Grey fox (Urocyon cinereogenteus) COSEWIC assessment and update update status report: chapter 8

Biology

General

U. cinereoargenteus is the most omnivorous of the North American canids. Vegetable matter, such as fruit, is an important year-round diet component. The grey fox has been observed to breed in Canada. The breeding season varies across their range, but Canadian grey foxes are thought to breed from mid-February to mid-March like populations in the northeastern U.S. Most female grey foxes become sexually mature and breed in their first year. Females have one litter of about 3 or 4 kits per year for their entire lives. Grey foxes are typically nocturnal or crepuscular. The basic social unit consists of an adult male and female and their offspring, and this group maintains a home range that is largely exclusive of other such groups. The adult sex ratio is usually 1:1 and grey foxes are generally assumed to be monogamous. It is unclear whether grey foxes are territorial, but they apparently use urine and faeces in communication.

Grey fox mortality in the wild is generally high and juveniles make up a large percentage of the population. The most important cause of mortality of grey foxes in the United States is human harvest by trapping and hunting, but fewer than 20 pelts are taken annually in Canada. Due to the lack of knowledge about numbers of grey foxes in Canada, the significance of this harvest pressure is unknown. Predation is not considered to be an important source of mortality for grey foxes. The diseases canine distemper and rabies, both prevalent in the wild and fatal to grey foxes, could severely affect populations, but likely only in cases of high grey fox density. The harsh Canadian climate may limit the northward spread of grey foxes (i.e. through the energy cost of locomotion in deep snow), but other factors, such as competition with other mid-sized carnivores, cannot be ruled out.

Reproduction

Little is known about grey fox reproduction in Canada. However, the grey fox breeding season is known to vary across its geographic distribution with more northern populations breeding later than more southern populations (Trapp and Hallberg 1975; Fritzell 1987). Therefore, Canadian populations of grey foxes should breed at a similar time, or later than populations in New York and Wisconsin whose peak breeding activity occurs from mid-February to mid-March (Layne and McKeon 1956; Jackson 1961). Most female grey foxes become sexually mature at around 10 months old (Fritzell 1987), breed in their first year (Wood 1958; Root and Payne 1985) and have one litter per year thereafter (Fritzell 1987). Litter sizes (based on counts of placental scars or embryos) range from 4.4 in New York (Layne and McKeon 1956) to 3.5 in Wisconsin (Root and Payne 1985). Fritzell (1987) calculated an average litter size of 3.8 from all published work on the subject. The grey fox gestation period has not been clearly determined, and is usually cited as being either 53 days, as in the red fox (Sheldon 1949; Wood 1958), or 63 days, as in the domestic dog (Grinnell et al. 1937, cited in Trapp and Hallberg 1975). However, a study of grey foxes bred in captivity indicates a gestation period of approximately 59 days (Altiere et al. unpubl. data, cited in Fritzell 1987). Whelping occurs in the den and young are weaned at approximately 4 months of age but remain with the family unit until they are about 8 months old (Nicholson et al. 1985).

Two reports of grey fox reproduction in Canada exist. The first is from a news release from the office of the District Forester, Department of Lands and Forests, in Kemptville, Ontario, dated 28 January 1952, which states, “It may be of interest to know that the gray fox is a fairly common visitor to this District. Since we have reports of litters being raised, it may be considered that this animal is a resident. One or two gray foxes are shot every year in the southern parts of Leeds, Grenville, Stormont, Dundas and Glengarry counties.” (Peterson et al. 1953, page 126). The second is of a six-week old kit found on Pelee Island in western Lake Erie in the week of 1 June 1998 (London Free Press 1998). Norm Beattie, an experienced naturalist and hunter who lives on Pelee Island, has observed grey fox den sites and estimates that there are possibly 12 to 15 breeding pairs on the island (N. Beattie, pers. comm.). These reports correspond to one of the two regions in which grey foxes are found in Canada (SO; Fig. 3a). There are no reports of grey fox reproduction from the WLS region (Fig. 3a), and the Manitoba Conservation Data Centre considers grey foxes to be accidental, that is, migrants from the U.S. (MBCDC 2000).

Survival

Few estimates of grey fox mortality in the wild exist, but it is generally reported as high (Trapp and Hallberg 1975). Prenatal mortality of embryos (due to resorption by the female) was estimated at 32% for grey foxes in Illinois (Layne 1958). Wood (1958) found that grey fox kits had a 50% probability of dying in their first summer, 90% probability of dying in their first winter and a 50% annual mortality rate thereafter. These high mortality rates have been supported by data from a variety of studies, which show that juveniles (<1 year old) make up a large proportion of grey fox populations, e.g. 61.5% in New York (Tullar and Berchielli 1982) and 66% in Wisconsin (Root and Payne 1985). These studies also show that there are typically very few foxes in the older age classes, e.g. 4.8% >3 years old in New York (Tullar and Berchielli 1982) and 3% >4.5 years old in Wisconsin (Root and Payne 1985).

The most important cause of mortality of grey foxes in the United States is human harvest by trapping and hunting (Tullar and Berchielli 1982; Fritzell 1987). Grey foxes are harvested in all Great Lakes and northeastern U.S. states (Table 1). The harvest season ranges from Sept. 16 to Mar. 15 in Minnesota, to Nov. 10 to Jan. 31 in Ohio and there are no bag limits for grey fox in any of the U.S. states bordering the Great Lakes and southeastern Canada for which this information was found (Table 1). Harvest rates vary quite widely among those northeastern U.S. states that provided data (Table 1). However, raw harvest data is often biased because of its dependence on factors such as the number of trappers and hunters and pelt price, and therefore may be only roughly indicative of population trends.

 

Table 1. Harvest rates, season dates and bag limits for Ontario and the northeastern and Great Lakes U.S. states.
Season Ontario MinnesotaFootnote a Wisconsin Michigan Ohio New York Vermont New Hampshire Maine
1979/80 1                
1980/81 4                
1981/82 2                
1982/83 4                
1983/84 11                
1984/85 2                
1985/86 6                
1986/87 7                
1987/88 3                
1988/89 3 7 000       13 980      
1989/90 2 5 000       7 529   58  
1990/91 7 6 000       4 407   63 73
1991/92 0 5 000       13 816   76 74
1992/93 0 5 000       4 328   86 58
1993/94 0 3 000       4 438   76 46
1994/95 0 2 000       5 008   97 50
1995/96 1 4 000       9 246   75 104
1996/97 1 NA       15 823   129 25
1997/98 7Footnote b 4 000 13 556 4 255   11 276   104 92
1998/99 17Footnote b 2 000 12 427     19 944   120 75
1999/00   3 000       32 429   89 82
Hunting Season Oct. 25 - Feb.28/29 Sept. 16 - Mar. 15 Oct. 14/28 - Feb. 15 Oct. 15 - Mar. 1 Nov. 10 - Jan. 31 Oct. 25 - Feb. 15Footnote c Oct. 27 - Feb. 10 Oct. 1 - Mar. 31 Oct. 16 - Feb. 28
Trapping Season Oct. 25 - Feb.28/29 Sept. 16 - Mar. 15 Oct. 14/28 - Feb. 15 Oct. 15 - Mar. 1 Nov. 10 - Jan. 31 Oct. 25 - Dec. 10Footnote c Oct. 27 - Dec. 31 ? Oct. 15 - Dec. 31
Bag Limit ? No Limit No Limit ? No Limit No Limit ? No Limit No Limit

Data for Ontario and the Great Lakes and northeastern U.S. states were taken from the following: Ontario (OMNR, unpubl. data), Minnesota (B. Berg, pers. comm.; MNDNR 2001), Wisconsin (WIDNR 2000), Michigan (T. Reis, pers. comm.; MIDNR 2001), Ohio (OHDNR 2001), New York (Gotie 2001; NYSDEC 2001a,b), Vermont (VTDFW 2001), New Hampshire (E. Orff, pers. comm.; NHFGD 2000a,b), Maine (W. Jakubas, pers. comm.; MEDIFW 2001).


Although the grey fox was declared a furbearer in Ontario in 1979 under an amendment to the Game and Fish Act (C. Heydon, pers. comm.), fewer than 20 pelts are traded annually in Ontario (Obbard et al. 1987; Table 1). From 1977 to 1989, biologists with the Ontario Ministry of Natural Resources attempted to verify every report of a grey fox trapped in Ontario; however, this is no longer being done (C. Heydon, pers. comm.). Approximately six to seven grey fox pelts are harvested each year from the Whiteshell Provincial Park region in southeastern Manitoba (I. McKay, pers. comm.). Quebec lists the grey fox as a furbearer so that managers can rapidly introduce regulations if necessary. However, it is currently illegal to harvest grey fox and although Quebec’s regulations stipulate that accidental captures must be reported to a wildlife conservation officer (Société de la faune et des parcs du Québec 2000), there is no incentive to report accidental captures (R. Lafond, pers. comm.). Given the important role that trappers and hunters often play in other jurisdictions in gathering information about species’ abundance and distribution, the absence of recent grey fox records in Quebec should be interpreted with caution.

Grey fox are hunted in Canada and there was a bounty on grey fox on Pelee Island until 15 years ago when it was removed because it did not comply with Ontario’s Game and Fish Act (N. Beattie, pers. comm.). Hunters on Pelee Island currently shoot six to ten grey foxes every winter, although there is no indication that this practice is having a negative effect on the population (N. Beattie, pers. comm.).

Roadway mortality has been reported for grey foxes, both in the U.S. (Tullar and Berchielli 1982) and in Canada (Ontario Natural Heritage Information Centre (ONHIC) unpubl. data, D. Coulson, pers. comm., P. Pratt, pers. comm.), but its significance for fox populations is unknown. In one study, grey foxes were less likely than red foxes to be shot or hit by cars, and the authors suggested that this was the result of more cryptic behaviour and smaller home ranges on the part of grey foxes (Tullar and Berchielli 1982).

Several predators have been reported to prey on grey foxes, including: golden eagles, Aquila chrysaetos (Mollhagen et al. 1972), domestic dogs, Canis familiaris (Tullar and Berchielli 1982), bobcats, Lynx rufus (Progulske 1955), and possibly coyotes, Canis latrans (Grinnell et al. 1937, cited in Fritzell 1987), but the effect of predation on grey fox populations is not considered important (Fritzell 1987).

Grey foxes are resistant to sarcoptic mange mites (Sarcoptes scabiei) and heartworm (Dirofilaria immitis), parasites that are fatal or seriously debilitating to most other wild canids (Stone et al. 1972; Simmons et al. 1980). The only diseases reported with significant frequency in U. cinereoargenteus are canine distemper and rabies (Gier 1948; Davidson et al. 1992 and references cited therein). Canine distemper and rabies are almost always fatal to grey foxes (Fritzell 1987; Yuan Chung Zee 1999) and thus have the potential to limit fox populations under conditions of high grey fox density. In a study of 157 sick or dead grey foxes from 1972 through 1989, canine distemper was found to be a more significant source of mortality than all other diseases combined (Davidson et al. 1992).

Physiology

The northern edge of the grey fox’s distribution may be limited by its ability to tolerate colder temperatures. U. cinereoargenteus is thought to be a southern, warm-adapted species (Waters 1964). Waters (1964) thought that historic fluctuations of the grey fox’s range might be explained by temperature fluctuations and there is some evidence that the current northward expansion of the grey fox’s range is due to a warming climate (see Distribution section above). At present, it is unknown how climate affects grey fox populations. Root and Payne (1985) suggested that the small average litter size (3.5) of a population in Wisconsin was due to a wider range of adverse environmental and nutritional conditions that exist on the northern periphery of the grey fox’s range. Also, the northward expansion of the grey fox’s range may be limited by the energy costs of locomotion in deep snow (M. Crête, pers. comm.).

Movements/dispersal

Grey foxes are most active at night (Follmann 1973, cited in Fritzell 1987; Yearsley and Samuel 1980; Haroldson and Fritzell 1984; Fritzell 1987; Harrison 1997). During its nightly activities, the adult male grey fox radiotracked in Ontario was often found making circuits of the woodlots within its home range (Bachmann and Lintack 1982).

Published estimates of grey fox home range sizes vary from 30 ha (Fuller 1978) to over 1000 ha (Haroldson and Fritzell 1984). However, home range estimates are known to increase with the length of time the animal is tracked (Haroldson and Fritzell 1984), indicating that care should be taken when interpreting results from these studies (Fritzell 1987). Home range size has been shown to increase during the breeding period (Follmann 1973, cited in Fuller 1978; Bachmann and Lintack 1982).

The extent of dispersal varies significantly among populations of U. cinereoargenteus (Fritzell 1987). Nicholson et al. (1985) found that, of 10 radio-collared juveniles in Alabama, only the males (N=3) dispersed from their natal areas. In contrast, 63% of female juveniles in New York dispersed from their natal areas, although male juveniles dispersed greater distances (Tullar and Berchielli 1982). Tullar and Berchielli (1982) also found that juveniles moved greater distances than adults. Sheldon (1953) recovered a marked vixen 84 km away from her natal area, 3 years after he had banded her as a juvenile.

Grey foxes have been dispersing into Canada from populations in the United States since the early decades of the 20th century (Downing 1946) and probably continue to do so. Given that the presence of the grey fox in Canada after a 350-year absence is due entirely to immigration from south of the border (Downing 1946), it seems likely that if the Canadian population were to disappear again, southern foxes would eventually repopulate the Canadian range.

Nutrition and interspecific interactions

U. cinereoargenteus is the most omnivorous of the North American canids, and their diet varies seasonally and geographically (Fritzell 1987). There have been a large number of studies that have catalogued the stomach contents of grey foxes (see references in Fritzell 1987 for detailed lists of organisms present in grey fox diet). Eastern cottontails (Silvilagus floridanus) and small rodents (e.g. Microtus spp. and Peromyscus spp.) are the grey fox’s principal prey (e.g. Hatfield 1939). Both Anderson (1946) and Palmer (1956) have suggested that the northward range extension of the grey fox is related to the northward spread of the cottontail rabbit (Sylvilagus spp.). However, the broad nature of the grey fox’s diet means that the range of this carnivore is unlikely to be dependent on any single prey item. Mammal prey are most important in the grey fox’s winter diet, possibly because other food sources such as insects and plants are less available. Plant material, such as persimmon (Doispyros virginiana), corn (Zea mays), apple (Malus pumila) and wild grape (Vitis spp.) can constitute up to 48% of the dry weight of fox stomach contents (e.g. Hockman and Chapman 1983). Vegetable matter is usually more prominent in the grey fox’s diet in fall than in winter, but still makes up an important component of the diet in the latter season. The main conclusion to be taken from these studies is that grey foxes are more omnivorous than any other canid (Fritzell 1987), and are opportunistic feeders that prey on food items according to their availability (Carey 1982).

Several studies have compared grey fox diet with other canids, presumably to investigate possible competition for food resources. The diet of the grey fox has a large degree of overlap with those of other species, such as coyotes and red foxes, but is more diverse and more omnivorous than the diets of these potential competitors (Hockman and Chapman 1983; Cypher 1993). Grey foxes are more efficient at digesting fruit, and have a narrower gape and smaller carnassials than red foxes. These features may allow grey foxes to be more effective herbivores, but less effective carnivores, than red foxes (Jaslow 1987). Grey foxes may avoid competition for food with other canids by changing their habitat use patterns (Cypher 1993; see Habitat section above, and following paragraph) in addition to their diet.

There is some evidence that coyotes may have a negative impact on sympatric grey fox populations. Grey foxes and other similar sized carnivores increased in numbers when coyotes were removed from experimental plots in western Texas (Henke and Bryant 1999). In southeastern Minnesota grey fox numbers have decreased while coyote numbers have increased, and at the same time, grey foxes have increased their range into northwestern Minnesota right up to the Manitoba border (B. Berg, pers. comm.). Bowhunter sighting records of grey foxes, red foxes and coyotes for the past 10 years in Ohio seem to suggest an inverse relationship between foxes and coyotes (Fig. 4a). But there is no clear relationship between foxes and coyotes in New Hampshire when using a standardized metric of trapping effort (catch per 100 trap nights) (Fig. 4b). Also, the mechanism by which coyotes might regulate grey fox populations (e.g. competition for food or other resources, or predation) is unclear.


Figure 4. Relationships between coyote, grey fox and red fox abundances for: a) Ohio (data provided by C. Dwyer) and b) New Hampshire (data from NHFGD 2000b).

Figure 4. Relationships between coyote, grey fox and red fox abundances.

Symbols and lines are as follows: grey fox (squares and dotted line), red fox (triangles and solid line), and coyote (circles and dash-dotted line).

Behaviour/adaptability

U. cinereoargenteus is typically nocturnal or crepuscular across its range (Fritzell 1987). The basic social unit consists of an adult male and female and their offspring, and this group maintains a home range that is largely exclusive of other such groups (Lord 1961; Haroldson and Fritzell 1984). The adult sex ratio is usually 1:1 (Wood 1958) and grey foxes are generally assumed to be monogamous, although there is no definitive evidence to support this (Fritzell 1987). On several occasions two females with their litters have been observed occupying the same den, which suggests that polygamy may occur (Sheldon 1949; Gerhardt and McAnnis Gerhardt 1995). Young grey foxes leave the den at about 2.5-3 months old, accompany their mother until 4 months old when they start to forage on their own, and become completely independent at about 7 months old (Nicholson et al. 1985). It is unclear whether grey foxes are territorial, but they apparently use urine and faeces in communication (Trapp and Hallberg 1975). Grey foxes can climb trees (Terres 1939) and may use them as a diurnal resting place or as a means of escape (Yeager 1938).

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