Fauna Series No. 7

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

RESULTS—THE TIMBER WOLF AND ITS ECOLOGY

Species of Lesser Significance

Isle Royale wolves influence species other than moose to a much lesser degree. Some of these are less important prey animals and others are scavengers. Probably none has a great effect on the wolf, but a consideration of wolf ecology must recognize these potential and actual relationships insofar as they are known.

BEAVER

The Isle Royale beaver population apparently reached its peak from 1945 to 1950 (page 19), and in the early 1950's a sharp decline was noticed by park residents. Today beavers are common, but compared to peak numbers, the population appears low. Since wolf sign first was noticed about the time the decrease occurred, a cause and effect relationship was postulated by residents of the island.

Although beavers have not been reported as primary food for any wolf population, they supplement the diets of wolves in many areas. Beaver remains occurred in 7 percent of 420 scats from the Rocky Mountain national parks of Canada (Cowan, 1947) and composed 10.5 percent of the occurrences in 76 wolf scats from Ontario (Peterson, 1955). On Isle Royale, beavers are the second most important wolf food (table 10); beaver remains composed over 10 percent of the occurrences in 438 scats collected during the present study. The data are segregated by year and compared with previous records from Isle Royale, in table 19.

TABLE 19.—OCCURRENCE OF BEAVER REMAINS COLLECTED IN WOLF SCATS ON ISLE ROYALE

The manner in which wolves hunt beavers is unknown, but possibly they follow shorelines until a beaver is found on land. Such an animal should be easy prey. In autumn and early spring, when beavers are most active on land, they probably are most vulnerable. For much of winter the animals are safe, but under certain conditions a few may be taken as early as March. During thaws, ice sometimes cracks along docks and islands, leaving crevices which beavers could enlarge. In 1961 when thaws occurred in late February and early March, much beaver activity was evident on March 3. Holes in the ice, trails to trees (some 100 feet from the holes) were seen along streams, islands, and docks, and in two of these places, wolves had killed beavers.

Figure 98—Beaver.

The first beaver kill was found near the northeast end of Washington Island. A trail led toward shore from a hole in the ice near a dock, and a few feet out from shore was a large blood-spattered area packed with wolf tracks. There was no sign of moose or moose remains in the vicinity. A nearby wolf was chewing what appeared to be an everted beaver hide. Two wolves were leaving the island, and four others rested about 2 miles away. We could not land and verify our aerial observations, but from the sign and the unusual behavior of the wolves, there was no doubt that they had killed a beaver.

On the same day, we found remains of a beaver killed by the pack of three near a small island in Tobin Harbor. The wolves had investigated two beaver houses situated against the island and had found trails leading from them to some fresh cuttings. A few feet from one trail there was a large bloody area covered with wolf tracks. Beaver fur was scattered about, and a well-chewed skull lay nearby.

The only literature encountered concerning predation on beaver in winter involved coyotes. Packard (1940) found tracks of a beaver that had emerged from under the ice and traveled half a mile before being killed by three coyotes. On Isle Royale, "Mr. Skadburg related the details of one incident in which coyotes killed a beaver that had been feeding upon a fallen birch from a hole in the ice" (Cole, 1952b). Extensive fieldwork in late March and April probably would produce more observations of predation on beaver.

Ever since wolves arrived on Isle Royale, they undoubtedly have been killing beavers, but whether wolves are directly responsible for the beaver decline is unknown. Before the wolf population was established, many colonies had been abandoned (Krefting, 1963), and supplies of aspen, the preferred food, in the southwest and central sections of the island were judged low (Krefting, 1951). There is a reserve of white birch on most of the island even today, and beavers certainly eat birch, but whether they can thrive on it has not been determined.

T h e following statements by Cowan (1947:169) indicate that, at least in his study area, wolves were not primarily responsible for a beaver decline:

In 1930 the Athabaska Valley of Jasper Park was superlative beaver range and bore a very heavy population despite the abundance of predatory animals. The removal of trees by these beavers at the time when the elk and moose were eliminating all seedling trees has had the inevitable result of rendering large areas of the park unsuitable for beaver.

There is little doubt that wolves and other predators are effective in reducing a beaver population that has eaten itself out.

I have seen no evidence that predators can prevent a beaver population from increasing until it is so large that safely available food becomes inadequate.

Figure 99—Snowshoe hare.

On Isle Royale, beavers that had depleted their aspen supply probably moved to new sites or traveled long distances to obtain aspen, increasingly exposing them to predation. In such a case, predation might be the immediate cause of a beaver decline, but the basic cause would be the shortage of preferred food. Undoubtedly any adverse conditions undergone by a beaver population will result in increased predation; the important distinction to make is whether wolves are primary or secondary causes of the decline. Little work on this aspect was done during the present study, but research is now underway on the ecology of Isle Royale beavers. It should furnish basic information from which a more definite statement can be made about wolf-beaver relations.

SNOWSHOE HARE

The Isle Royale hare population conceivably is a potential food source for the wolves. In British Columbia, Stanwell-Fletcher (1942:139) found that when the snow crusted ". . . snowshoe rabbits became once more a part of the wolf diet." Murie (1944:58) maintained that "if hares were plentiful they would probably supplement the food supply of wolves considerably." However, it appears that, on Isle Royale, hares are eaten only incidentally and that even a high population hardly would affect the wolf.

Isle Royale hare numbers increased considerably during the present study, as evidenced by increased percentages of hare remains in fox scats, reports from island residents, and observations of hares and tracks (table 20). Even so, the percentage of occurrence of hare remains in 438 wolf scats was only 3.1 percent, and no increase was evident from 1958 to 1960. The low representation of hare is at variance with findings by Cole (1952a) that hare remains composed 24 percent of the winter food of Isle Royale wolves, based on 87 scats. However, since coyotes still were present in 1952, possibly some scats were from coyotes, for Thompson (1952) demonstrated a great overlap in scat sizes of these species. The present figures do agree with statistics by Cowan (1947) and Thompson (1952), who reported that hare remains composed 7 percent of 420 scats from the Rocky Mountain national parks of Canada, and 5 percent of 435 scats from Wisconsin, respectively.

TABLE 20.—EVIDENCE OF INCREASE IN HARE NUMBERS

a Seen from aircraft during moose census or observations of wolves hunting.

Observations of wolf-hare encounters also indicate that hares are not important to Isle Royale wolves. On February 6, 1961, the pack of 15 accidentally flushed three hares but paid no attention. Even a lone wolf, hunting moose, showed no interest in three hares it flushed, on February 3, 1961. In one of these instances, a hare circled counterclockwise and passed in front of the wolf; 20 seconds later the wolf crossed its trail without investigating.

To substitute enough hares for the average of 10 to 13 pounds of moose consumed daily, each wolf would have to take two or three a day, which might be an arduous task. Wolves probably have more difficulty catching hares than do foxes (which feed regularly on them), because the latter can follow hares into thicker cover and smaller hiding places, and over deep snowdrifts. It probably is more efficient for wolves to concentrate on moose.

RED FOX

Foxes seem to be common on Isle Royale but not abundant. A strong limitation on numbers probably is winter food supply, for mice are relatively unavailable, and at least during the first year of the study, hares were scarce. Information on the diet of the fox was obtained by analysis of 295 scats collected from trails during three summers (table 21). Probably the sample was biased toward winter, for summer scats containing berries disintegrate quickly. Nevertheless, the dependence of foxes on hares, at least during winter, is evident. Since the hare population is growing, perhaps fox numbers also will increase.

TABLE 21.—ANALYSIS OF FOOD REMAINS IN 295 FOX SCATS COLLECTED FROM TRAILS

It is interesting that the most foxes observed from the air per day (10, on February 27) were seen in 1961. This is six more than the maximum per day seen during the first two winters. Although complete figures were not kept on number of animals observed per study period, until 1961, the highest number (29) definitely was seen in 1961. However, conclusions on population changes should not be based on observations alone, for biases are many. Most foxes are seen at moose remains on lakes, bays, and shores, where they are most apparent from the air. Variations in amount of time spent over any of these locations could produce widely different totals of animals seen. Since foxes were studied only incidentally, no evidence was obtained that allows absolute, or even relative, estimates of population size.

Figure 100—Red fox. Photo by Don Murray.

The most important relationship between the fox and the wolf involves the food gleaned by foxes from remains of wolf-killed moose. Although moose remains composed only 6 percent of the occurrences in the fox scats collected, a higher percentage of the winter diet may consist of moose. Scats on trails (the only places where scats were collected) would be left primarily by traveling animals, but many foxes apparently remain near moose kills for days; most of their scats would not be represented in the collection. During winter, foxes feed on almost every kill soon after the wolves leave, and on one occasion four animals were seen feeding on a carcass.

Although wolves generally benefit foxes, they sometimes kill them. Twice, foxes were seen to flee instantly upon sensing wolves. Another time, on March 15, 1960, I watched a wolf kill a fox. About 5:05 p.m. the large pack was heading through a spruce swamp about a mile southwest of Halloran Lake when suddenly the lead animal sprang toward a running fox 125 yards away. As the wolf passed a moose carcass, from which the animal had run, a second fox scurried off. Within about 15 yards the wolf caught the fox and shook it violently. It then carried the limp carcass under some trees. Half an hour later, I found that the wolf had ripped out the intestines of the fox and abandoned the animal, at least temporarily. The next day the carcass was gone; it may have been eaten or just carried back under the trees.

A contrasting observation was made on the day after the fox was killed. While the wolves fed on the moose carcass, a fox lay curled up about 100 feet away, apparently fast asleep. On another occasion, a fox ventured to within 100 feet of a lone wolf feeding on a moose carcass. Cole (1957) twice saw foxes closely approach wolves near moose carcasses, and found that "they seemed to have little fear of the larger animals when abundant supplies of food existed nearby." Murie (1944) also reported instances in which foxes showed no fear of wolves. He concluded, in his Mount McKinley study, that "the relationship between the wolf and the fox seems to be one of mutual gain." Foxes secure food from wolf kills, and wolves enlarge fox dens for their own use.

On Isle Royale six fox pups were produced in 1961 in a den which certainly was large enough for a wolf den (page 71 and figure 60). In the previous 2 years it was vacant, and its original occupants remain unknown. This was the only den found, so nothing is known about this aspect of wolf-fox relationships on Isle Royale.

RAVEN

A peculiar relationship exists between the large pack of wolves and a flock of ravens. At every fresh kill, ravens perched in nearby trees, waiting to feed. The instant the wolves finished eating, these scavengers alighted on the carcass. Apparently kill remains provide the primary winter food for Isle Royale ravens.

Probably because these birds are so dependent on the wolves, small flocks regularly accompany the animals during their travels. They fly ahead of the pack, perch in trees until the wolves pass, and then "leap frog" them again. Frequently, I even saw ravens tracking wolves. A bird so engaged flies directly over a string of tracks. Upon discovering a wolf scat, it thoroughly picks this apart and presumably swallows all edible portions, and then continues along the trail. I cannot remember any raven backtracking, so perhaps the birds deliberately track the wolves to overtake them and only feast on scats incidentally.

Once while the wolves attacked a moose, the ravens swirled around them excitedly. After the wolves wounded the moose, one bird sat in a tree and cawed as they tried to make the moose run. Sometimes the scavengers joined wolves in eating bloody snow.

Wolves and ravens often seem to play together, especially when the wolves rest on the ice, fully gorged (figure 101). The following account of activity noted on March 5, 1961, includes the range of "playful" behavior witnessed between wolves and ravens. As the pack traveled across a harbor, a few wolves lingered to rest, and four or five accompanying ravens began to pester them. The birds would dive at a wolf's head or tail, and the wolf would duck and then leap at them. Sometimes the ravens chased the wolves, flying just above their heads, and once, a raven waddled to a resting wolf, pecked its tail, and jumped aside as the wolf snapped at it. When the wolf retaliated by stalking the raven, the bird allowed it within a foot before arising. Then it landed a few feet beyond the wolf, and repeated the prank.

Figure 101—Ravens about to tease some resting wolves.

Crisler (1958) who observed similar activity from the ground, described it as follows:

He [a raven] let the pups trot to within six feet of him, then rose and settled a few feet away to await them again. He played this raven tag for ten minutes at a time. If the wolves ever tired of it, he sat squawking till they came over to him again.

Although Isle Royale wolves almost caught teasing ravens several times, I never saw them succeed; neither were raven remains found in any of the 438 wolf scats analyzed. Therefore, it appears that either the ravens are thoroughly familiar with the wolf's capabilities, or the wolves do not seriously attempt to capture the ravens.

COYOTE

Coyotes were present on Isle Royale from the early 1900's, but by February 1957, very few remained (page 19). Since no coyote or coyote sign was found during the present study, undoubtedly the species has been extirpated from Isle Royale. The cause of this is unknown, but the wolf may be responsible. During a study of British Columbia fauna, Munro (1947) recorded a report from a native who had found remains of a coyote killed and eaten by wolves. The man believed that where wolves invade an area, they drive the coyotes out. Minnesota wardens also discovered a coyote killed by wolves (Stenlund, 1955:46): "The male coyote had run onto the lake from the woods and was immediately killed by the wolves [three] which were running on the ice." Since coyotes and wolves are closely related and since wolves are strongly territorial, it is not unlikely that on a limited range, such as Isle Royale, wolves would chase, and probably kill, every coyote encountered.

Last Modified: Thurs, Jul 4 2002 10:00:00 pm PDT
http://www.cr.nps.gov/history/online_books/fauna7/fauna5l.htm