Swift fox (Vulpes velox) COSEWIC assessment and status report 2009: chapter 6

Biology

Population Dynamics

Swift Foxes generally live in socially monogamous pairs (Kilgore 1969), although extra–pair mating also occurs (Kitchen et al. 2006). A typical social group consists of a mated pair with kits; additional non–breeding females may act as helpers to raise kits (Kilgore 1969; Covell 1992; Olson et al. 1997; Sovada et al. 2003; Tannerfeldt et al. 2003). In Alberta, a male was observed with the litters of two different females on the same day (Moehrenschlager 2000). Concurrent den sharing and home range use (particularly during the mating season) occurs in Alberta and Saskatchewan (Carbyn et al. 1994). Kamler et al. (2004) assessed the relationship between Swift Fox density,mating systems and group structure in northwestern Texas. Although high– and low–density populations were separated by only 40 km, and vegetation and diets were similar between sites, polygynous groups, communal denning, and non–breeding females occurred in the high density area, whereas only monogamous pairs occurred at the low density site.

Swift Foxes begin breeding in mid–February in Canada (Moehrenschlager and Macdonald 2003). Females are monoestrous (Kilgore 1969). Gestation is about 51 days (Schroeder 1985) and litter size is usually about four kits (Kilgore 1969; Hillman and Sharps 1978; Moehrenschlager 2000; Olson and Lindzey 2002), but can be as high as eight (Moehrenschlager 2000). Both parents care for the young. Litters are larger when helpers are present (Covell 1992). Kits weigh 50–100 g at birth, their eyes open at 10–15 days. They emerge from the natal den at about four weeks and are weaned at 6–7 weeks (Kilgore 1969; Hines 1980). Although kits break close associations with their parents at 4–6 months (Covell 1992), Moehrenschlager (2000) found that only 33% (n=12) of juveniles had left natal home ranges by 9.5 months. By 18 months, all had dispersed. Both males and females are sexually mature by the end of their first year (Kilgore 1969; Morgan Ernest 2003), but not all first–year females breed (Carbyn 1998).

Swift Foxes generally live 3–7 years in the wild (Reid 2006). In captivity, an individual lived to nearly 13 (Kilgore 1969; Egoscue 1979). In the border region, annual adult survival rates range from 0.38 (95% CI = 0.15–0.60) to 0.63 (95% CI = 0.23–0.86) (Moehrenschlager et al. 2007b), similar to estimates from the core range in the US (Covell 1992; Sovada et al. 1998; Kitchen et al. 1999; Olson and Lindzey 2002; Schauster et al. 2002; Zimmerman et al. 2003). Juvenile survival in the border region (0–0.50, Moehrenschlager et al. 2007b) is also similar to the US (0.13–0.69, Covell 1992; Sovada et al. 1998; Kitchen et al. 1999; Schauster et al. 2002). Moehrenschlager and Macdonald (2003) reported that survival rates of translocated Swift Foxes were similar to wild residents. Mortality is primarily attributable to natural predation or human causes. There is no evidence that disease is an important source of mortality (Moehrenschlager et al.2004). Direct human–caused mortality is caused by poison, shooting, collisions with vehicles, and trapping (Kilgore 1969; Carbyn et al. 1994; Pruss 1994; Sovada et al.1998). Predation by Coyotes is the principal cause of mortality (Covell 1992; Carbyn et al. 1994; Sovada et al. 1998; Kitchen et al. 1999; Andersen et al. 2003; Moehrenschlager et al. 2007b).

Denning

A major factor influencing maintenance of viable populations is availability of suitable dens (Egoscue 1979; Pruss 1999; Harrison and Whitaker Hoagland 2003). Swift Foxes use dens throughout the year (Kilgore 1969; Egoscue 1979; Hines 1980; Tannerfeldt et al. 2003). They serve as refugia from predators, protection from extreme climatic conditions, prevention of water loss, periodic rest sites, and secure places to raise young (Cypher et al. 2003; Tannerfeldt et al. 2003). Swift Foxes use many dens concurrently (Moehrenschlager et al. 2004), usually concentrated within a 1–2 km² area (Pruss 1994; Tannerfeldt et al.2003), but throughout the home range (Moehrenschlager et al. 2004). Swift Foxes will use other appropriately sized holes to evade predators (Pruss 1999). In Canada, pairs may use up to eight multiple entrance dens when rearing young (Pruss 1994; Tannerfeldt et al. 2003) and up to 22 dens per year (Moehrenschlager 2000). They reuse den sites in subsequent years (Pruss 1994; Moehrenschlager 2000; Tannerfeldt et al. 2003) and change dens frequently (Kilgore 1969).

Swift Fox dens primarily occur in short– or mixed–grass prairie (Kilgore 1969; Uresk and Sharps 1986). In Canada, they are almost exclusively on native prairie with unobstructed views of the surroundings (Carbyn 1998). Dens typically occur in areas with well–drained soils, usually on the tops of hills with gradual slopes (Cutter 1958a; Uresk and Sharps 1986; Pruss 1999). Natal dens in Canada are on average 267 m from the nearest road and 1 km from the nearest water source (Pruss 1999). Burrow openings are about 20 cm in diameter (McGee et al. 2006a) which likely precludes entry by Coyotes. In spring and summer, dens used for raising kits typically have one or two central chambers extending to a depth of 1 m, and connected to the surface by many tunnels (Cutter 1958a; Kilgore 1969). Dens used for shelter and predator avoidance have fewer entrances (Hillman and Sharps 1978; Pruss 1999; Moehrenschlager 2000; Olson 2000).

Dirt tailings and entrance size distinguish Swift Fox dens from Coyote and Red Fox dens (McGee et al. 2006a). Ninety–six percent (n=74) of Swift Fox dens had conspicuous dirt tailings at entrances and den openings smaller than those of Coyotes and Red Foxes.

Swift Foxes can excavate their own dens, but frequently enlarge burrows of ground squirrels, prairie dogs (Cynomys spp.), or American Badgers (Cutter 1958a; Kilgore 1969). Although most dens are built in natural substrates, human–made structures such as pipes, culverts, buildings, and rubble piles may also be used (Moehrenschlager et al.2004). Pruss (1994) found that shifts between dens did not exceed 500 m; however, Moehrenschlager (2000) recorded a pair moving a litter of seven kits 1.9 km.

Diet and Foraging Behaviour

Swift Foxes are opportunistic foragers, primarily eating mammals, but also birds and their eggs, insects, plants, and carrion (Cutter 1958b; Kilgore 1969; Hines 1980; Cameron 1984; Uresk and Sharps 1986; Hines and Case 1991; Pruss 1994; Zimmerman 1998; Kitchen et al.1999; Sovada et al.2001). In the border region, scat analysis revealed that mammals are the most frequent prey (68.3%: rodents – 59.5%; lagomorphs – 7.1%; large mammals [presumably carrion] – 1.7%), insects (23.8%), and birds (8.0%) (Moehrenschlager et al. 2007b). Reynolds et al. (1991) found that small rodents and carrion from ungulates were most important with a small portion from lagomorphs and ground squirrels. Typically, relatively high frequencies of insects and plant matter occur in the diet; but these items constitute a small portion of total biomass (Kilgore 1969; Harrison 2003; Moehrenschlager et al. 2004; Kamler et al. 2007a).

Swift Fox diets vary geographically, depending on the local assemblage and prey abundance. Murid rodents were the most frequent prey of Swift Foxes in Kansas and Nebraska (Hines and Case 1991; Sovada et al. 2001), although leporids are also common (Cutter 1958b; Cameron 1984; Zumbaugh et al. 1985). Black–tailed Prairie Dogs (Cynomys ludovicianus) are most important in South Dakota (Uresk and Sharps 1986); but unlikely to be important (Carbyn 1998). In general, prey are not larger than White–tailed Jackrabbits (Lepus townsendii; 3–4 kg).

In the northern part of their range, prey selection reflects differences in availability between summer and winter. For example, ground squirrels and grasshoppers (Melanoplus spp.) are important in summer (Carbyn 1998). In winter, Swift Foxes probably switch to lagomorphs.

Swift Foxes are mostly solitary hunters, exhibiting seasonal variation in activity periods (Carbyn 1998). Activity is correlated with day length and varies with temperature (Carbyn 1998). Swift Foxes are largely nocturnal in winter, but become more crepuscular in summer (Moehrenschlager et al. 2004). In spring and summer, Pruss (1994) reported activity over extended periods during day and night. Swift Foxes tend to follow predictable routes while hunting, such as along fence lines, ridges, roads, and trails (Carbyn 1998). Food is sometimes cached (Pruss 1994; Sovada et al. 2001).

Physiology

Temperatures on the prairies range from near −40°C in winter to +40°C in summer (National Climate Data and Information Archive 2002). Behaviourally, Swift Foxes thermoregulate and conserve water by timing activity bouts and using dens. On cold, sunny winter days, they often sun themselves at den entrances (Carbyn 1998). When free water is available, a Swift Fox requires about 210 g of food per day; in the absence of water, moisture requirements can be satisfied solely from food, by eating about 330 g per day (Flaherty and Plaake 1986).

Movement and Dispersal

Although homes ranges can overlap, there is nearly total separation of core activity areas of adjacent same–sex individuals, suggesting territoriality (Pechacek et al. 2000; Andersen et al. 2003; Sovada et al. 2003). For example, Canadian Swift Fox home ranges (including Montana) overlapped by 74% among mates and only 29% between neighbours (Moehrenschlager et al. 2007b).

In Canada, young Swift Foxes begin to disperse in August (Pruss 1994; Moehrenschlager 2000). However, 6 weeks before the subsequent breeding season, up to two–thirds of juveniles in Alberta and Saskatchewan remained within natal home ranges (Moehrenschlager et al. 2004). By 18 months of age, all foxes had dispersed (Moehrenschlager and Macdonald 2003). Normal dispersal distance is <15 km (Covell 1992; Moehrenschlager 2000; Schauster 2001; Sovada et al.2003). Long distance dispersal of five wild–born juveniles from the Blackfoot Reservation (range=43.1–55.8 km, with one covering a distance of 190.9 km) were recorded (Ausband and Moehrenschlager 2009). Swift Foxes translocated from Wyoming to Colorado dispersed mean distances of 27 km (adults) and 19 km (juveniles) (Moehrenschlager 2000). Males disperse farther than females (Covell 1992; Moehrenschlager 2000). In Texas, Nicholson et al. (2007) found that dispersal direction reflected land use practices with animals usually dispersing to rangeland and away from anthropogenic features.

Swift Fox home range size, while correlated with prey abundance, varies significantly with method of calculation, seasons, and sample size (Hines and Case 1991; Kitchen et al. 1999; Pechacek et al. 2000; Andersen et al. 2003; Harrison 2003; Sovada et al. 2003; Zimmerman et al. 2003). Home range size of 36 Swift Foxes in the border region based on the fixed kernel method was 31.9 ± 4.8 km² (Moehrenschlager et al. 2007b). Average home ranges of Canadian Swift Foxes, residing at the northern periphery of the global range of the species, are approximately three times larger than Canadian kit fox from Mexico (Moehrenschlager et al. 2007).

Interspecific Interactions

Coyotes are important predators of Swift Foxes in Canada and with Red Foxes likely compete with Swift Foxes (see Limiting Factors and Threatssection).

Adaptability

Swift Fox adaptations for life on the prairies include their opportunistic foraging strategy and use of dens for shelter and protection from predators (Pruss 1999; Allardyce and Sovada 2003; Harrison and Whittaker Hoagland 2003; Tannerfeldt et al. 2003). In response to the extreme climate of the prairies, they thermoregulate and reduce water loss by adjusting activity periods and burrow use (Pruss 1994). Their feet are well–insulated with the foot pads being almost entirely covered by fur (Egoscue 1979).

As their name suggests, Swift Foxes are fast runners, able to reach speeds of 60 km/h. Their slender skeleton and long legs are adapted for running (Egoscue 1979), which helps them evade predators and hunt fast prey e.g., lagomorphs.

Page details

Date modified: