By Betsy Robinson and Steve Gehman, from Yellowstone Science, Vol. 6, No. 3, Summer 1998
Wolverines (Gulo gulo) are among the least-studied and most poorly understood fur-bearing animals in North America. This largest terrestrial member of the weasel family (Mustelidae) is renowned for its ferocity in story and legend, but indeed only two scienctific studies of wolverines have been conducted in the lower 48 states. Hornocker and Hash (1981) conducted a seven-year study of wolverines in northwestern Montana during the 1970s, and Copeland (1996) studied wolverines in central Idaho from the winter of 1992-93 through 1995.
Wolverines may never have been numerous, but their numbers and distribution have been drastically reduced in the lower 48 states since the arrival of European humans. Outside of Alaska, the largest wolverine populations in the United States are thought to be in Montana and Idaho, with sightings also reported in Wyoming, Colorado, California, Oregon, and Washington. Montana and Alaska are the only states that still allow wolverines to be legally trapped. Currently, an average of eight wolverines are trapped in Montana each year.
Information about the historic and present abundance and distribution of wolverines in and around Yellowstone National Park is scant. Schullery and Whittlesey (1992) documented 12 reports of wolverine sightings between 1806 and 1883, and noted three additional statements about wolverine presence. Consolo Murphy and Meagher (in press) searched park records from 1883 through 1995 for evidence from in and around the park and found 104 sightings, 25 track reports, 4 additional records, and 1 museum specimen. However, records were often lacking in the detail necessary to evaluate their reliability and accuracy. They concluded that there was a likelihood that Yellowstone National park helped support a resident wolverine population and that more information was needed on this rare carnivore's status and distribution.
Natural History of the "Skunk Bear"
Wolverines are known as "skunk bears" because of physical features and behavioral characteristics that remind people of both skunks and bears: light stripes that often extend from the face down the sides of the wolverine; a habit of marking carcasses on which they are feeding with musk or urine; a stocky, low-slung body and broad head; incredible olfactory abilities; and scavenging habits. Wolverines weigh between 30 and 60 pounds, possess long, sharp claws that allow them to dig through deep, frozen snow, and have extremely powerful jaws that can crush frozen bones.
During winter, wolverines are known to visit avalanche chutes where unwary bighorn sheep, mountain goats, elk or moose might have been caught and buried by a snow slide. In the winter of 1993-1994, Steve discovered just such a scene in upper Cache Creek. A bull elk had been buried in several feet of snow by an avalanche. Two wolverines used their acute sense of smell to locate the carcass, their long claws to excavate the frozen animal, and their powerful jaws to gradually consume it. By the time Steve happened on the scene all that was left was the elk's skull and piles of hits hair, along with a series of trails made by the wolverines as they visited the carcass over a period of days or perhaps weeks. Wolverines feed almost exclusively on carcasses during the winter, but are omnivorous the rest of the year, consuming berries, insect larvae, bird eggs, and even porcupines.
Wolverines seem to require true wilderness, and in sizeable chunks. Some of the animals in Copeland's study had home ranges of 770 square miles, and wandered up to 125 miles while dispersing. There figures put wolverines in the company of other wide-ranging carnivores such as grizzly bears and wolves. Male wolverines require larger home ranges than females, and often a single male's range overlaps with the ranges of several females.
One adaptation that helps wolverines cover such large ranges in winter is the large size of their paws in relation to their body. All members of the weasel family have five toes, compared to four toes for the canids and felids. In addition, the wolverine has a distinctive chevron-shaped interdigital pad. The wolverine tracks that we have found in the Yellowstone area measured 4 to 4 ½ inches wide.
Wolverines have an unusual reproductive strategy that is shared by other mutelids as well as bears. In all cases, the animals mate in spring or early summer, the egg is fertilized and develops for a short time, and then development is suspended for many months. In the case of the wolverine, the period of suspended development may last for almost a year before the fetus implants in the mother's uterus and development continues. Once that happens, gestation lasts approximately a month. Young are born in February or March and stay with their mother through the summer. Evidence from Copeland's study suggests that extended family groups may stay together even longer. He found wolverines visiting den sites of animals believed to be cousins, and gathers seemingly sharing parental duties. Perhaps the most surprising discovery of the study is that wolverines are not loners, as once was assumed.
The Yellowstone Study: Slowly Accumulating Evidence
For the past five winters we have searched for wolverines and other carnivores on the northern range of Yellowstone National Park and the adjacent Shoshone and Gallatin National Forests. Our work has focused on determining the presence/absence of a number of medium-sized mammalian carnivores: weasels, pine martens, fishers, river otters, wolverines, bobcats, mountain lions, lynx, foxes, coyotes, and wolves in various locations and habitats across the northern ranges. In particular, we have been interested in determining the extent to which the three rarest of these carnivores (fisher, wolverine, and lynx) are present in the northern portion of the ecosystem.
The northern Yellowstone carnivore study was begun by Sue Consolo Murphy at Yellowstone National Park's Center for Resources using hair-snagging devices, then expanded by Dr. Robert Crabtree of Yellowstone Ecosystem Studies, under whom we have been conducting the study. We have employed three methods in searching for our target species: hair-snares, remote camera stations, and snow-track transects.
Hair snares consisted of barbed wire spirals surrounded and encased by cylindrical tubes of wire mesh. Snares were placed under fallen trees, root systems, or dense branches to minimize snow accumulation on them, and were baited with small amounts of ungulate flesh, fish, or processed sardines. Commercial trapping lure was applied to vegetation near each site to lure animals.
Each remote camera station consisted of a Trailmaster Infrared Game Monitoring System, a bait package, and an application of trapping lure. The caera was triggered when an animal broke the infrared beam position under the suspended bait. The system was capable of daytime and nighttime photographs, and recorded the date and time of all animals visits.
We used two types of transects to collect carnivore track data. Detection or reconnaissance surveys were conducted primarily to cover as much distance as possible in areas of suspected high-quality habitat. Enumeration surveys were conducted to document all carnivore tracks observed while following predetermined transect routes, so that track densities could be compared among habitat categories and among years.
Results of Our Search Efforts
Wolverines or their tracks were detected 19 times during the first five winters of the survey efforts. No confirmed wolverine hairs were collected during the 2,668 nights of hair-snare operations at 42 sites. However, Consolo Murphy did collect a wolverine guard hair from a snare located on the north slope of Mt. Washburn during the winter of 1989-90. Wolverine was one of six carnivore species to visit our 55 camera stations. During 2,600 total nights of camera operation, wolverines made two visits and were photographed eight times. Most of our wolverine data were obtained from track observations. Wolverine tracks were observed five times during 140 snow transects that covered 403 miles (648 km), and an additional 12 times during other aspects of the project.
Our first wolverine photos were obtained during the winter of 1993-94 in Cinnabar Basin, approximately 3 miles north of the park. We had received several reports of wolverine activity in that area, and decided that it would be a good place to test our skill at using the camera system. After 51 nights of camera operation, a wolverine showed up and took two photographs of itself. This incident taught us a valuable lesson about the level of patience required to obtain data on these wide-ranging animals. In January 1997, we obtained a second set of photographs of a wolverine, taken south of Cooke City approximately 3.7 miles (6km) from the park boundary. Betsy discovered the tracks while making her way to check a remote camera, and followed them into the camera site. The camera system indicated that the wolverine had investigated the site at 10:30 a.m. the previous day.
In December 1997 we shifted our carnivore survey efforts from Yellowstone's northern range to the northwestern corner of the park and nearby portions of the Gallatin and Madison ranges. Between December 3, 1997 and March 18, 1998, we used 10 camera systems at 20 sites, and conducted 53 track transects covering 155 miles (250 km).
A ski trip along the Specimen Creek drainage in February, 1998, yielded some exciting and mysterious findings. After discovering fresh tracks made by a group of four or five wolves in the lower reaches of the drainage, we skied on and found tracks of a wandering wolverine that intersected the trail three times in a 1.2-mile (2 km) segment. Upon reaching our camera station located approximately 4.3 miles (7km) up the trail, we immediately noticed that something was amiss. All that remained of the of the infrared transmitter unit was its back plate and nylon strap that held it to a tree. The main body of the transmitter had been broken off, though four stout screws had originally attached the back of the unit to the main body. There were no human tracks in the vicinity of the camera site, and all other components of the camera station remained undisturbed. What animals could possibly have done the damage? Perhaps a moose or elk kicked it. But then the transmitter body should have been lying nearby in the snow; we searched the area thoroughly, digging down through the top 16 inches (40cm) of snow, but found nothing. Perhaps the wolverine whose tracks we saw earlier in the day was the culprit; its tracks came within 0.3 mile (0.5km) of the camera site that day; maybe it had visited the site two or three weeks earlier when the damage was done. Unfortunately the camera system was malfunctioning at the time of the incident and no photographs were obtained of the destructive animal. We will never know what happened, but we place our bets on the skunk bear.
We documented an additional three wolverine track sets during our efforts this past winter: as second set in the Specimen Creek drainage, and two sets in the Gallatin National Forest within 22 miles (35 km) of the northwest corner of Yellowstone National Park.
Since mid-1995 the park has received 19 reports of 24 wolverine sightings and two additional reports of tracks, bringing the total to 164 observation records - only slightly more than one for each year of the park's history.
The Future of Wolverines in Greater Yellowstone
Wolverines have the potential to be important indicators of ecosystem health and integrity. We know that, like other carnivores, they have been affected by human activities. Their historic numbers and distribution were drastically reduced, probably as a result of some combination of factors such as decimation of prey populations, widespread predator control programs, and habitat alteration and fragmentation. Recently, Copeland documented a wolverine abandoning her den in response to a skier traveling through a mountain bowl where her den was located, indicating the wolverine's vulnerability to human presence.
With increasing pressure being placed upon wildland habitats by recreationists, industry, and land developers, the potential for further impacts to wolverine populations is significant. In August 1994, the U.S. Fish and Wildlife Service was petitioned by several environmental groups to list the wolverine as a threatened or endangered species under the Endangered Species Act. Ironically, the petition was refused in April 1995 on the grounds that not enough information existed regarding their current distribution and population status. In order to use the wolverine as an indicator species, we must first develop a more complete database on its abundance and distribution. Our goal is to assist in the compilation of such a database; we hope to spend the next ten winters helping to survey the Yellowstone Ecosystem for wolverines, as well as for fishers and lynx. The more we learn about these rare carnivores, the better we will be able to protect them and their habitat for long-term survival in greater Yellowstone.