Sciurus arizonensisArizona gray squirrel

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Geographic Range

Sciurus arizonensis occupies mountain ranges in central Arizona, western New Mexico, and Sonora, Mexico, along the U.S. border. (Best and Riedel 1995)

Habitat

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)

Physical Description

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)

Reproduction

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

Behavior

The Arizona gray squirrel often runs on the ground and jumps from branch to branch in trees. Depending on its feeding habits and the time of year, the squirrel may be secretive or easily observed. During the breeding season, when females are pregnant or nursing young, S. arizonensis is hard to locate. At other times of the year, it is conspicuous. When it wants to avoid detection, the squirrel may stand perfectly still for long periods of time. The Arizona gray squirrel is essentially a silent animal. Its only vocalizations are chucking and barking alarm calls, which the animal ventures only when safely perched in a tree. The species is generally not afraid of humans or dogs, but it will vocalize warnings if it detects the presence of spotted skunks, snakes of various kinds, or housecats. In Arizona, the squirrel builds conspicuous leaf nests in the crotch between a tree's trunk and a major branch. Each squirrel may have several leaf nests or none at all. During the winter, several adults may occupy a nest. (Best and Riedel 1995)

Communication and Perception

Food Habits

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)

Economic Importance for Humans: Positive

This species may be important as a disperser of tree seeds and the spores of mycorrhizal fungi.

Conservation Status

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)

Other Comments

The Arizona gray squirrel is a harmless rodent that co-exists peacefully with humans and other animals in a highly circumscribed habitat. Its future in the United States depends on the preservation of the canyon forests where it makes it home.

Contributors

Peter Munoz (author), University of Michigan-Ann Arbor.

Glossary

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

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.

forest

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

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

sexual

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

tactile

uses touch to communicate

References

Best, T. L. and S. Riedel, 1995. "Sciurus arizonensis," Mammalian Species 496:1-5.

Cockrum, E.L., 1992. Mammals of the Southwest. University of Arizona Press: Tucson, Arizona. pp. 90-91.

Findley, J.S., 1975. The Natural History of New Mexico Mammals. The University of New Mexico Press: Albuquerque, New Mexico. pp. 63-64.

Hall, E.R., 1959. The Mammals of North America. John Wiley & Sons: New York, N.Y. pp. 432-433.

Vaughan, T.A., 1986. Mammalogy. Third Edition. Saunders College Publishing: Fort Worth, Texas. vii+576 pp.