Fauna Series No. 7

Fauna of the National Parks — No. 7 The Wolves of Isle Royale

SUMMARY

THE PRIMARY objective of this 3-year investigation was the appraisal of wolf-moose relationships in Isle Royale National Park, a 210-square-mile island in northwestern Lake Superior. The island has supported a moose herd since the early 1900's and a wolf population since about 1948; no other big game or large carnivore is present. The use of a light aircraft for counting moose and following wolves during 435 hours in February and March 1959—61 facilitated gathering the most significant information; field work during three springs and summers provided supplementary data. Sixty-five weeks were spent in the field.

The primary wolf pack, composed of 15 to 16 animals, was the same size each winter, as was a pack of 3. An additional pack, of two wolves, probably also was present each year. Copulation was observed in 1959 and 1960 in the large pack, but apparently no young survived to the following winters. Reasons for the lack of increase remain unknown. The large pack traveled over the entire island, but most of its activity occurred on about half the area. The small packs did not frequent this section but traveled extensively in the other half. Strife between the large pack and the other wolves was additional evidence of territoriality.

During 31 days, from February 4 to March 7, 1960, when the entire route of the large pack was known, the animals traveled approximately 277 miles, or 9 miles per day. However, during 22 of those days the wolves fed on kills and did not journey far. Thus in 9 days of traveling, the animals averaged 31 miles per day. The normal pace was about 5 m.p.h. During the entire study, the longest distance known to have been traveled in 24 hours was approximately 45 miles. On the basis of 25 observations, the maximum distance traveled between kills was 67 miles; the minimum, 0; and the average, 26.5.

The moose herd numbers about 600 in late winter. Probably most moose are host to the winter tick (Dermacentor albipictus), and a substantial number of older animals are infected with hydatid cysts (Echinococcus granulosus) and actinomycosis, all of which undoubtedly are important in predisposing moose to wolf predation. The tapeworm Taenia hydatigena and the lungworm Dictyocaulus sp. also are present in the herd, but the incidence of infection and the effect of these parasites are not known.

The primary moose-mortality factor is wolf predation, since the large pack alone killed an average of one moose per 3 days during the winter study periods. Average daily consumption per wolf, based on estimated weights of kills, ranged from 9.7 pounds in 1960 to 13.9 pounds in 1961. Individuals apparently ate as much as 20 pounds at a meal but sometimes went 5 days without food.

Special effort was made to observe hunts by the large pack, and in 68 hours 66 hunts involving 132 moose were witnessed. The pack actually tested 77 moose (held them at bay or chased them long distances) and killed only 6 of these, a "predation efficiency" of 7.8 percent. Running is the first defense of the moose, but if the wolves are not discovered soon enough, many moose stand and defy the pack. Of the 36 that stood at bay until the wolves gave up, none were killed, but 5 of the 41 that ran were dispatched. Defense of the calf is strong and stereotyped. The cow protects the rear of the calf, which seems to be the favorite point of attack. If any wolf closes in, the cow charges and sends it scurrying.

Nine hunts were observed in which moose were killed or wounded. To kill a moose, the wolves attack its rump and flanks. They cling to the animal and slow it down. Meanwhile, one wolf grabs the nose of the moose and occupies the animal's attention until the others inflict significant damage to the rump. Usually moose are killed within 10 minutes, but some wounded animals manage to hold off the wolves for several hours.

On the basis of the winter kill rate, annual adult moose mortality approximates 83 animals. About 17 percent of the moose herd is composed of yearlings in late winter, so the annual increment just before calving season should be 85 yearlings. Thus the herd is believed to be relatively stable. The total annual kill is calculated to be 142 calves and 83 adults. On the basis of consumption figures, it is estimated that approximately 5,823,300 pounds of browse are required annually to support the moose herd that produces the 89,425 pounds of moose consumed by about 1,512 pounds of wolves.

The wolves appear to have kept the moose herd within its food supply, culled out undesirable individuals, and stimulated reproduction. Wolves and moose probably will remain in dynamic equilibrium, although the moose herd may decline in the next decade because a large proportion of the browse is growing out of reach of the moose.

When possible, wolf kills were examined on the ground. Fifty-seven kills were found and 51 examined; 18 were calves, but most of the others were 8 to 15 years old. None was 1 to 6 years old. Only bones of most kills could be examined, but 39 percent of the adults showed symptoms of debilitating conditions. One of the two intact, wolf-killed adults examined harbored 57 golf-ball-sized hydatid cysts in its lungs, and the other 35.

Last Modified: Thurs, Jul 4 2002 10:00:00 pm PDT
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