Wolverines give birth to their young during a period when winter conditions prevail, usually February-March, so dens must provide not only protection from predators but also protection from cold temperatures. Wolverines are known to den in alpine, subalpine, taiga, boreal forest, and tundra habitats. Sites where wolverine dens have been found include ravines or drainages where snow accumulates (Pulliainen 1968, Bjärvall 1982, Serebryakov 1984, Magoun 1985), snow-covered rocky scree or boulder talus (Haglund 1966, Myrberget 1968, Pulliainen 1968, Copeland 1996, Lee and Niptanatiak 1996), snow-covered fallen trees usually near timberline, including trees downed by avalanches (Pulliainen 1968, Zyryanov 1989, Copeland 1996, Inman etal. 2007), taiga peat bogs or conifer forest with rocky areas and fallen trees (Pulliainen 1968, Dawson et al. in press), and mountain birch woodlands near fells or alpine areas (Pulliainen 1968, Landa et al. 1998). Magoun and Copeland (1998) suggested that a critical feature of wolverine denning habitat is the dependability of deep snow throughout the denning season (February-May). Snow greater than 1 m deep, distributed uniformly or accumulated in drifts, provides protection from cold temperatures. Long, complex snow tunnels in hardened snowdrifts characterize den sites in tundra and alpine areas, and in some cases, the tunnels lead down to entrances under boulders that provide additional protection for kits. In forested areas where snow is deep and soft, dens are located under fallen trees or boulders that provide added structure to the den, preventing snow tunnels from slumping.
Magoun and Copeland (1998) described two types of reproductive dens, natal dens where young are born and maternal dens where the mother may move the kits if conditions are no longer suitable at the natal den. In tundra and alpine areas where wind-hardened snowdrifts are used as natal dens, the dens have a complex structure involving branching snow tunnels about 30-40 cm in width that are up to 60 m in total length and contain a number of enlarged beds 40-90 cm wide that are used as lairs for the kits, food storage, and latrine depots (Myrberget 1968, Serebryakov 1984, Magoun 1985). The floor of the tunnel system where the kits are kept can be as much as 3-5 m under the surface of the snowdrift or a little as 1 m. Kits often rest directly on the snow surface or on bare ground if the tunnels go all the way to the ground. Sometimes there is vegetation on the floor of the bed where kits are kept, although it is not clear if the female purposefully places vegetation in the bed or if the vegetation on the floor of the bed results from the female chewing off shrubby vegetation contained within the snow layer while constructing the bed. If boulders or rocky cliffs are present at the den site, the snow tunnels will often run along the edge of the boulders or cliffs where snow has accumulated in deep drifts and the boulders and rocky shelves of the cliffs may be used to shelter the kits. If rocky areas are extensive enough, the kits may be located well within the rocks where they are inaccessible to larger predators. In forested habitats, natal dens are often located under fallen trees, either under a single large tree that has fallen or a group of trees that have blown down or been sheared off by an avalanche. The trees are covered with deep snow and dens are formed in snow tunnels that incorporate subnivean spaces under the tree trunks. Many natal dens are located on north-facing slopes where snow melt is delayed in the spring months, but reported aspect and slope of wolverine natal dens are quite variable, especially for dens in tundra and alpine habitats. Sites used for maternal dens are often close to the natal den and have similar structure, although the distance between the natal den and maternal den can be 3-4 km away (Magoun and Copeland 1998) and as much as 6 km (Myrberget 1968). The length of the tunnel systems in maternal dens can be considerably shorter than at the natal den site, especially if the female moved her kits to the maternal den as a result of disturbance at the natal den (Myrberget 1968).
Wolverine kits are kept at reproductive dens until they are able to follow the mother, at least for short distances, usually around the time they are weaned at 9-10 weeks of age. In some cases, kits are kept at the natal den until weaning (e.g., Magoun 1985) but females may use multiple maternal dens prior to weaning. Why natal dens become unsuitable is not well understood. Magoun (1985) suggested that den abandonment in the arctic was probably forced by spring snow melt. In Idaho, abandonment of natal dens coincided with a period when maximum daily temperatures rose above freezing for the first time since denning commenced (Magoun and Copeland 1998), suggesting that den abandonment may be a response to increased moisture in the dens or collapsing snow tunnels. Den abandonment could also be caused by disturbance at the den site. Myrberget (1968) mentions 4 instances of den abandonment due to human disturbance and suggested that secondary dens may be less suitable than the natal den. Den abandonment by females in Idaho and Alaska appeared to be caused by human presence at maternal den sites whereas repeated human presence at a natal den site in Alaska did not cause den abandonment (Magoun and Copeland 1998). Magoun and Copeland (1998) suggested that maternal dens may not be as secure as natal dens and use of maternal dens may be easily disrupted by the presence of humans nearby. Whatever the cause of den abandonment, the repeated use of some sites for natal dens (Lee and Niptanatiak 1996, Copeland 1996) suggests that there are critical denning requirements that are only met at specific sites. Copeland et al. (2010) suggested that areas where snow lasts well into May, across the worldwide range of the wolverine, define the habitat niche of the wolverine, citing the distribution of documented den sites.
Reproductive females may also be influenced in their selection of denning habitat by its suitability as rearing habitat for kits after weaning (Magoun and Copeland 1998). Factors contributing to successful rearing of kits have not been well studied. Magoun and Copeland (1998) suggested that wolverines may select denning areas that have low populations of predators and provide adequate food for kits in summer. Researchers have documented shifts in wolverine activity from lower elevations to higher elevations in summer (Hornocker and Hash 1981, Whitman et al. 1986, Banci 1987, Copeland 1996) or reported selection by reproductive females for high elevation habitats with marmots and ground squirrels (Krebs et al. 2007, Lofroth et al. 2007). However, Bevanger (1992) stated that opportunities for hunting are severely reduced for wolverines in summer and that wolverines rely on food previously cached in swamps, rocky screes, and snowdrifts. High elevation and high latitude habitats probably provide more options for food storage in summer than do low-elevation habitats (Magoun and Copeland 1998). Long-lasting snow beds and other cool areas where wolverines are known to cache food may be important features of wolverine kit-rearing areas, especially in spring when they may also serve as rendezvous sites for the mother and kits (Magoun 1985). Once winter food caches are exhausted in early summer and the kits are more mobile, wolverines may rely more heavily on predation on small mammals and birds than on scavenging (Magoun 1987, Myrberget and Sørumgård 1979, Copeland 1996, Landa et al. 1997). The relevance of summer food availability to the selection of denning areas by reproductive female wolverines needs further research.
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