2.4 Ermine or Short-tailed Weasel (Mustela erminea) M-MUER
The ermine, or short-tailed weasel is intermediate in size between the long-tailed weasel and the least weasel. In summer, its pelage is reddish-brown above and creamy white below, changing entirely to white in winter, with the exception of the black tail-tip. Body weights of adult ermine in North America range from 56 to 206 g, with smaller animals in the south. Like other weasels, the ermine exhibits sexual dimorphism, with males being 40 - 80% heavier than females. The skull of the male may be 9 - 24% larger than that of the female (King 1983).
The ermine inhabits a variety of habitats. In North America, it is most abundant in boreal, montane, and Pacific Coast coniferous forests (Fagerstone 1987). Ermines avoid dense forests and settle in successional or forest-edge habitats, wet meadows, marshes, ditches, riparian woodlands, or river banks with high densities of small mammals (Simms 1979a; Simms 1979b; King 1983, Fagerstone 1987). Simms (1979a) found that ermine exhibited a decided preference for early successional communities and avoided forested habitats. Simms (1979a) also found that male ermine were more often associated with shrubs than were females. Males generally occupy a wider range of habitats than females and both male and female ermines occupy more habitat types during spring and summer than during fall and winter (Fagerstone 1987).
Ermine diets appear to be influenced by prey availability, and hence, habitat. Simms (1978) found the remains of collared and brown lemmings in ermine scat from the central Arctic. Simms (1979a) discusses the ability of female ermines to exploit the subnivean environment due to their optimum size (small cranium), whereas males, are often too large. Simms (1979a) also suggested that ermine are vole specialists, which is consistent with other literature (e.g., Erlinge 1977, Fitzgerald 1977). Fagerstone (1987) mentions rats, chipmunks, pikas, nesting birds, amphibians, reptiles, earthworms, and insects as prey. King (1983) cites an account of ermine snatching fish from a stream. Variable dietary composition is likely, changing with prey availability (Fagerstone 1987).
Ermine are well adapted to snowy environments and they range into alpine areas (King 1983). Fitzgerald (1977) found ermine living year-round at elevations of 2000 - 3000 m in the Sierra Nevadas. The long, thin shape of ermine increases its thermoregulatory demands in cold climates, and snow provides vital insulation against extremes in air temperature (Chappell 1980).
Size differences may affect the reproductive success of both males and females. Males are polygynous, and larger males are presumed to have an advantage in obtaining mates because they will be more capable of over powering a rival male who is smaller (King 1983). Because females raise their young alone, smaller females may be more efficient when hunting small prey, therefore their energy requirements for hunting would be lower than that of larger female, providing that the abundance of small prey is sufficient to sustain the smaller female. In addition, smaller females have smaller absolute (but not relative) energy requirements because they are not required to expand their territory to search for larger prey items. This suggests selection for larger males and smaller females in ideal situations (when smaller males do not obtain mates and small prey sustains smaller weasels). As with other ecological traits, these assumptions may not always hold true.
Reproduction is discussed by King (1983). Ovulation is believed to be induced by copulation. Delayed implantation results in a mean gestation off 283 days, with a maximum recorded of 378 days (from captive-bred ermine). Active gestation from implantation to parturition takes four weeks. A single litter is produced in the spring, most often in April. The number of offspring per litter ranges from four to thirteen, but four to eight is the usual number, with equal sex ratio. Reproductive success is strongly influenced by food supplies prior to parturition.
The ermine has a circumboreal range throughout the Holarctic and has been introduced into New Zealand (King 1983).
Ermine occur throughout British Columbia, including Vancouver Island and the Queen Charlottes (Cowan and Guiguet 1965). Local distribution of ermine is broadly related to that of small rodents
and lagomorphs (King 1983). Because ermine are well adapted to living in snowy conditions, snow presents little obstacle to the distribution (King 1983).
Mustela erminea haidarum is globally ranked as 'G5T2'. This indicates that M. e. haidarum is an imperiled subspecies of an otherwise common species. Provincially red listed, its ranking is 'S2', indicating that this subspecies is Imperiled.
Mustela erminea anguinae globally ranked as 'G5T3', indicating that M. e. anguinae is an imperiled subspecies of an otherwise common species. Provincially blue listed, its ranking is 'S3', which indicates that this subspecies is Vulnerable.
The other three subspecies, M. e. fallenda, M. e. invicta, and M. e. richardsonii are found on the provincial "Yellow List".
Home ranges of males are larger than females by factors of two to six, and may overlap those of several females (Fagerstone 1987). Females exclude other females from their home ranges, and tend to avoid any male on whose range they live (King 1983). Where prey is abundant, home ranges tend to be small. Estimates of home range size for individual males vary from 0.04 - 2.0 km2, with most averaging 0.10 - 0.40 km2 (King 1983). In southern Ontario, male home ranges averaged 0.20 - 0.25 km2, and female home ranges averaged 0.10 - 0.15 km2 (Simms 1979b). In North America, home range size varies with latitude; ermines in the south tend to have smaller home ranges than those in the north (Fagerstone 1987). Ranges are marked by scent from the anal glands. By continually visiting all parts of its home range a resident male may patrol the boundaries and maintain fresh scent marks in the course of foraging (King 1983).
The density and structure of ermine populations are unstable, because life spans are short and reproductive capacity is high (King 1983). Populations are greatly influenced by fluctuations in the supply of prey, especially of small mammals. Estimates of population densities have ranged from one ermine / 0.17 km2 (Simms 1979b) to one / 0.50 km2 (King 1983). Large-scale movements during breeding season are not uncommon, as juvenile males attempt to establish breeding territories (King 1983).
Although ermine may be active day or night, they are primarily nocturnal (Fagerstone 1987). They typically move rapidly, investigating every hole and crevice, stopping often to survey their surroundings. Ermine are active for short periods, alternating between activity and rest periods every three to five hours (King 1983, Fagerstone 1987). Three types of vocalizations are described: a screech, an aggressive squeal, and a trill. Vocalizations and visual signs may have some significance in territorial defense (Fagerstone 1987).