Arizona Gray Squirrel

The Arizona gray squirrel (Sciurus arizonensis) is a large tree squirrel restricted to mountain forests in Arizona, western New Mexico and northeastern Sonora in Mexico. Adults measure roughly 49–57 cm including the tail and commonly weigh about 540–740 g.

The upperparts are grey, the belly is white to cream and the bushy tail is dark above with pale edges and a yellowish, reddish or dark underside. It differs from Abert's squirrel (Sciurus aberti) in lacking the long ear tufts and in its closer association with deciduous riparian forest rather than ponderosa-pine forest.

The species is currently retained as Sciurus arizonensis by the Mammal Diversity Database. A proposed rearrangement of the squirrel tribe Sciurini would move this species, together with several relatives, to the genus Parasciurus.

The Mammal Diversity Database does not currently adopt that change and treats the proposed generic arrangement as tentative pending further investigation. Sciurus arizonensis therefore remains the current scientific name.

Arizona gray squirrels inhabit riparian deciduous and mixed forest in mountain canyons and valleys, usually where large oaks, Arizona walnut, sycamore, cottonwood, juniper and pine provide food, nest sites and connected canopy. In the United States they are often found from about 1,500–1,900 m, although the species occurs lower in parts of Mexico.

The squirrels are particularly dependent on mature riparian forest with several canopy layers and good connections between trees. Dense woodland along streams also provides escape cover in landscapes that can otherwise be dry and open.

The diet changes strongly with season and year. Acorns, juniper seeds and walnuts were the main foods during fieldwork in the Huachuca Mountains in 2007–2008, but pine seeds, flowers, buds, fruit, fungi, insect galls and occasional other animal matter are also eaten.

Arizona gray squirrels feed both in trees and on the ground, moving between mature food trees and the forest floor as different resources become available. The importance of major foods changed markedly between the two years in the Huachuca Mountains, reflecting strong variation in rainfall and seed and nut production. Poor food years can therefore affect both body condition and reproductive success.

The Arizona gray squirrel is diurnal and individuals usually move alone, although home ranges overlap and several squirrels may use the same highly productive food tree.

A study published in 2010 from the Huachuca Mountains found that males had larger home ranges than females, and that both home-range size and overlap increased during the mating period. Male space use was especially associated with access to females, while female space use more closely tracked food resources.

Dense canopy and riparian undergrowth provide important cover from predators. Adult survival can be relatively high in good habitat, but a study published in 2014 from the Huachuca Mountains found predation to be the most frequent known cause of death.

Reproductive timing and success vary with rainfall and food production. During fieldwork in 2007–2008, males remained reproductively active over a broad January–July period, while the timing of female receptivity followed changes in food availability. More females reproduced in the year with better resource conditions.

Several males may chase one receptive female during mating. After a gestation of about two months, the female gives birth in a leaf-and-twig nest high in a tree or, less often, in a suitable tree cavity. Litters normally contain two to four young.

Not every adult female produces a litter each year, especially when food is scarce. Nest placement is strongly associated with mature trees and complex canopy structure, linking reproduction directly to the quality of riparian forest.

The Arizona gray squirrel is classified as data deficient on the IUCN Red List. Arizona also lacks enough data to say confidently whether the statewide population is declining or how it is changing over time.

The species is nevertheless listed as Tier 2 among Arizona's Species of Greatest Conservation Need. For the Arizona gray squirrel, this reflects factors including its isolated distribution and the large proportion of its U.S. range that lies within Arizona. The lack of information also makes local changes difficult to detect without regular surveys and long-term monitoring.

Protecting mature riparian woodland is the central conservation need. Loss of streamside forest, drought, logging, development and severe wildfire can remove canopy cover, food trees and nest sites while isolating the remaining habitat patches.

A study published in 2017 after large fires found that native Arizona gray squirrels remained closely associated with unburned riparian forest, whereas introduced Abert's squirrels used more open post-fire habitat more readily. The direct effect of Abert's squirrels on Arizona gray squirrel populations remains uncertain, but habitat change can increase overlap between the species.

Maintaining dense canopy, large deciduous trees and continuous unburned woodland along mountain streams, together with better long-term monitoring, will be important for assessing and reducing future risk.