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COSEWIC assessment and update status report on the wolverine Gulo gulo in Canada

Biology

General

This document is not intended to provide a comprehensive literature review of wolverine biology, since many published reviews exist (Wilson 1982, Hash 1987, Hatler 1989, Banci 1994, Petersen 1997, Biodiversity Legal Foundation 2000, Dawson 2000, Fortin et al. 2002). Rather, recent literature and personal communications, which provide essential information for a scientific assessment of the current status of wolverines in Canada, are assigned priority. Despite the apparent wealth of literature on wolverines, they remain one of the least studied North American medium to large-sized carnivores.

Habitat and population changes since the previous COSEWIC assessment are important criteria for determining the current status. Aspects of the species biology which contribute to the wolverine’s resiliency to harvest, sensitivity to habitat change and ability to re-colonize unsaturated or vacant habitats are also vital for assessing population vulnerability.


Reproduction

The reproductive rate and hence the population resiliency of wolverines is relatively low. Most wolverines become sexually mature at two years of age (Rausch and Pearson 1972, Banci and Harestad 1988). The proportion of adult female carcasses that were pregnant ranged from 92% (Rausch and Pearson 1972) to 74% (Banci and Harestad 1988). The pregnancy rate declined from 92% for 2 year olds to 37% for combined 6 year old and older age classes. Delayed implantation of the blastocyst permits breeding in summer when females tend to be more sedentary. The average number of fetuses ranged from 2.8 to 3.4 in a Yukon study (Banci and Harestad 1988). The latter study was based on carcasses, therefore the actual litter size, following the re-absorption of fetuses and stillbirths, was lower. The size of litters is greatest for females over the age of 6 (Banci 1994), but the pregnancy rate for those older females is lower. Reproductive rates observed in Alaska and Idaho were 0.69 and 0.89 kits per female per year respectively (Magoun 1985, Copeland 1996), since females gave birth 2 or more years apart. The rate of mortality among kits and the most successful age classes of females at raising kits to weaning are unknown factors.

The fact that female reproductive rates are highest in early adult years has implications for trapline management using untrapped areas as population refugia. The maintenance of refugia from trapping will ensure that older females are conserved, since trapping removes resident animals and vacant habitats are generally colonized by dispersing juveniles.

There is some debate over whether wolverine populations fluctuate with snowshoe hare (Lepus americanus) cycles, or if increased trappability leads to the perception of wolverine fluctuations (Hatler 1989). Hares are an important component of the wolverines’ diet (Banci 1987). Banci (1987) noted an increase in the harvest of adult males in March 1983 following the snowshoe hare population crash in 1982-83.


Survival

Wolverines are preyed on by bears, wolves, cougars (Puma concolor), golden eagles and other wolverines. These other predators may be encountered with greater frequency at carrion. In a summary of mortality rates of radio-collared wolverines from 12 studies, J. Krebs (pers. com., 2002) found that human caused mortality from trapping and road/rail kill accounted for 46% of deaths. Natural sources of mortality included predation by wolves, cougars and conspecifics (30% of non-human causes), and starvation (49% of non-human causes). Survival was < 0.75 among all age/sex classes in trapped areas, and > 0.84 in areas where trapping did not occur. Intrinsic rates of increase (λ) were estimated at 0.88 in trapped populations and 1.06 in untrapped populations. Survival was highest among adult females (0.88 in untrapped areas, 0.73 in trapped areas) and lowest among subadult males (0.45 in trapped areas) (Krebs, pers. com., 2002). This evidence suggests that trapped populations would decline without immigration from refugia from trapping.


Physiology

Wolverine kits reach adult body size by 7 months of age (Magoun 1985). Rapid growth of the energy-producing tissues (liver, heart, brain, and kidneys) requires a high metabolic rate early in life, which in turn places high energetic demands on the mother (Wilson 1982). The high metabolic rate should improve kits ability to thermoregulate during long foraging trips by the mother.  Kits are weaned at 9-10 weeks (Banci 1994). Both mother and kits may require more carbohydrates near the time of weaning when energy requirements peak (Wilson 1982).


Movements/Dispersal

Wolverines typically occupy home ranges that vary from about 50-400 km2 for females and 230-1580 km2 for males (Hornocker and Hash 1981, Gardner 1985, Magoun 1985, Whitman et al. 1986, Banci 1987, Copeland 1996). Lofroth (2001) documented an average home range of 3500 km2 for dispersing subadult males. There may be home range overlap between members of the same and opposite sexes (Krebs and Lewis 2000, Lofroth, pers. com., 2002), however this is poorly understood. Intersexual overlap is considered to be more common. A proportion of the population is transient at any given time. Transients are typically, but not always, yearlings. Yearling females tend to establish home ranges nearer their natal ranges than yearling males do, although both sexes are capable of long distance movements (Magoun 1985, Gardner et al. 1986, Mulders 2000). Males can disperse over 200 km (Copeland 1996; n=3), 378 km (Gardner et al. 1986), and 73 to 326 km (Mulders 2000; n=3). Magoun (1985) reported a 300 km movement by a female of unknown age, and Mulders (2000) reported movements by 5 females between 69 and 225 km. When estimating population densities, home range overlap and the presence of transients must be considered. Simple home range extrapolation (e.g., Banci 1994) results in an underestimation of population size.

Vangen et al. (2001) attributed sex-biased dispersal patterns to resource competition between females and competition for mates by males. Wolverines are able to traverse rugged terrain, including tundra and glaciers that would act as barriers to many species of mammals. Dispersal characteristics likely gave wolverines the capacity to recolonize gaps in their distribution in Scandinavia (Vangen et al. 2001). However, long distance movements place individuals at greater risk of mortality due to predation, trapping, accident or starvation (Copeland 1996).

The large home range size of wolverines increases its susceptibility to trapping. Refugia from trapping must necessarily be large enough to protect entire ranges of wolverines. The propensity of juveniles to disperse long distances is a key factor in gene flow.


Nutrition and Interspecific Interactions

Wolverines are scavengers and predators, opportunistically feeding on abundant or readily procurable food. Fresh prey are eaten more during summer and carrion, including cached items, is used more in winter (Magoun 1987). Prey species may include rodents, snowshoe hares, birds, and young ungulates. The most common sources of carrion are caribou, moose (Alces alces), mountain sheep (Ovis dalli and O. canadensis), mountain goats (Oreamnos americanus), deer (Odocoileus hemionus and O. virginianus), and elk (Cervus elaphus) (Gardner 1985, Banci 1987, Magoun 1987, Mulders 2000, Lofroth 2001). Fish and marine mammals are also scavenged. Wolverines will also eat plant material such as berries. Studies in northern and southeastern British Columbia have shown that caribou and marmots (Marmota spp.) are important foods for denning females (Lofroth, pers. com., 2002). Marmots and other sciurids are important to adult females throughout the summer.

Large carnivores such as grizzly bears, wolves and cougars require large tracts of wilderness ecosystems which mirror the wolverine’s requirements but, more importantly, they generate carrion for wolverines. These carnivores compete with wolverines at kill sites, and are a potential source of wolverine mortality.

The highest densities of wolverines occur in the mountainous areas of the Yukon Territory, Northwest Territories, British Columbia and Alberta, where habitats, prey species, and ungulates are most diverse and abundant (Figure 3).


Behaviour/Adaptability

Wolverines prefer pristine areas; however, home ranges frequently overlap active traplines, cross-country ski trails, busy roads such as logging roads (E. Lofroth, per. com. 2002) and the edges of communities. However, large highways and other transportation corridors may act as barriers to movements and dispersal, and are significant sources of mortality. Austin (1998) found that wolverines avoided areas within 100 m of the Trans Canada Highway and preferred areas more than 1100 m away. They selected narrow crossings (<100 m). Lofroth (2001) hypothesized that wolverines prefer uncut forest stands within a matrix of cut and uncut stands, particularly where forest roads are active, a behaviour that increases trap vulnerability.

Wolverines are curious, and will investigate campsites, food caches and even cabins when humans are not present. Their propensity to “rob” food caches and cabins is a contributing factor in their mythology. Wolverines will opportunistically use snowmobile trails for travel and scavenging trapped animals and hunter kills. Wolverines are thought to be highly secretive; however, they are occasionally observed at a distance by people hiking or skiing above treeline and on glaciers, and hiking and rafting in the Arctic. In the Arctic, wolverines are frequently hunted from snowmobiles.

Denning females are sensitive to disturbance caused by researchers (Magoun and Copeland 1998), therefore snowmobiling and back-country skiing may similarly impact reproductive success (Heinemeyer et al. 2001, Lofroth, pers. com., 2002). Den relocation or litter abandonment may result.