].
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 31. Reynolds, H. G. 1963. Western goshawk takes abert squirrel in Arizona. Journal of Forestry. 61: 839. [25362]

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Known predators

Sciurus aberti (Kaibab squirrel) is prey of:
Buteo jamaicensis
Accipiter gentilis

Based on studies in:
USA: Arizona (Forest, Montane)

This list may not be complete but is based on published studies.
  • D. I. Rasmussen, Biotic communities of Kaibab Plateau, Arizona, Ecol. Monogr. 11(3):228-275, from p. 261 (1941).
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Known prey organisms

Sciurus aberti (Kaibab squirrel) preys on:
Pinus

Based on studies in:
USA: Arizona (Forest, Montane)

This list may not be complete but is based on published studies.
  • D. I. Rasmussen, Biotic communities of Kaibab Plateau, Arizona, Ecol. Monogr. 11(3):228-275, from p. 261 (1941).
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General Ecology

Basically solitary late spring through fall, nonterritorial (Hall 1981). Average home range in Arizona was estimated at 7.3 ha during summer and fall, 2.0 ha in winter; 18-45 ha in uncut forest in another study. Most of time may be spent in limited area of home range (Hall 1981). Populations seem to fluctuate widely over time. In Arizona, density was estimated at 30/sq km, 2.5-5/sq km, and 30-65/sq km. Feeding by squirrels appears to have neglible effect on ponderosa pine growth (Hall 1981).

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Habitat-related Fire Effects

More info for the term: low-severity fire

Ponderosa pine is dependent on frequent, low-severity fire for
maintenance and reproduction. Such fires also benefit Abert's squirrels
since they are dependent on ponderosa pine. See the ponderosa pine
write-ups (P. ponderosa var. arizonica and P. ponderosa var. scopulorum)
for further information on the effects of fire in ponderosa pine
habitats. The immediate effect of low-severity fire in ponderosa pine
on Abert's squirrels is probably negligible.

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Timing of Major Life History Events

More info for the term: litter

Active Period: 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 [17]. They are
used year-round by most Abert's squirrels for nightly shelter [37].

Nesting: Nests are built by the female Abert's squirrel out of pine
twigs 0.5 inch (1.3 cm) or less in diameter and 6 to 24 inches (15-61
cm) long. Nests are lined with a variety of materials [17]. Nests are
roughly spherical and a small platform often extends beyond the bowl
edge on one side [35]. Females often move the litter to a larger nest
when the young are 3 to 6 weeks old [17].

Breeding and Gestation: In central Arizona breeding occurs from May 1
to June 1 and there are young in the nest from June 10 to July 27 [17].
Farantinos [6] reported a 46-day gestation period.

Litter Size and Development of Young: In central Arizona eight litters
were composed of two to five young each [6,17]. Three or four young per
liter is typical [16]. 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 [17].

Female Abert's squirrels usually bear only one litter per year [17].
Hall and Kelson [16] however, reported that two litter are often borne
per year in the southern parts of Abert's squirrel range.

Mortality: The most apparent causes of Abert's squirrel mortality are
food shortage and injuries (such as broken teeth) that lead to mortality
[17].
  • 6. Farentinos, R. C. 1972. Observations on the ecology of the tassel-eared squirrel. Journal of Wildlife Management. 36(4): 1234-1239. [25359]
  • 16. Hall, E. Raymond; Kelson, Keith R. 1959. The mammals of North America, Volume II. New York: The Ronald Press Company. 79 p. [21460]
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 35. Skinner, T. H.; Klemmedson, J. O. 1978. Abert squirrels influence nutrient transfer through litterfall in a ponderosa pine forest. Res. Note RM-353. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 8 p. [18448]
  • 37. Snyder, Marc A.; Linhart, Yan B. 1994. Nest-site selection by Abert's squirrel: chemical characteristics of nest trees. Journal of Mammalogy. 75(1): 136-141. [23150]

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Life History and Behavior

Cyclicity

Comments: Active throughout the year. Diurnal activity begins shortly before sunrise; squirrels return to nests before sunset. In Arizona, most active during the first 4 hours after dawn in summer; midday rest period followed by late afternoon feeding (Hall 1981).

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Life Expectancy

Lifespan, longevity, and ageing

Observations: Little is known about the longevity of these animals, but one specimen lived 7 years in captivity (Richard Weigl 2005).
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Reproduction

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).

Average birth mass: 12 g.

Average gestation period: 43 days.

Average number of offspring: 3.5.

Average age at sexual or reproductive maturity (female)

Sex: female:
327 days.

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On Mogollon Plateau, Arizona, breeds mostly late April-May. Young are born in June and early July. Litter size is 2-5. Gestation lasts about 40 days. Young are weaned at about 10 weeks, out and about in August. (Hoffmeister 1986).

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Conservation

Conservation Status

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).

IUCN Red List of Threatened Species: least concern

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IUCN Red List Assessment


Red List Category
LC
Least Concern

Red List Criteria

Version
3.1

Year Assessed
2008

Assessor/s
Linzey, A.V.

Reviewer/s
McKnight, M. (Global Mammal Assessment Team), Amori, G., Koprowski, J. & Roth, L. (Small Nonvolant Mammal Red List Authority)

Contributor/s

Justification
Listed as Least Concern in view of its wide distribution, presumed large population, occurrence in a number of protected areas, and because it is unlikely to be declining at nearly the rate required to qualify for listing in a threatened category.
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National NatureServe Conservation Status

United States

Rounded National Status Rank: N5 - Secure

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NatureServe Conservation Status

Rounded Global Status Rank: G5 - Secure

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Abert's squirrel is listed by the State of Wyoming as a Priority III
species: the species does not warrant intensive management programs but
its needs should be accomodated in resource management planning [40].
  • 40. Finch, Deborah M. 1992. Threatened, endangered, and vulnerable species of terrestrial vertebrates in the Rocky Mountain Region. Gen. Tech. Rep. RM-215. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 38 p. [18440]

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Population

Population
This species is sparse in much of its range, but is abundant at some times and/or in some localities. Recorded densities range from 2 - 114 individuals per square kilometre.

Population Trend
Stable
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Threats

Major Threats
This species habitat is threatened by clear-cutting over large areas. Populations are relatively isolated due to the distribution of ponderosa pine habitat.
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Management

Conservation Actions

Conservation Actions
This species is present in several protected areas within its patchy range. Forest management plans that require logging in small blocks would help maintain viable populations.
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Management Requirements: Effects of timber harvest on squirrels can be reduced by protecting small groups of trees and nest trees, feed trees, and water sources (Patton et al. 1985). Pederson et al. (1987) recommended pine harvest be limited to small (less than 20 ha) selective blocks. Occasional ground fires may be a necessary ingredient of prime habitat (Hall 1981).

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Management Considerations

More info for the terms: cover, density, litter, selection, tree

Population Stability: Abert's squirrel abundance fluctuates with
ponderosa pine cone crops. The population of Kaibab squirrels
fluctuates noticeably with the amount of ponderosa pine cones available
in a year [6]. Abert's squirrels are sufficiently abundant to withstand
some hunting in Colorado, Arizona, and New Mexico [11].

Cone Crop consumption: Larson and Schubert [20] reported that Abert's
squirrels consumed seed of 0.3 to 74.7 percent of the cone crop in any
single year. They estimated that Abert's squirrels reduced total
ponderosa pine seed production by at least 20 percent over a 10-year
period, combining estimates of cone consumption and clipping of twigs
bearing immature conelets [20]. Allred and Gaud [1] estimated that
Abert's squirrels in Arizona consumed the seeds of 5,357 cones per
hectare (when cones were abundant 1986 and 1988) within the 42-month
period between 1986 and 1990. This represented 95 percent of the cone
production. They estimated the Abert's squirrel population density of
at least one per hectare [1]. In central Arizona in a year of low
Abert's squirrel population and a good cone crop, Abert's squirrels used
less than 10 percent of the cones [17]. Reynolds [31] estimated that
Abert's squirrels cause about 10 percent loss. Squillace [39] reported
that Abert's squirrels consumed the seed from 60 to 89 percent of
ponderosa pine cones in poor or fair seed years.

Impact on Feed Trees: The inner bark of twigs is often harvested by
Abert's squirrels only from specific, individual ponderosa pines
referred to by researchers as "feed trees" [8,17]. Selection of feed
trees is associated with specific chemical and physiological
characteristics of individual trees: feed trees are lower in certain
monoterpenes and have higher amounts of sugars in phloem tissue [36].
Preferred trees have no obvious external distinguishing characteristics.
Abert's squirrels prefer twigs of trees of cone-bearing age [17]. Feed
tree density varies, reflecting Abert's squirrel numbers [24]. On three
sites in Colorado Abert's squirrels used less than 10 percent of
ponderosa pine as feed trees [21]. In Utah Abert's squirrels used an
average of 0.6 tree per acre in cut forests and 4.7 trees per acre in
uncut stands [24]. Kaibab squirrels in northwestern Arizona clipped 41
percent of ponderosa pines 8.7 inches (22 cm) d.b.h. and larger in 1980,
and 68 percent of such trees in 1984 [38]. In north-central Arizona
only 4 percent of trees in the study area were used heavily. Trees used
in one year as feed trees were not always used in consecutive years, and
nonfeed trees were sometimes used as feed trees in subsequent years.
Abert's squirrels usually alternate feed tree use with 2 to 3 years of
no use [10].

In central Arizona 74 and 59 percent of trees were clipped by Abert's
squirrels in 2 consecutive years. Most of the clipped trees had the
remains of less than 100 twigs at their bases; each of 2 trees had over
1,200 twigs clipped [17]. In Colorado Abert's squirrels clipped an
average of 64.9 twigs per tree over an entire winter [36]. Clipping is
usually concentrated in the upper crown [10]. Feed trees that had been
partly defoliated had slightly lower growth rates than adjacent
ponderosa pines, but it was concluded that the overall impact of
defoliation was slight on prime sites for ponderosa pine growth [15].
Of 180 feed trees in Colorado only one was defoliated so severely that
it died [36]. Nonfeed trees in northwestern Arizona had lower growth
rates than feed trees prior to Kaibab squirrel introduction; it was
inferred that the production of chemical defenses present in nonfeed
trees reduced growth rates. Conversely, after Kaibab squirrels began
feeding on specific trees the growth rates of these feed trees slowed
until it was lower than that of nonfeed trees. In this area, which is
marginal for ponderosa pine growth, impact of squirrel feeding on
individual ponderosa pines is substantial [38]. Nest trees are rarely
defoliated [37].

Abert's squirrels affect the rate of nutrient transfer in ponderosa pine
stands by increasing the amount of litter under feed trees. Increased
litter and increased nitrogen and carbon in the litter (because clipped
twigs are often actively growing) increase nitrogen cycling [35].

Habitat management: Management for quality Abert's squirrel habitat is
management for large diameter, cone-producing ponderosa pines [11,24].
Optimum habitat for Abert's squirrels consists of stands of large
ponderosa pine at densities greater than 200 trees per acre [26].
Timber harvest in ponderosa pine stands is not incompatible with Abert's
squirrel Habitat management. Management goals should include
maintenance of small, uneven-aged groups of large trees [26]. The
recommended harvest type is group selection, with retention of ponderosa
pine 15 to 20 inches (38-51 cm) d.b.h. in groups suitable for nesting
[3,10,24,29]. Pederson and others [29] also recommended the following:
established Abert's squirrel nesting and feeding sites should be
avoided, harvesting should occur in late summer to early fall (after
juveniles have left nests), logging units should be broken into small
blocks and worked checkerboard fashion (to minimize direct disturbance
of squirrels), and slash should not be piled and burned.

Indiscriminate logging can degrade Abert's squirrel habitat. Lower
numbers of Abert's squirrels and lower recruitment rates occur in areas
where large pines are harvested than in unharvested areas. In Utah
Abert's squirrels fed less in logged ponderosa pine plots than in
control plots. Abert's squirrels moved away from logged areas to
unharvested stands. Plots had been logged with either a 10-inch (25 cm)
or 12-inch (30 cm) minimum diameter cut [29]. Abert's squirrels
consumed more hypogeous fungi in uncut stands than in logged stands.
Fewer fungi were produced in logged stands, probably because crown
reduction increased drying out of litter and decreased the amount of
litter [28].

Silvicultural treatment appears to have little effect on feed tree
selection [10]. Outbreaks of northern Kaibab pandora moths that cause
defoliation of ponderosa pines probably have adverse effects on Kaibab
squirrels [33]. Gambel oak, also an important tree for Abert's
squirrels in some areas, is altered by disturbance. Disturbances
enhance brushy growth forms of Gambel oak. Patch cutting (opening less
than 10 acres) provides the greatest diversity of oak forms (brush and
tree forms) [18]. Kruse [18] suggested that selectively cutting oaks
less than 8 inches (20 cm) d.b.h. and greater than 15 inches (38 cm)
d.b.h. would provide the greatest benefit to wildlife habitat, including
that for Abert's squirrels. A model of the food and cover requirements
for use in management decisions was constructed by Patton [26].

Abert's squirrels have been successfully transplanted to suitable
habitats [11]. Kaibab squirrels were introduced to an extensive pinyon
(Pinus spp.)-juniper-Gambel oak community in northwestern Arizona. In
10 years the transplanted population had reached a density similar to
that of Kaibab squirrels on the Kaibab Plateau [38].

A few parasites of Abert's squirrels were discussed by Keith [17].

REFERENCES :
NO-ENTRY
  • 1. Allred, W. Sylvester; Gaud, William S. 1993. Green foliage losses from ponderosa pines induced by Abert squirrels and snowstorms: a comparison. Western Journal of Applied Forestry. 8(1): 16-18. [20020]
  • 3. 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, WY. Laramie, WY: University of Wyoming, Department of Range Management, Wyoming Shrub Ecology Workshop: 11-21. [13913]
  • 6. Farentinos, R. C. 1972. Observations on the ecology of the tassel-eared squirrel. Journal of Wildlife Management. 36(4): 1234-1239. [25359]
  • 8. Farentinos, R. C.; Capretta, P. J.; Kepner, R. E.; Littlefield, V. M. 1981. Selective herbivory in tassel-eared squirrels: role of monoterpenes in ponderosa pines chosen as feeding trees. Science. 213: 1273-1275. [25361]
  • 10. Ffolliott, Peter F.; Patton, David R. 1978. Abert squirrel use of ponderosa pine as feed trees. Res. Note RM-362. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 4 p. [18449]
  • 11. Flyger, Vagn; Gates, J. Edward. 1982. Pine squirrels: Tamiasciurus hudsonicus and T. douglasii. In: Chapman, Joseph A.; Feldhamer, George A., eds. Wild mammals of North America: Biology, management, and economics. Baltimore, MD: The Johns Hopkins University Press: 230-238. [25232]
  • 15. Hall, Joseph G. 1981. A field study of the Kaibab squirrel in Grand Canyon National Park. Wildlife Monographs. 75: 1-54. [25473]
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 20. 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, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 15 p. [4714]
  • 21. Linhart, Yan B.; Snyder, Marc A.; Habeck, Susan A. 1989. The influence of animals on genetic variability within ponderosa pine stands, illustrated by the effects of Abert's squirrel and porcupine. In: Multiresource management of ponderosa pine: Proceedings of a conference; 1989 November 14-16; Flagstaff, AZ. Gen. Tech. Rep. RM-185. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 141-148. [25364]
  • 24. Patton, David R. 1975. Abert squirrel cover requirements in Southwestern ponderosa pine. Res. Pap. RM-145. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 12 p. [25366]
  • 26. Patton, David R. 1984. A model to evaluate abert squirrel habitat in uneven-aged ponderosa pine. Wildlife Society Bulletin. 12(4): 408-414. [25363]
  • 28. Pederson, Jordan C.; Farentinos, R.C.; Littlefield, Victoria M. 1987. Effects of logging on habitat quality and feeding patterns of Abert squirrels. The Great Basin Naturalist. 47(2): 252-258. [6904]
  • 29. Pederson, Jordan C.; Hasenyager, Robert N.; Heggen, Albert W. 1976. Habitat requirements of the Abert squirrel (Sciurus aberti navajo) on the Monticello District, Manti-Lasal National Forest of Utah. Publication No. 76-9. Salt Lake City, UT: Utah Department of Natural Resouces, Division of Wildlife Resources. 108 p. [25206]
  • 31. Reynolds, H. G. 1963. Western goshawk takes abert squirrel in Arizona. Journal of Forestry. 61: 839. [25362]
  • 33. Schmid, J. M.; Bennett, D. D. 1988. The North Kaibab pandora moth outbreak, 1978-1984. Gen. Tech. Rep. RM-153. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 18 p. [3301]
  • 35. Skinner, T. H.; Klemmedson, J. O. 1978. Abert squirrels influence nutrient transfer through litterfall in a ponderosa pine forest. Res. Note RM-353. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 8 p. [18448]
  • 36. Snyder, Marc A. 1993. Interactions between Abert's squirrel and ponderosa pine: the relationship between selective herbivory and host plant fitness. The American Naturalist. 141(6): 866-879. [22172]
  • 37. Snyder, Marc A.; Linhart, Yan B. 1994. Nest-site selection by Abert's squirrel: chemical characteristics of nest trees. Journal of Mammalogy. 75(1): 136-141. [23150]
  • 38. Soderquist, Todd R. 1987. The impact of tassel-eared squirrel defoliation on ecotonal ponderosa pine. Journal of Mammalogy. 68(2): 398-401. [4677]
  • 39. Squillace, A. E. 1953. Effect of squirrels on the supply of ponderosa pine seed. Research Note No. 131. Missoula, MT: U.S. Department of Agriculture, Forest Service, Northern Rocky Mountain Forest and Range Experiment Station. 4 p. [28266]
  • 18. Kruse, William H. 1992. Quantifying wildlife habitats within Gambel oak/forest/woodland vegetation associations in Arizona. In: Ffolliott, Peter F.; Gottfried, Gerald J.; Bennett, Duane A.; [and others], technical coordinators. Ecology and management of oak and associated woodlands: perspectives in the sw United States & n Mexico: Proceedings; 1992 April 27-30; Sierra Vista, AZ. Gen. Tech. Rep. RM-218. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 182-186. [19762]

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Use of Fire in Population Management

Prescribed fire in ponderosa pine can be used to reduce woody
understories and encourage ponderosa pine reproduction, growth, and
productivity [11].
  • 11. Flyger, Vagn; Gates, J. Edward. 1982. Pine squirrels: Tamiasciurus hudsonicus and T. douglasii. In: Chapman, Joseph A.; Feldhamer, George A., eds. Wild mammals of North America: Biology, management, and economics. Baltimore, MD: The Johns Hopkins University Press: 230-238. [25232]

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Relevance to Humans and Ecosystems

Benefits

Economic Importance for Humans: Negative

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).

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Economic Importance for Humans: Positive

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.

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Wikipedia

Abert's squirrel

Abert's squirrel (or tassel-eared squirrel) (Sciurus aberti) is a tree squirrel in the genus Sciurus endemic to the Rocky Mountains from United States to Mexico, with concentrations found in Arizona, The Grand Canyon, New Mexico, and southwestern Colorado.

Etymology[edit]

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[edit]

The currently accepted scientific name for Abert's squirrel is Sciurus aberti Woodhouse.[3] 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.[4][5]

Distribution[edit]

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.[4] Abert's squirrel also extends a short distance into Wyoming where ponderosa pine (Pinus ponderosa) is present. Abert's squirrels transplanted to the Graham and Santa Catalina mountains of Arizona have established stable populations. Mellott and Choate[6] reported Abert's squirrels present in the Spanish Peaks State Wildlife Area, 43 miles (72 km) 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.[7] Subspecies distributions are as follows:[4]

  • S. a. aberti (Woodhouse) – northern Arizona
  • S. a. barberi (Allen) – northwestern Chihuahua
  • S. a. chuscensis (Goldman) – New Mexico-Arizona border area
  • S. a. durangi (Thomas) – Durango
  • S. a. ferreus (True) – Rocky Mountains, central Colorado
  • S. a. kaibabensis – Kaibab Plateau, northern Arizona
  • S. a. mimus (Merriam) – New Mexico-Colorado border area
  • S. a. navajo (Durrant and Kelson) – southeastern Utah
  • S. a. phaeurus (Allen) – Durango and extreme southern Chihuahua

Plant communities[edit]

Ponderosa pine groove
Ponderosa pine foliage and cones

Abert's squirrel is closely associated with, and nearly confined to cool, dry interior ponderosa pine forests.[4] In Arizona, ponderosa pine forests are most extensive between 5,500 and 8,500 feet (1,676–2,590 m) elevation.[8] 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).[4] Findley and others [9] mention that Abert's squirrels are common in mixed conifer canyons in New Mexico.

Preferred habitat[edit]

Abert's squirrels make almost exclusive use of ponderosa pine for cover, nesting, and food.[4] Optimum Abert's squirrel habitat is composed of all-aged ponderosa pine stands with trees in even-aged groups, densities of 168 to 250 trees per acre (496–618/ha), and 150 to 200 square feet per acre (34.4–45.3 sq m/ha) basal area. In optimum habitat average diameter of ponderosa pines is 11 to 13 inches (28–33 cm), with Gambel oaks in the 11.8- to 14-inch (30–36 cm) diameter at breast height (d.b.h.) range.[10] Optimum habitat has some ponderosa pine over 20 inches (51 cm) d.b.h., which are the best cone producers.[11] Larson and Schubert [12] reported that ponderosa pine 36 to 40 inches (91–102 cm) d.b.h. produced an average of 446 cones per tree per crop. Trees less than 24 inches (61 cm) d.b.h. produced less than 100 cones per crop.

In central Arizona, Abert's squirrel summer home ranges averaged 18 acres (7.3 ha) and ranged from 10 to 24 acres (24.7–59.3 ha). Ranges were somewhat smaller in winter.[4] Ramey [13] reported that the mean Abert's squirrel home range for spring and summer was 20 acres (8.13 ha) in Black Forest County, Colorado. Subadult males had spring home ranges of about 27 acres (11 ha), and adult females had somewhat larger summer home ranges than adult males.[13] Patton [14] reported the ranges of three squirrels as 10, 30, and 60 acres (4.0, 12.2, and 24.4 ha) in Arizona. Hall [15] reported the home range of an adult female as 29 acres (11.8 ha).

In Colorado, Ramey [13] 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 [16] estimated 227 squirrels per square mile (82/km2) in fall 1970 and 317 per square mile (114/km2) in fall 1971.

Cover requirements[edit]

Summer nests are built by female 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. In central Arizona nest trees ranged from 12 to 41 inches d.b.h. and were 20 to 110 feet (33.5 m) tall.[4] In another Arizona study, nest trees ranged from 11.6 to 36.6 inches (29.4–93 cm) d.b.h. Most nests are placed in the upper third of the tree crown.[17] Nests were placed from 16 to 90 feet (4.9–27) above the ground, usually on a large limb against the bole, or in the forks of smaller branches. Nests were most often built on the southern to southeastern side of the tree.[4] Patton [7] reported that nest trees in Arizona had crowns that were 35% to 55% of the total tree height, and most often were 14 to 16 inches (36–41 cm) d.b.h. Nests are built in trees occurring as part of a grouping of trees with interlocking crowns.[7][11] Dwarf mistletoe (Arceuthobium vaginatum) infestations that cause the formation of "witches brooms" are often incorporated into or support Abert's squirrel nests.[18]

In winter, pairs of Abert's squirrels, usually an adult female and one subadult (presumed) offspring, use the same nest for shelter.[4]

Physical characteristics[edit]

Abert's squirrel collecting nesting material
View of an Abert's squirrel showing rusty/reddish stripe on back.
Sciurus aberti ferreus: foothills west of Denver

Abert's squirrels are 46–58 cm long with a tail of 19–25 cm. The most noticeable characteristic would be their hair 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 fur coloration. They typically have a gray coat with a white underbelly and a very noticeable rusty/reddish colored strip down their back. Aberts found in Colorado rocky mountain foothills appear black all over as shown in the image to the right.

Timing of major life events[edit]

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.[4] They are used year-round by most Abert's squirrels for nightly shelter.[17]

Nests are built by the female Abert's squirrel out of pine twigs 0.5 inch (1.3 cm) or less in diameter and 6 to 24 inches (15–61 cm) long. Nests are lined with a variety of materials.[4] Nests are roughly spherical and a small platform often extends beyond the bowl edge on one side.[19] Females often move the litter to a larger nest when the young are 3 to 6 weeks old.[4]

In central Arizona, breeding occurs from May 1 to June 1 and there are young in the nest from June 10 to July 27.[4] Farantinos [20] reported a 46-day gestation period. Eight litters were composed of two to five young each.[4][20] Three or four young per litter is typical.[21] 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.[4] Female Abert's squirrels usually bear only one litter per year.[4] Hall and Kelson [21] however, reported that two litter are often borne per year in the southern parts of Abert's squirrel range.

The most apparent causes of Abert's squirrel mortality are food shortage and injuries (such as broken teeth) that lead to mortality.[4]

Biology and behavior[edit]

Abert's squirrel typically builds its nest in the branches of the ponderosa pine in groups of twigs infected with dwarf mistletoe. They are strictly diurnal. Abert's squirrel does not store its food like other North American squirrels.

Food habits[edit]

Abert's squirrel eating a ponderosa pinecone

Abert's squirrels consume ponderosa pine year-round. Parts eaten include seeds, which are the most highly preferred item, inner bark (particularly of young twigs), terminal buds, staminate buds, and pollen cones. Other foods include fleshy fungi (particularly hypogeous fungi), carrion, bones, and antlers. Severe weather is not always a deterrent to feeding activity.[4] Where Mexican pinyon (Pinus cembroides) seeds are available, Abert's squirrels consume them in preference to ponderosa pine seeds.[15] Gambel oak acorns may also provide substantial food for Abert's squirrels.[22]

Ponderosa pines produce large cone crops every 3 to 4 years; cones are virtually absent about 1 year out of 4. Abert's squirrels begin eating immature seed shortly after cone development begins in late May. Seeds are eaten through the summer as the cones mature. Seeds from up to 75 cones may be eaten per day per squirrel during the months when seeds form the squirrels' major food. Seeds are disseminated from cones in October and November. Abert's squirrels continue to consume seed from late maturing cones and collect single seeds from the ground. The succulent inner bark of twigs is eaten all year, but most heavily in winter. Needle clusters are clipped from the twigs, the outer bark is removed, the inner bark is consumed, and then the twig is discarded. In winter a single squirrel consumes about 45 twigs per day.[4] Most feed trees range from 11 to 30 inches (30–76 cm) d.b.h.[8] After seeds have been disseminated Abert's squirrels are dependent on inner bark, which forms the bulk of the diet from November to April. The soft inner tissue of small apical buds is also a preferred item. In May, staminate buds and cones and immature ovules are consumed as available. New staminate cones are entirely consumed; only the pollen is eaten from dried cones. The bark of areas infected with dwarf mistletoe also appears to be preferred.[4]

Fleshy fungi consumed include members of the following genera: Agaricus, Amanita, Boletus, Hypholoma, Lepiota, Lycopedon, Russula and Tuber. Mushrooms poisonous to humans are consumed by Abert's squirrels without difficulty, including destroying angels (A. muscaria and A. vaginata) and a species of Russula.[4]

Water is obtained mostly from food, but Abert's squirrels sometimes drink at stock ponds or other standing water (i.e., rain puddles).[4]

Predators[edit]

Reynolds [23] suggested that northern goshawks (Accipiter gentilis) may take enough Abert's squirrels to regulate Abert's squirrel populations. Hawks (Accipitridae and Falconidae) prey on Abert's squirrels in central Arizona, but even though other potential predators are present, i.e., gray fox (Urocyon cinereoargenteus), bobcat (Lynx rufus), coyote (Canis latrans), there is no evidence that they prey on Abert's squirrels.[4]

References[edit]

 This article incorporates public domain material from the United States Department of Agriculture document "Sciurus aberti".

  1. ^ Linzey, A. V. (2008). Sciurus aberti. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 6 January 2009.
  2. ^ Thorington, R.W., Jr.; Hoffmann, R.S. (2005). "Sciurus (Otosciurus) aberti". In Wilson, D.E.; Reeder, D.M. Mammal Species of the World: a taxonomic and geographic reference (3rd ed.). The Johns Hopkins University Press. pp. 754–818. ISBN 0-8018-8221-4. OCLC 26158608. 
  3. ^ Hall, E. Raymond. 1981. The mammals of North America. 2nd ed. Vol. 2. New York: John Wiley and Sons
  4. ^ a b c d e f g h i j k l m n o p q r s t u v w x Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150–163
  5. ^ Nash, Donald J.; Seaman, Richard N. 1977. Sciurus aberti. Mammalian Species. 80: 1–5
  6. ^ Mellott, Ron S.; Choate, Jerry R. 1984. Sciurus aberti and Microtus montanus on foothills of the Culebra Range in southern Colorado. The Southwestern Naturalist. 29(1): 135–137
  7. ^ a b c Patton, David R. 1975. Abert squirrel cover requirements in Southwestern ponderosa pine. Res. Pap. RM-145. Fort Collins, Colorado: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station
  8. ^ a b 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
  9. ^ Findley, James S.; Harris, Arthur H.; Wilson, Don E.; Jones, Clyde. 1975. Mammals of New Mexico. Albuquerque, New Mexico: University of New Mexico Press.
  10. ^ Flyger, Vagn; Gates, J. Edward. 1982. Pine squirrels: Tamiasciurus hudsonicus and T. douglasii. In: Chapman, Joseph A.; Feldhamer, George A., eds. Wild mammals of North America: Biology, management, and economics. Baltimore, Maryland: The Johns Hopkins University Press: 230–238.
  11. ^ a b 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
  12. ^ 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
  13. ^ a b c 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
  14. ^ 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
  15. ^ a b 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
  16. ^ Farentinos, R (1972). "Social dominance and mating activity in the tassel-eared squirrel (Sciurus aberti ferreus)". Animal Behaviour 20 (2): 316–26. doi:10.1016/S0003-3472(72)80053-8. PMID 4674670. 
  17. ^ a b Snyder, Marc A.; Linhart, Yan B. 1994. Nest-site selection by Abert's squirrel: chemical characteristics of nest trees. Journal of Mammalogy. 75(1): 136–141
  18. ^ Farentinos, R. C. 1972. Nests of the tassel-eared squirrel. Journal of Mammalogy. 53(4): 900–903
  19. ^ Skinner, T. H.; Klemmedson, J. O. 1978. Abert squirrels influence nutrient transfer through litterfall in a ponderosa pine forest. Res. Note RM-353. Fort Collins, Colorado: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station
  20. ^ a b Farentinos, R. C. 1972. Observations on the ecology of the tassel-eared squirrel. Journal of Wildlife Management. 36(4): 1234–1239
  21. ^ a b Hall, E. Raymond; Kelson, Keith R. 1959. The mammals of North America, Volume II. New York: The Ronald Press Company
  22. ^ Reynolds, Hudson G.; Clary, Warren P.; Ffolliott, Peter F. 1970. Gambel oak for Southwestern wildlife. Journal of Forestry. 68(9): 545–547
  23. ^ Reynolds, H. G. 1963. Western goshawk take abert squirrel in Arizona. Journal of Forestry. 61: 839
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Names and Taxonomy

Taxonomy

Comments: Phylogeography based on mtDNA data does not conform well with current subspecies designations; there are two phylogeographic assemblages: eastern (Mexico; New Mexico-Colorado-Utah) and western (Arizona-southwestern New Mexico) (Lamb et al. 1997).

Subspecies kaibabensis formerly was regarded as a distinct species.

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Common Names

Abert's squirrel
Abert squirrel
tassel-eared squirrel
Kaibab squirrel

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The currently accepted scientific name for Abert's squirrel is Sciurus
aberti Woodhouse [14,17]. There are nine recognized subspecies
including the Kaibab squirrel (S. a. kaibabensis), formerly recognized
as a separate species, S. kaibabensis. Subspecies are as follows
[17,23]:

S. a. aberti Woodhouse
S. a. barberi Allen
S. a. chuscensis Goldman
S. a. durangi Thomas
S. a. ferreus True
S. a. kaibabensis
S. a. mimus Merriam
S. a. navajo Durrant and Kelson
S. a. phaeurus Allen
  • 17. Keith, James O. 1965. The Abert squirrel and its dependence on ponderosa pine. Ecology. 46: 150-163. [1320]
  • 14. Hall, E. Raymond. 1981. The mammals of North America. 2nd ed. Vol. 2. New York: John Wiley and Sons. 1271 p. [14765]
  • 23. Nash, Donald J.; Seaman, Richard N. 1977. Sciurus aberti. Mammalian Species. 80: 1-5. [25474]

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