Ryan Babe
BIO 378
Urocyon cinereoargenteus
Gray fox
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.
(Fritzell and Haroldson 1982)
Figure 2. Wisconsin gray fox distribution.
(Crossley 1985)
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)
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BIO 378
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).
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BIO 378
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.
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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.
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