Surplus killing of white-tailed deer by wolves northcentral Minnesota

Abstract

During winters (November-May) of 1990-1991 to 1995-1996, I investigated factors that may predispose white-tailed deer (Odocoileus virginianus) to surplus and excessive killing by gray wolves (Canis lupus). Each year, 22-75 female deer were monitored with radiotelemetry (total = 172). Forty-one deer with radiocollars were killed by wolves; annual mortality rate in November-May ranged from 3.3% (1992-1993) to 22.7% (1995-1996). Surplus and excessive killing of deer by wolves was observed only during winter 1995-1996 (11 of 17 deer investigated). There was no difference in the number of days elapsed (3-4) between estimated dates of death and ground examination of the kill site during winters of 1990-1991 to 1994-1995 versus 1995-1996; median ages of wolf-killed deer were similar (9.8 versus 7.8 years). Median marrow fat in femurs was lower during winter 1995-1996 (8.0%) than during winters when surplus and excessive killing did not occur (84.1%). Winter 1995-1996 was historically severe with mean weekly snow depths ≥70 cm from late January to mid-March and ≥60 cm through early April. Evidence indicates that nutrition and extreme deterioration of condition of deer is the major link between winter severity (penetrability and depth of snow) and excessive killing by wolves, which may be predicted when snow depth exceeds 70 cm for 4-8 weeks.