Winter Dormancy in Black Bears in Southwestern Washington

Abstract

Despite relatively mild weather, black bears (Ursus americanus) in southwestern Washington entered dens and remained for an average of 126 days. Bears entered their dens during a 5-week period that began on 21 October. A significant difference (P < 0.05) was found among the average dates of den entrance of adult females, yearlings, and adult males. Adult females were the first bears to enter their dens; yearlings entered next, and adult males entered last. The stimulus to enter a den probably was provided by the cumulative effect of weather. The actual period of winter dormancy was preceded and followed by periods of increased inactivity. Inactivity during the predenning and postdenning periods was correlated with daily weather, principally maximum daily temperature and precipitation. J. WILDL. MANAGE. 40(3):408-415 Through most of their range, black bears enter dens and remain inactive during winter months; this trait apparently allows them to occupy areas where scarce winter food supplies might otherwise preclude their presence. The timing of entrance into the den and emergence from the den in the spring may be influenced by weather (Northcott and Elsey 1971) or physical condition (Spencer 1955). Erickson (1964) and Jonkel and Cowan (1971) presented data that suggest the timing of these events may differ among the various sex and age groups in a population. Because the black bear is managed almost entirely by hunter-oriented programs, a knowledge of the timing of winter dormancy is important. Also, because estimates of population parameters of the black bear frequently are derived from hunter-killed samples (Willey 1971, P-R Job Prog. Rep., Proj. W-38-R, Vermont Fish Game Dept., Montpelier, Mcllroy 1972), the knowledge of possible sources of bias such as disproportionate availability, which might be caused by differential entry and eme gence dates of the various sex and age groupings, is desirable. Published accounts of winter dormancy in the black bear, with few exceptions, have come from regions of their range characteriz d by climatically severe winters. Poelker and Hartwell (1973) presented the first documented accounts of winter dormancy of the black bear in southwestern Washington, an area of mild winters. They concluded, after following the movements of a subadult female, that the period of winter dormancy was nearly 3 months. The bear, however, had been aroused from dens on n ne occasions, which may have biased the accurate appraisal of both the intensity and duration of winter dormancy. Our objective is to report on the temporal aspect of winter do mancy and associated periods of inactivity of 13 black bears. S. D. Lindzey, R. E. Walker, A. K. Miles, and R. Libby assisted with field work. The Weyerhaeuser Company and Washington Department of Game provided permits necessary to conduct this study. J. M. Welch, Manager, Willapa National Wildlife Refuge, and H. Hardesty provided logistical support during the term of the study. H. C. Black, J. A. Crawford, and A. W. Erickson offered valuable criticism of the manuscript. The late H. M. Wight, Leader, Oregon Coopera1 Study conducted and supported under the auspices of the Oregon Cooperative Wildlife Research Unit: Oregon Department of Fish and Wildlife, Oregon State University, U.S. Fish and Wildlife Service, and The Wildlife Management Institute cooperating. Oregon State University Agricultural Experiment Station Technical Paper No. 4123. 408 J. Wildl. Manage. 40 (3):1976 This content downloaded from 157.55.39.17 on Wed, 31 Aug 2016 04:28:01 UTC All use subject to http://about.jstor.org/terms WINTER DORMANCY IN BLACK BEARS * Lindzey and Meslow 409 tive Wildlife Research Unit, provided stimulus and direction to our efforts.