Fauna Series No. 7

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

DYNAMICS OF WOLF-MOOSE COACTION

Maintenance of a Healthy Herd

An obvious result of intensive predation on Isle Royale moose is the elimination of heavily parasitized, diseased, old, or otherwise inferior individuals. Since 14 of 36 wolf-killed adults (39 percent) showed debilitating conditions even though only bones were examined from most, it seems safe to assume that every adult killed is either inferior or a victim of some circumstance predisposing it to predation. This becomes especially evident when one considers that the 15 to 16 wolves tested an average of 13 moose for each one they killed while under observation (page 144). If this ratio applies from November 1 to May 20, when a calculated 146 moose should be killed, approximately 1,898 moose would be tested in that period and undoubtedly many others are tested during summer. Since the wolves travel to every part of the island (figures 46, 47, and 3, showing the foot trails used by wolves in summer), they should detect any weak or inferior moose in a short time. Culling benefits any population, but it probably is especially important to Isle Royale's dense herd. It may even be the reason that such a high population has survived. Inferior animals undoubtedly use food less efficiently and reproduce less effectively, so in a herd crowding its environment, these animals would be least desirable.

Research in several other locations has shown that predation on big game exerts a culling effect. Murie's classic study (1944) of wolves and Dall sheep proved this beyond question in Mount McKinley National Park. Although evidence from other studies is not as conclusive, collectively it strongly supports the hypothesis. Hibben (1937) found that all of the 11 puma-killed deer he examined were either ill-proportioned, diseased, parasitized, or otherwise significantly abnormal compared to 74 hunter-killed deer. Cowan (1956) reported that on Vancouver Island, British Columbia, domestic dogs preyed primarily on malnourished and heavily parasitized deer. In the Canadian Northwest Territories, Banfield (1954) studied the hunting techniques of wolves in caribou country and concluded that weak or inferior caribou would be among those most likely to succumb to the wolves' methods. Other results similar to these (Cowan, 1947; Peterson, 1955; Crisler, 1956, 1958) already have been discussed, as has work by Stenlund (1955) and Burkholder (1959), who found no evidence of a culling effect. However, one should remember that seldom are intact carcasses of wolf-killed animals available for examination, and even if they were, the psychological or behavioral factors that might predispose big game to predation still would go undetected.

A heavily cropped herd composed of healthy animals with sufficient food should reproduce vigorously. Probably one of the most sensitive indicators of a moose population's reproductive abilities is the twinning rate. Pimlott (1959b) summarized results of many studies, including his own, and found wide variation (2 to 28 percent) in rates of twinning; he also discussed the effect of nutrition on reproduction and concluded that "variations in adult fecundity may be caused by a number of nutritional factors that differ from one range to another." In Alberta and British Columbia, Cowan (1950) found that elk on overgrazed range had a twinning rate of less than 1 percent, whereas herds in better nutritive condition had a rate of 25 percent.

At present, Isle Royale moose appear to have one of the highest twinning rates reported. Of 53 cows seen with calves in the summer of 1959, 20 were accompanied by twins, a rate of 38 percent (figure 105). (If only 25 different cows were seen, the 95 percent confidence limits would be 19 percent and 57 percent.) In 1960, which appeared to be a year of unusually low production, the twinning rate was 15 percent, on the basis of 47 observations of cows with calves. (The 95 percent confidence limits would be 1 percent and 29 percent if 25 different cows were observed.) In contrast, in 1929 when wolves were not present and moose overpopulated the island, Murie (1934) observed that only 1 of 45 cows with calves was accompanied by twins.

Figure 105—Cow and twin calves in early September.

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