Urocyon cinereoargenteus - Gray fox

Skull Pictures:    
Dorsal      Mandible
Lateral     Ventral

Physical Description:

The common name gray fox fits its scientific name well. Urocyon is Greek for “tailed dog” and cinereoargenteus is Latin for “silver” or “gray” (Crossley 1985). The gray fox is distinguished from most other canids by their grizzled upper parts, cinnamon to buff neck and under parts, black tipped tail, and dorsal median mane of black tipped hairs (Jackson 1961). The grizzled appearance results from individual guard hairs that are banded with white, gray, and black (Crossley 1985). Black-tipped hairs near the center of the back form a dark stripe that extends into a black mane of coarse hair on top of the black-tipped tail. Parts of the neck, sides, and limbs are cinnamon-red. The ears, throat, chest, belly, and hind legs contain traces of white. The face has black, white, and red markings. The underfur is predominantly buff and gray. Guard hairs range from 50-70 mm in length, and underfur from 30-40 mm long (Fritzell and Haroldson 1982). A single annual molt occurs from summer through autumn.

The skull can be distinguished from other canids by widely separated temporal ridges that approach each other posteriorly to produce a u-shaped form, with a slightly down-curved supraorbital crest that has a distinct concave depression on the upper surface (Fritzell and Haroldson 1982). The temporals are roughened below the temporal ridges, and the dentary has a prominent “step” near the posterior ventral border. Skull length ranges from 11-13 cm. The dental formula for the gray fox is 3/3 1/1 4/4 2/3 = 42 (Jackson 1961).

Fritzell and Haroldson (1982) reported total body length of the gray fox to range from 800-1125 mm, with a 275-443 mm tail, and 100-150 mm hind-foot length. They also found that adults range form 3-7 kg, with males being slightly larger than females. Males were found to have longer pelves, longer and wider scapulae, more massive limb bones, and longer calcanea than females of corresponding age classes. The tail gland of the gray fox extends from 1/3-1/2 of the tail length, which is the largest in North American canids. Six mammae are present in the gray fox (Fritzell and Haroldson 1982).

Distribution:

The gray fox inhabits wooded, brushy, and rocky habitats from the southern fringe of Canada down to northern Venezuela and Columbia, excluding the northern Rocky Mountains (figure 1). Since the beginning of the twentieth century, their range has expanded into areas formerly unoccupied or where they were extirpated (Petersen et al. 1977). Fossil records have been found at around 40 Pleistocene locations, the earliest being in the Irvingtonian epoch. The gray fox was widely distributed from California to Florida during the Wisconsin glaciation, with a northern limit in Pennsylvania (Chapman and Feldhamer 1982). 

Figure 1. U.S. Gray fox distribution.

Figure 2. Wisconsin gray fox distribution.

Little is know about gray fox distribution in Wisconsin prior to 1900. Since then, records indicate that the gray fox has been relatively common in the southern part of the state. The greatest populations are found in the driftless area of southwestern Wisconsin (figure 2). Range was expanded into northern Wisconsin between 1900 and 1960, but has been in decline since (figure 3). High pelt values caused a large percentage of harvest of the gray fox population (Petersen et al. 1977). It is unlikely that the gray fox will ever be abundant in northern Wisconsin, since this is the northern limit of its ecological range (Crossley 1985).

 Figure 3. Historical gray fox distribution in Wisconsin (Peterson et al. 1975)

Ontogeny and Reproduction:

The breeding season for gray foxes varies geographically. In Wisconsin breeding activity occurs from mid February through March (Crossley 1985). Where gray and red foxes (Vulpes vulpes) are sympatric, gray foxes breed 2 to 4 weeks later than red foxes (Fritzell and Haroldson 1982). The gestation period of the gray fox is 51-54 days. Males become fertile earlier in the year and remain fertile longer than females. Females are sexually mature around 10 months of age. Out of 54 10-month-old females examined in Wisconsin, approximately only 10 were not sexually mature (Root 1981). Testes of 4-month-old males were found to be similar to adults, including the presence of primary spermatocytes, which indicates that testicular activity begins around this time (Fritzell and Haroldson 1982).

Mean litter size of the gray fox in southwestern Wisconsin is 3.5.  Litters of 3-4 are most common, but sizes from 1 to 9 have been documented (Root 1981). Pups weigh 3 ounces at birth, are dark-skinned, blind, and naked (Crossley 1985). Fuzzy fur begins to develop and the eyes open around 10-12 days. At 3 months, young accompany parents away from the den to forage, and by 4 months young begin to forage independently. Pups begin to disperse from their parents in fall. Gray foxes may reach 14-15 years old in the wild (Fritzell and Haroldson 1982).

Fritzel and Haroldson (1982) described many methods of estimating age of the gray fox. Counts of cementum annuli are considered to be the most accurate. Tooth wear has been used to determine age categories by the amount of wear on the conules of M1. Closure of skull sutures has been used based upon the closure of the basisphenoid-presphenoid suture at 10-11months. At 24 months, the vomer is fully developed and continuous with the presphenoid. Distal epiphyses of the ulna and radius ossify between 8 and 9 months. Eye lens weight, body weight, and eruption of check teeth have also been used to determine age classes in the gray fox.

Ecology and Behavior:

The gray fox is closely associated with the deciduous forest. It prefers a variety of habitat types, rather than large homogeneous areas. Crosley (1985) wrote that in Wisconsin the gray fox prefes brush covered bluffs and hills intermingled with woodland and farmland. This provides a large amount of edge habitat. Old fields are used most during summer and fall when fruits and insects are numerous. Yearlong, woodlands are used more during the day than at night.

Dens are used throughout the year, but most use occurs during the whelping season in spring (Fritzell and Haroldson 1982). Dens are generally located in brushy or wooded habitats. According to Crosley (1985), most dens In Wisconsin are located on east, southeast, or south-facing slopes to take advantage of heat from the sun. Den sites include hollow logs and trees, rock outcroppings, underground burrows, cavities under rocks, abandoned buildings, wood, sawdust, and brush. Most underground dens are abandoned dens of other animals. Gray fox dens are much harder to detect than those of the red fox.

Radio telemetry has shown that average home-range size varies according to sex and geographic location. Crosley (1985) reported that home range sizes in Wisconsin vary from 31-765 acres. Size of home range is related to quantity and quality of suitable food and habitat. Home range increases for both sexes in late fall and winter, but decreases for females during the whelping period, as male home-range peaks. Estimated densities of the gray fox range from 1.2 to 2.1 per km2 (Fritzell and Haroldson 1982).

Specific diet of the gray fox in the wild varies by location and season. In the eastern and central U.S., winter diet is composed primarily of mammals. Cottontail rabbits are the most important winter food source in Wisconsin (Crossley 1985). Mice, rats, and other rodents are also eaten in winter. Summer and fall diets include invertebrates (mainly orthopterans), grapes, apples, corn, elderberries, acorns, and grain. Birds, squirrels, opossum, and deer carrion are other less commonly eaten foods.

The gray fox is primarily nocturnal or crepuscular, but is occasionally observed during broad daylight (Crossley 1985). It is considered to be more social than the red or arctic fox, because only the gray fox exhibits allogrooming (Fritzell and Haroldson 1982). The gray fox is assumed to be monogamous, and family units consisting of a male, female, and possible juveniles, have been indirectly observed through radio telemetry and relative trap success (Root 1981). These family units are thought to occupy separate spatial or temporal home ranges than other groups. Urine and feces play a role in communication by deposition in areas such as bare ground, on logs, rocks, elevated sites, or along trails (Fritzell and Haroldson 1982). Scats are often deposited in groups.

Fritzel and Harrison (1982) note that the gray fox is also well known for its tree climbing behavior, which is facilitated by sharp and recurved claws. Observations have been made of them climbing trees up to 18 m tall by grasping trees with their forefeet and pushing with their hind feet, or even by jumping from branch to branch. Trees are descended by backing down vertical trees, or running down sloping trees. Climbing is done for the purpose of foraging, resting, or escape.

The top three causes of mortality for the gray fox in Wisconsin are hunting, trapping, and road kills (Petersen et al. 1977). Peterson et al. (1977) reported that the overall mortality rate is 66%, with 70-80% of that caused by hunting and trapping. Dogs, coyotes, bobcats, some hawks, and diseases such as canine distemper and rabies are sources of natural mortality in the gray fox (Fritzell and Haroldson 1982). They are also host to a variety of fleas, ticks, chiggers, lice, mites, and internal parasites; however, the gray fox is highly resistant to sarcoptic mange, a disease that affects red foxes.

Remarks:

Original name of Urocyon cinereoargenteus: Canis cinereo argenteus Schreber, 1775 (Wilson and Reeder 1993).

Synonyms for cinereoargenteus: boreales, californicus, colimensis, costaricensis,floridanus, fraterculus, furvus, guatemalae, inyoensis, nigrirostris, ocythous, orinomus, parvidens, peninsularis, pensylvanicus, scottii, sequoiensis, texensis, townsendi, virginianus (Wilson and Reeder 1993).

Literature Cited:

Chapman, J. A., and G. A. Feldhamer, editors. 1982. Wild animals of North America. The Johns Hopkins University Press, Baltimore, Maryland, USA.

Crossley, A. 1985. The gray fox (Urocyon cinereoargenteus). Wisconsin Department of Natural Resources Publication WM-005-85.

Fritzell, E. K., and K. J. Haroldson. 1982. Urocyon cinereoargenteus. Mammalian Species. 189.

Jackson, H. T. 1961. Mammals of Wisconsin. University of Wisconsin Press, Madison, Wisconsin, USA.

Petersen, L. R., M. A. Martin, and C. M. Pils. 1977. Status of gray foxes in Wisconsin. Wisconsin Department of Natural Resources Resource Report 94.

Root, D. A. 1981. Productivity and mortality of gray foxes and raccoons in southwestern Wisconsin. Thesis, University of Wisconsin Stevens Point, Stevens Point, Wisconsin, USA.

Wilson, D. E., and D. M. Reeder, editors. 1993. Mammal Species of the World. Smithsonian Institution Press,Washington, D.C., USA.

Reference written by Ryan Babe, Biology 378 student.  Edited by Christopher Yahnke.
Page last updated 4-29-04.