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COSEWIC assessment and update status report on the swift fox (Vulpes velox) in Canada (2000)

General Biology

Morphology, description and taxonomy

The swift fox is the smallest of the North American canids (Egoscue 1979) and one of 3 species belonging to the genus Vulpes.  The animal is the size of a large house cat with measurements ranging around 840 mm (total length); 280 mm, tail, 30 mm, hind foot, and 80 mm, ear length.  The black tip on its tail, and black around the muzzle, distinguishes swift foxes from young coyotes or light colour phased red foxes.  Winter pelage is buffy gray with some red (orange tan) coloration in abdominal areas.  Summer fur is short and more reddish.  Average weight of adult males is about 2.5 to 3 kg, and females 2.0 to 2.4 kg, respectively.  Body size of males is about 8% heavier than females (Egoscue 1979).  Swift foxes have an elongated skull with small widely‑spaced teeth.  Skull sizes that have been recorded for Colorado foxes (males) were measured as 112 mm, zygomatic breadth (64 mm) and interorbital constriction (24 mm), post orbital constriction (23 mm).

The three members of the genus Vulpes in North America are red fox (V. vulpes), kit fox (V. macrotis) and swift fox (V. velox).  Kit and swift foxes are considered the "arid land" or prairie/desert fox complex.  The exact taxonomic distinction between the two species has been under review (Samuel and Nelson 1982; Dragoo et al., 1990; Mercure et al., 1983; Wayne, in prep.)

The swift fox differs from the kit fox in appearance by a broader skull, shorter ears, shorter tail length and slightly larger body size.  Swift fox are residents of grassland regions, while kit fox occupy the desert environments west of the Rocky Mountain Area and associated mountain ranges.  A review of the subspecies designation was summarized by Knowles (Fauna West 1991).  He noted that Merriam, an early taxonomist prone to excessive subspecies designations, described two subspecies, the northern swift fox (Vulpes velox hebes), and the southern swift fox (Vulpes velox velox) (Merriam 1902).  This classification was the basis of a brief listing of the northern subspecies as an endangered species (U.S. Fish and Wildlife Service 1979 and 1982).  The northern swift fox was delisted when it was decided that valid subspecies variation did not exist (Stromberg and Boyce 1986).  However, those authors cautioned that there was significant geographic variation among the specimens examined and that this variation may reflect genetic differences.  They advised that conservation efforts to restore swift foxes to former portions of their range consider this geographic variation.  The issue precipitated a debate in the literature between Stromberg and Boyce, who critically reviewed the Canadian reintroduction program, and (Herrero et al., 1986) who defended it.  A similar study conducted by (Dragoo et al., 1990) also concluded that the subspecific designation, as proposed by Merriam (1902), was not valid.

Hall (1981) suggests that the swift and kit foxes are conspecifics.  Dragoo et al. (1990) present data to support this contention.  They assessed the relationships of these two foxes by morphometric and protein-electrophoretic methods.  In the latter case, they found genetic divergence to be negligible with a high degree of genetic similarity among all subspecies examined.  Morphometric analysis were only able to distinguish between the swift and kit fox and not between any of the previously proposed subspecies.  Dragoo et al.(1990) propose reclassifying  the swift and kit foxes as a single species - Vulpes velox - with only two recognized subspecies - the swift fox, Vulpes velox velox, and the kit fox, Vulpes velox macrotis.

 

Diet and foraging behaviour

Earliest description of the food habits was from Baird, who noted that "mice and grasshoppers" were eaten (Baird 1858).  We know from various studies that swift foxes opportunistically prey on a variety of food sources (Pruss 1994).  A list of identified food items from material collected in Oklahoma included 13 species of mammals, 4 species of birds, one species each of amphibians and reptiles and 30 species of invertebrates  (Kilgore 1969).  Jack rabbits (Lepus townsendii) are the largest prey species in Canada. Ground squirrels (Spermophilus spp.) likely are seasonally very important.  Black-tailed prairie dog distribution (Cynomys ludovicianus) in the Canadian prairies is very limited, therefore of no consequence to swift fox survival in most areas.

Current studies (Moehrenschlager, Michie and Moehrenschlager, in prep.) involve an analysis of scat samples collected on the Canadian prairies from 1994 to 1997.  Results will be particularly important in identifying winter food habits in the northern extent of the range of the species.  Work on small mammal availability in winter resulted in interesting data for 3 regions in which swift foxes have been released (Klausz 1997). Biomass values in winter were low for upland, roadside and coulee areas.  Foxes probably seek out appropriate micro-environments with concentrations of voles/mice and insects and vulnerable components (e.g. young hares) within the prairie ecosystem.

Much work needs to be carried out on the foraging behaviour of swift foxes in northern ranges.  Swift foxes are largely nocturnal in winter.  Onset of activity appears to be correlated with light (sunset) but varies with the temperature.  It is not uncommon to see foxes sunning themselves at den entrances during cold, sunny days in winter.  It is likely that foxes travel along predictable routes (fence lines; ridges, cattle trails, etc.) in foraging trips through their home ranges.  Pruss (1994) found that during the spring/summer period, swift fox are active over extended periods during both the day and night.  A total of 10 natal dens (5 per study season) were watched from May‑August 1991-1992.

Northern swift foxes switch their food requirements from winter to summer based on availability.  Young foxes in summer forage on grasshoppers.  Similarly, foxes soon after release were seen to forage on insects.  Adult foxes have been observed feeding young with ground squirrels.  Ground squirrels and live grasshoppers are not available in the winter months. 

 

Denning activities

Swift foxes are the most den-dependent of all canids in North America.  Even though dens are used year round, the use of specific sites shifts (Chambers 1978; Hillman and Sharps 1978; Hines and Case 1991; Kilgore 1969).

On the Canadian prairies, all dens investigated were in native prairie.  These sites provide clear views of the surroundings, which could be a hedge against predation.  Pruss (1994) compared the location and physical characteristics of 32 occupied swift fox natal/rearing dens and 33 unoccupied (typically badger) burrows.  A stepwise discriminant function analysis identified 5 potential discriminators between occupied and unoccupied sites (i.e. position on hill, height of new grass, distance to water, distance to roads and slope).  Dimensions of den openings in South Dakota were an average 19 cm wide and 22 cm high (Hillman and Sharps 1978).  Number of den entrances is greater for natal dens than for escape dens.

 

Mating systems, growth and reproduction

Females are monestrous, with oestrous occurring from late December to February. Swift foxes have a gestation period of about 55 days.  The pups are born from mid-April to June (C. Smeeton pers. comm.), and weigh about 200 grams at two weeks of age (P. West pers. comm.).  The pup's eyes and ears are open at about two weeks, and the pups are fully weaned by six weeks.  They reach their adult weight by mid-summer (J. Creviston pers. comm.).  Pups have a soft woolly coat for the first month, but later develop the adult pelage.  Males reach sexual maturity before the end of their first year; however, not all of the first year vixens will breed.

Foxes usually mate for life in a monogamous relationship, but observations of some burrows containing one male and two females have been made (Nowak and Paradiso 1983; Covell 1992; Carbyn et al., 1994).  Covell also found one case of 3 females in association with one male.  Observations, particularly during the mating season, of concurrent den sharing and home-range use was recorded by various investigators in the Lost River Ranch/Border area in southern Alberta and Saskatchewan (Carbyn et al., 1994).