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Winter Severity and Wolf Predation on
a Formerly Wolf-free Elk Herd


We conducted our study from 17 March to 15 April 1997 and from 2 to 31 March 1998. These are periods when prey generally are in their poorest nutritional condition and thus most vulnerable to wolf predation (Mech and Frenzel 1971, Mech 1977, DelGiudice 1998, Mech et al. 1998).

Daily, we attempted to locate each pack. Three teams (1 per pack) of 2 people each were assigned to find and observe the wolf packs between dawn and dusk each day with radiotracking equipment and spotting scopes. When weather permitted, a fixed-wing aircraft was used daily to locate and observe the 3 study packs. If wolves were seen hunting elk, we attempted to monitor the hunts and determine outcomes. The distances between ground observers and wolves varied from about 200 to 2,000 m. Carcass locations were recorded on 1/24,000-scale USGS maps. Four other YNP packs were located from aircraft as often as time and weather allowed, but the only data we used from these wolves involved kill rates.

After it appeared that a pack had abandoned a kill, we attempted to determine whether the wolves were scavenging or had made the kill (from fresh blood and tracks; Mech et al. 1998:35). Probable kills were defined as carcasses for which such evidence was not quite as certain. The crews also checked each carcass for species, age (calf or adult), and sex; estimated amount eaten; and collected bone marrow and mandibles. Kill rates and amounts eaten were estimated based on the following assumed weights (kg) for elk: cow, 226; yearling, 165; calf, 103; and bull, 266 (K. M. Murphy, National Park Service, unpublished data). Femur marrow was assayed to determine fat content (Neiland 1970). Prey age was estimated by examining annulations in incisiform teeth (Matson's Laboratory, Milltown, Montana, USA).

The 1997 winter was 1 of the severest on record with a Winter Severity Index (WSI) of –2.6 on a scale of –4.0 to +4.0. The 1998 winter was relatively mild with a WSI of +2.9 (P. Farnes, Snowcap Hydrology, personal communication). Winter severity in 1997 was exacerbated by freezing rain on 1 January that limited ungulates from accessing forage until spring thaw.

The data for much of our study represented complete counts or nearly complete counts rather than random samples. For example, we located nearly all of the kills that each of our 3 main study packs made during our study. Thus, any differences we observed are real and not a result of sampling error.

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