Estimated Age Structure of Wolves in Northeastern Minnesota

Information about population age structures is useful to understand survival rates, longevity, and population turnover. However, little such information is available about wolf (Canis lupus) populations. Mech (1970) estimated age structures of wolf-population age structure from pup:adult ratios applying various demographic assumptions, but no direct information has been published to test his estimate. Mech et al. (1998) aged 94 live wolves darted in Denali National Park and Preserve, Alaska, USA, but the estimates of wolf ages were based on educated guesses because no technique was available for aging live wolves. Since then, Gipson et al. (2000) published criteria for aging live wolves based on tooth wear of known-aged animals. This method allows data to be gathered from populations of live animals, which can facilitate examination of the age structure of populations. My objective was to use this method to estimate the age structure of a population of wolves in the Superior National Forest (SNF) of northeastern Minnesota, USA. The study area encompassed 2,060 km 2 of wilderness immediately east of Ely in the east-central SNF (488N, 928W) of Minnesota and was an arbitrarily defined portion of a much larger region, contiguous to Canada, all of which historically supported wolves without interruption (Sten-lund 1955). Topography varied from large stretches of swamps to rocky ridges, with elevation ranging from 325– 700 m above sea level. Winter temperatures ,À358C were not unusual, and snow depths (usually from about mid-Nov through mid-Apr) generally ranged from 50–75 cm on the level. Temperatures in summer rarely exceeded 358C. Conifers predominated in the forest overstory, with the following species present: jack pine (Pinus banksiana), white pine (P. strobus), red pine (P. resinosa), black spruce (Picea mariana), white spruce (P. glauca), balsam fir (Abies balsamea), northern white cedar (Thuja occidentalis), and tamarack (Larix laricina). However, as a result of extensive cutting and fires, much of the conifer was interspersed with large stands of paper birch (Betula papyrifera) and aspen (Populus tremuloides). Detailed descriptions of the forest vegetation were presented by Ohmann and Ream (1969). Wolves in the study area fed primarily on white-tailed deer (Odocoileus virginianus), moose (Alces alces), and beavers (Castor canadensis; Frenzel 1974). The wolves have been legally protected since 1974, although some have occasionally been killed accidentally or illegally by humans (Mech 1977). The wolf population in the study area has remained relatively stable since about 1975 and since 1990 has numbered about 55 6 …