Washington's wild one
An authoritative article on wolverine by USFS research wildlife biologist Keith Aubry. Originally published in the Fall 2008 edition of Conservation Northwest Quarterly.
by Keith Aubry, USFS research wildlife biologist
The wolverine (Gulo gulo) is one of the rarest and least-known mammals in North America. They generally are found in areas remote from human development, occur at very low densities, and are secretive and difficult to observe.
The wolverine’s reputation as an especially fierce and dangerous carnivore is largely a myth. Wolverines feed primarily by scavenging on the carcasses of mountain goats, deer, elk and other ungulates, especially during the winter. However, they are capable of killing large prey when deep snow conditions give them a predatory advantage, and they often prey on smaller mammals, such as marmots and ground squirrels, during the snow-free months.
The wolverine is a species of special conservation concern in the contiguous United States. A petition to list all wolverine populations in the lower 48 under the Endangered Species Act was denied in March 2008, but the petitioners, including Conservation Northwest, in 2008 filed a notice of intent to sue over that decision. The wolverine is considered a candidate species for listing as threatened or endangered by the state of Washington, and a sensitive species by the Pacific Northwest Region of the US Forest Service.
Several colleagues and I recently completed a comprehensive study of the wolverine’s historical distribution and broad-scale habitat relations in the contiguous US. Our results indicate that the historical range of wolverines in the Pacific states was limited to populations in the northern Cascade Range in Washington (which were connected to populations in British Columbia), and an isolated population in the southern Sierra Nevada of California. Intervening areas in southern Washington, Oregon, and northern California have a very scanty or nonexistent record of wolverine occurrence. Analysis of genetic data from museum specimens showed that California wolverines had unique genetic characteristics and had been isolated from other populations in North America for 2,000 to 10,000 years.
We believe this distributional gap existed because unoccupied areas lack the habitat conditions associated with wolverine occurrence in southern boreal forests of the contiguous US—relatively broad expanses of alpine habitat conditions and snow cover that persists through the end of the late-spring denning period (mid-May).
The California wolverine was extirpated by the 1920s, probably due to high levels of human-caused mortality from trapping and poison-baiting target ting other animals, combined with a lack of immigration from other populations. Consequently, biologists were amazed to learn that a remote-camera photo had been taken of a wolverine near Lake Tahoe in February of 2008. However, genetic analyses of hair and scat obtained from this animal showed it to be a male that originated in the northern Rockies. We don’t know if he made the journey on his own or had been intentionally translocated, but this record represents the first physical evidence of wolverine occurrence in California in over 85 years.
In the early 1900s, wolverines were also becoming scarce in other portions of their range; by the 1940s, many biologists believed that the wolverine had been extirpated from most if not all of the contiguous US.
Beginning in the 1960s, however, reliable and persistent evidence of wolverine occurrence began to appear in the North Cascades and the northern Rocky Mountains of Idaho and Montana. Wolverines currently occur in northern Washington, north-central Idaho, northwestern Montana, and the Greater Yellowstone Ecosystem in Wyoming. However, all other portions of their historical range in the US—California, Utah, Colorado, and the Great Lakes region—remain unoccupied.
We don’t know if reestablishment of wolverine populations in the northwestern US was due to recolonization from populations in Canada or if small, remnant populations expanded their range after regulatory protections were established. In either case, the North Cascades Ecosystem was not only a focal area of wolverine occurrence historically, it now represents the southernmost extent of wolverine distribution in the Pacific states.
Wolverines always give birth in snow caves, which is one of the reasons they are closely tied to areas that have relatively deep spring snow cover. In the western mountains of the contiguous US, wolverines are naturally vulnerable to environmental changes because their primary habitat is fragmented to begin with—it occurs in an archipelago of montane “islands” at high elevations.
Because the geographic extent of spring snow cover will be affected by global warming in predictable ways, the wolverine’s association with spring snow enables us to evaluate the extent to which their primary habitat may be affected by global climate change. Our preliminary results are rather alarming and suggest that if current trends continue, much of the wolverine’s potential range in the contiguous US will eventually become unsuitable for them.
During the winter of 2005-06, we initiated a radiotelemetry study of the wolverine in the North Cascades in Washington; ours is the first wolverine study ever conducted in the Pacific states. Wolverines are extremely difficult to study in this area due to their low densities and limited access into the unroaded wilderness areas where they live. However, the development of satellite-based radiotelemetry, which enables wildlife researchers to monitor animals remotely via an internet-based data-collection system, has greatly enhanced our ability to conduct field research on wolverines.
We trap wolverines during the winter in areas that are accessible by snowmobile using wooden box traps that provide a warm and secure place for captured animals to await the arrival of our field crew. If severe weather prevents us from reaching the trap within the next day, captured wolverines can escape from the trap by chewing their way out!
To date, we have captured and monitored the movements of five wolverines (3 males and 2 females). Our study animals occupied areas along the Cascade crest from the Canadian border south to Lake Chelan, including the southeastern portion of North Cascades National Park.
In February 2007, we were particularly excited to discover that “Melanie,” one of our female study animals, was pregnant, because it represented the first documented evidence of reproduction by wolverines in Washington. We initially captured Melanie in 2006, but her collar malfunctioned after one week. After recapturing her in 2007, we were able to track her movements for four months before the battery in her satellite collar ran out. Although we were unable to recapture her in 2008, she was detected at a remote-camera station (identified by her uniquely colored ear tags).
Our preliminary results indicate that both male and female home ranges in Washington are quite large (>500 mi2). One interesting pattern we’ve observed is the tendency for wolverines to travel along a roughly circular or “figure-eight” route within their home range over the course of 10 to 12 days, often returning very close to their starting point. Such routes may reflect the most efficient way to mark and patrol their territories or to locate ungulate carcasses.
We plan to continue our research for at least three more years, and will be expanding the geographic scope of our study area into British Columbia in collaboration with wolverine biologists at the BC Ministry of Environment. We are also currently working with volunteers at Conservation Northwest to conduct remote-camera surveys for wolverines along the western edge of North Cascades National Park to evaluate opportunities for expanding our study area to the west. Studies like these help bring us closer to understanding the mysterious wolverine.