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Northern Prairie Wildlife Research Center

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Patterns of Prey Selection by Wolves
in Denali National Park, Alaska


We aerially located radio-tagged wolves to find their kills, and examined the remains from the ground. Wolves were captured primarily by darting from a helicopter and were fitted with radio collars. Collared wolves were located an average of three times per month with antennae-equipped PA-18 Supercub and Cessna 185 aircraft. Not all packs were located during each flight, and some packs were located more frequently than others. Wolves were located more often in late winter and spring than during late autumn and early winter; summer data were especially sparse.

Kill sites were examined from the ground to determine the species, cause of death, age, sex, and condition of prey. The position of the carcass, presence of blood, signs of a struggle, and manner of feeding provided clues as to whether wolves killed or merely scavenged a particular carcass. Sex was determined by the presence of antlers or antler pedicels (moose), antler size (caribou), horn shape and size (sheep), mandible, metatarsus, or metacarpus length (caribou and sheep), or pelvis shape (all species). Teeth, usually incisors, were collected for age determination by sectioning and counting cementum annuli (Matson's Laboratory, Milltown, Montana). Age of younger animals was determined by the pattern of tooth eruption. Sheep were aged by annular rings on the horns and by tooth eruption and tooth sectioning. Marrow (usually femur marrow) was collected, dried, and weighed to determine percent fat content (Neiland 1970). The reported fat percentage is probably higher than actual because some specimens were not retrieved from kills until long after the animal had died.

All bones found at kill sites were examined for abnormalities, and certain bones were collected, if available, for subsequent cleaning and examination.

Because of the disparity in sizes of the wolf's prey, intermittent sampling of wolf locations would result in a bias toward locating the largest prey, which provide more food and occupy wolves longer. Therefore, we used two approaches to analyzing the kill data for determining the proportions of moose, caribou, and sheep killed. First we examined the observed data directly. Second, we assumed that the time wolves spent on a given age, sex, and species of prey was directly proportional to the weight of that class of prey, so the relative proportions of kill classes were adjusted accordingly. The observed number of each age, sex, and species of prey was multiplied by the reciprocal of the assumed weight of each prey class to arrive at an adjusted proportion of each kill class. For example, assuming bull moose weighed four times as much as caribou cows, we multiplied the number of caribou cows by one and bull moose by one quarter. Sample sizes of prey remains data vary for each type of data because not all types of data could be collected from every kill or prey carcass found. We performed two analyses of the monthly kill proportions because of strong bias during summer against finding calf kills and disproportionate sampling of certain packs. One year-round analysis excluded calves, and one analysis for October through April included calves.

To determine if any class of prey was killed disproportionately in any given month, we used the Chi² test to compare monthly proportions of each prey class against the mean of the monthly proportions of the year-round sample. We performed this test for adults only and adult and calf samples. Significance for a given month was assumed if P≤0.05 for that month. For comparing age structures, we used the Kolmogorov-Smirnov test (Hollander and Wolfe 1973).

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