Mammal Species of the World
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Sciurus arizonensis occupies mountain ranges in central Arizona, western New Mexico, and Sonora, Mexico, along the U.S. border. (Best and Riedel 1995)
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: Central and southeastern Arizona (southern and western slopes of the Mogollon Plateau from north of Sedona, Coconino County, to Blue, Greenlee County, and many isolated mountain ranges to the south, including the Prescott, Bradshaw, Pine, Mazatzal, Sierra Ancha, Santa Catalina, Rincon, Santa Rita, Patagonia, Pajarito, and Atascosa mountains); west-central New Mexico (watersheds of the San Francisco and Gila rivers in Catron County, Pinos Altos Mountains in Grant County); northeastern Sonora, Mexico (Sierra de los Ajos, Sierra Azul, Sierra de la Madera, Sierra Patagonia, Sierra de Pinitos, and mountains northeast of Cucurpe) (Hoffmann et al., in Wilson and Reeder 1993, Best and Riedel 1995); 1120 m to above 2700 m (Best and Riedel 1995).
Known commonly as the Arizona gray squirrel, Sciurus arizonensis is gray in color throughout most of its upper body. Patches of yellow are sometimes present behind the ears. The tail is black dorsally and yellow to brown ventrally. The two sides of the tail are separated by white edging, and the underparts of the squirrel also are white. The squirrel's gray pelage darkens during the winter, and its underparts and feet are often stained from walnut juice (see Food Habits). Total body length for the species, including the tail, averages 21 inches. (Best and Riedel 1995) (Cockrum 1992)
Other Physical Features: endothermic ; bilateral symmetry
Length: 57 cm
Weight: 706 grams
Size in North America
Range: 455-574 mm
Range: 527-884 g
See Best and Riedel (1995).
Catalog Number: USNM 8475
Collection: Smithsonian Institution, National Museum of Natural History, Department of Vertebrate Zoology, Division of Mammals
Preparation: Skin; Skull
Collector(s): E. Coues
Year Collected: 1865
Locality: Fort Whipple, Yavapai County, Arizona, United States, North America
- Type: Coues, E. 1867 Sep. American Naturalist. 1: 357.
Arizona Mountains Forests Habitat
This taxon is found in the Arizona Mountain Forests, which extend from the Kaibab Plateau in northern Arizona to south of the Mogollon Plateau into portions of southwestern Mexico and eastern Arizona, USA. The species richness in this ecoregion is moderate, with vertebrate taxa numbering 375 species. The topography consists chiefly of steep foothills and mountains, but includes some deeply dissected high plateaus. Soil types have not been well defined; however, most soils are entisols, with alfisols and inceptisols in upland areas. Stony terrain and rock outcrops occupy large areas on the mountains and foothills.
The Transition Zone in this region (1980 to 2440 m in elevation) comprises a strong Mexican fasciation, including Chihuahua Pine (Pinus leiophylla) and Apache Pine (P. engelmannii) and unique varieties of Ponderosa Pine (P. ponderosa var. arizonica). Such forests are open and park-like and contain many bird species from Mexico seldom seen in the U.S.. The Canadian Zone (above 2000 m) includes mostly Rocky Mountain species of mixed-conifer communities such as Douglas-fir (Pseudotsuga menziesii), Engelmann Spruce (Picea engelmanni), Subalpine Fir (Abies lasiocarpa), and Corkbark Fir (A. lasiocarpa var. arizonica). Dwarf Juniper (Juniperus communis) is an understory shrubby closely associated with spruce/fir forests. Exposed sites include Chihuahua White Pine (Pinus strobiformis), while disturbed north-facing sites consists primarily of Lodgepole Pine (Pinus contorta) or Quaking Aspen (Populus tremuloides).
There are a variety of mammalian species found in this ecoregion, including the endemic Arizona Gray Squirrel (Sciurus arizonensis), an herbivore who feeds on a wide spectrum of berries, bark and other vegetable material. Non-endemic mammals occurring in the ecoregion include: the Banner-tailed Kangaroo Rat (Dipodomys spectabilis NT); Desert Pocket Gopher (Geomys arenarius NT). In addition, there is great potential for restoring Mexican Wolf (Canis lupus) and Grizzly Bear (Ursus arctos horribilis) populations in the area because of its remoteness and juxtaposition to other ecoregions where these species were formerly prevalent.
There are few amphibians found in the Arizona mountain forests. Anuran species occurring here are: Red-spotted Toad (Anaxyrus punctatus); Southwestern Toad (Anaxyrus microscaphus); New Mexico Spadefoot Toad (Spea multiplicata); Woodhouse's Toad (Anaxyrus woodhousii); Northern Leopard Frog (Lithobates pipiens); Chiricahua Leopard Frog (Lithobates chiricahuensis VU); Madrean Treefrog (Hyla eximia), a montane anuran found at the northern limit of its range in this ecoregion; Boreal Chorus Frog (Anaxyrus woodhousii); Western Chorus Frog (Pseudacris triseriata); and Canyon Treefrog (Hyla arenicolor). The Jemez Mountains Salamander (Plethodon neomexicanus NT) is an ecoregion endemic, found only in the Jemez Mountains of Los Alamos and Sandoval counties, New Mexico. Another salamander occurring in the ecoregion is the Tiger Salamander (Ambystoma tigrinum).
A number of reptilian taxa occur in the Arizona mountains forests, including: Gila Monster (Heloderma suspectum NT), often associated with cacti or desert scrub type vegetation; Narrow-headed Garter Snake (Thamnophis rufipunctatus), a near-endemic found chiefly in the Mogollon Rim area; Sonoran Mud Turtle (Kinosternon sonoriense NT).
Habitat and Ecology
In Arizona, S. arizonensis occurs in dense, mixed-broadleaf communities of riparian-deciduous forest. Usually, the species is restricted to elevations of 1,500 to 1,900 meters above sea level. Favored habitats are groves of old cavity-prone Arizona sycamores and other large deciduous trees. In New Mexico, the squirrel is confined to deep canyons with water, where black walnuts and acorns are abundant. In Mexico, the squirrel occupies riparian forests at lower elevations than does its Arizona cousin. (Best and Riedel 1995) (Cockrum 1992) (Findley 1987)
Terrestrial Biomes: forest ; mountains
Comments: Hardwood and mixed oak and pine forests. Found in river valleys and canyons. Found where black walnuts and acorns are abundant. Also in cottonwood and sycamore groves. Makes leaf nests in trees.
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.
Sciurus arizonensis may feed on a wide variety of vegetable material, including nuts, fruits, bark, berries, flowers, and fungi. The actual breadth of the diet depends on the availability of food sources and the particular geographic range of the animal. In New Mexico, S. arizonensis feeds almost exclusively on walnuts, supplemented by roots. In Arizona and Mexico, the gray squirrel eats walnuts, but also acorns, juniper berries, hackberries, pine seeds, and fungi. The diet of these squirrels is more varied on a seasonal basis as well. In late summer and early autumn, the Arizona- and Mexico-based squirrels take in insects and other animal matter. Walnuts are a staple for S. arizonensis regardless of geographic range, and several individuals often harvest these nuts in the same tree. Another dietary habit shared by all members of the species is the consumption of flower parts in late winter and early spring. This seasonal food source is thought to be linked to reproductive activity (see Reproduction). (Best and Riedel 1995)
Comments: Mast, especially walnuts and acorns, pine seeds, flowers, buds, fungi, and other vegetation. May also eat insects.
Populations seem to fluctuate, but more information is needed.
Life History and Behavior
Perception Channels: tactile ; chemical
The onset of breeding activity in the Arizona gray squirrel is correlated with flower emergence and the inclusion of flower parts in the diet. It is theorized that the flower parts contain vitamin A and other vitamins that stimulate reproductive activity. Estrus occurs in females in April and early May. Mating chases also occur during this time, with several males pursuing a single female. Not all females breed each year. Gestation usually lasts about two months, and the litter size ranges from two to four offspring. (Best and Riedel 1995)
Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual
Sexual activity apprently extends from January to June. Juveniles taken in mid-August suggest births in mid-June. No evidence of second annual litter or fall litter (Hoffmeister 1986).
IUCN Red List Assessment
Red List Category
Red List Criteria
- 1996Lower Risk/near threatened (LR/nt)
The population of Arizona gray squirrels in the United States is fairly small, a situation that may be connected to competition from Sciurus aberti, a hardier squirrel and a close relative. The United States Department of the Interior, Fish and Wildlife Service classifies the Arizona gray squirrel as a "Category 2" species, which is reserved for taxa that may be eligible for threatened or endangered status. In Mexico, S. arizonensis has suffered severe habitat loss due to logging and the clearing of forests for agricultural use. The squirrel is rare in Mexico and is considered a threatened species in the that country. (Best and Riedel 1995)
IUCN Red List of Threatened Species: data deficient
National NatureServe Conservation Status
Rounded National Status Rank: N4 - Apparently Secure
NatureServe Conservation Status
Rounded Global Status Rank: G4 - Apparently Secure
Relevance to Humans and Ecosystems
This species may be important as a disperser of tree seeds and the spores of mycorrhizal fungi.
Arizona gray squirrel
The Arizona gray squirrel (Sciurus arizonensis) is a tree squirrel in the genus Sciurus endemic to the canyons and valleys surrounded by deciduous and mixed forests in eastern Arizona and northern Mexico.
It is threatened by habitat loss. The only other large squirrel that is within its range is Abert's squirrel, which has ear tufts and lives in pine forests. Although they act and look like other gray squirrels, the Arizona gray squirrel is actually more closely related to the fox squirrel.
- Linzey, A. V., Timm, R., Álvarez-Castañeda, S. T., Castro-Arellano, I. & Lacher, T. (2008). Sciurus arizonensis. In: IUCN 2008. IUCN Red List of Threatened Species. Retrieved 6 January 2009.
- Thorington, R.W., Jr.; Hoffmann, R.S. (2005). "Sciurus (Sciurus) arizonensis". 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.
- Youth,Howard. Publishing date unknown. Enjoying Squirrels More (or Less!). Pp 11. Pardson Corporation, Marietta, Ohio.
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Names and Taxonomy
Comments: Sciurus arizonensis may be conspecific with S. nayaritensis (Hoffmeister 1986). Scirurus nayaritensis and S. arizonensis may be subspecies of S. niger (see Best and Riedel 1995). Hoffmeister (1986) examined geographic morphological variation and concluded that S. arizonensis is best treated as monotypic (though a somewhat variable unit). Thorington and Hoffman (in Wilson and Reeder 2005) recognized arizonensis, nayaritensis, and niger as distinct species and recognized three subspecies of S. arizonensis (arizonensis, catalinae, and huachuca).