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Effects of Food Supply and Kinship on Social Behavior Movements and Population Growth of Black Bears in Northeastern Minnesota USA

Lynn L. Rogers

Document Type

Article

Journal/Book Title/Conference

Wildlife Monographs

Volume

97

First Page

1

Last Page

72

Publication Date

1987

Abstract

Black bears (Ursus americanus) were studied in northeastern Minnesota from 1969 through 1985 to determine (1) the annual cycle of foraging and social behavior, (2) changes in mother-offspring relationships from birth through maturity, (3) the influence of food availability on social behavior, movements, and population growth, and (4) important habitat components. Studies were most intensive during 1971-76 when 28 males and 55 females were radio-tracked. The study area had relatively few fruit and mast species and a frost-free growing season of only 118 days. Crop failures were common and reduced the reproductive rate to less than half the biological potential. Females first reproduced at 4-8 years of age (average 6.3 years), depending upon food supply, and produced subsequent litters at 2-4-year intervals (average 2.28 years). Litters averaged 2.38 cubs. Cub survival was 59-88%, depending on food supply in the year of conception and the year of birth. Bears that supplemented their diets with garbage reproduced nearly twice as fast as the above. Density was 1 bear/4.1−6.3 k⁢m2, including cubs. The annual cycle of behavior was closely tied to the annual cycle of plant growth and fruiting. Bears emerged from dens in late March or April, depending in part on weather, but females and subadults remained lethargic until late April or early May when aspen (Populus spp.) catkins and young green vegetation became available. Mature females then re-established territories averaging 9.6 k⁢m2, and mature males used overlapping mating ranges averaging 75 k⁢m2. Mating began about 10 June and was essentially completed by 9 July when fruit began to ripen in abundance. Foraging then became the main activity until fruit and nuts disappeared in late summer. Sixty-seven percent of the males and 40% of the females foraged >7 km outside their territories or mating ranges in late summer or early fall, moving up to 200 km away before returning for denning. Adult males that remained in their mating ranges after the mating season spent a disproportionate amount of time in the buffer zones between female territories rather than inside them. Despite generally solitary habits, bears behaved in accordance with kinship theory within a social order governed mainly by the distribution and abundance of food. Mothers recognized their independent offspring and tolerated them in their territories. Mothers avoided areas where their yearlings concentrated their activities, thereby allowing them to have nearly exclusive feeding areas. Young males voluntarily dispersed as subadults and settled 13-219 km away (average 61 km). Young females usually expanded their yearling ranges into adult territories. Mothers shifted away as their daughters expanded their ranges, thereby aiding daughters in obtaining territories. Adults of both sexes deterred immigration. The bears behaved as described above where food was dispersed, but formed hierarchies where food was clumped as at garbage dumps. There, female territories overlapped slightly, and males commonly tolerated individual distances of <2 m. Play was common among bears up to 4.7 years old around garbage dumps but was not seen elsewhere except among cubs. Adult females used garbage dumps in their territories but avoided other dumps where unfamiliar adult males congregated. The denning period was 5-7 months. Very fat bears abandoned food and entered dens in late September. Less fat bears that found food continued to feed as late as early November. Bears in northeastern Minnesota achieved a deeper hibernation than has been reported from regions with shorter denning periods. Hibernating bears commonly did not awaken in midwinter until after several minutes of prodding. Overwinter mortality was <1% regardless of den type, but lightweight yearlings starved after emergence in spring. Starvation was the main cause of death among cubs and yearlings, and gunshot was the main cause of death among older bears. Bears commonly roamed farther in northeastern Minnesota than has been reported from other regions. A larger area of unbroken habitat may be needed to maintain this population than would be needed where food is more reliable and abundant. Habitat management practices beneficial to bears in northeastern Minnesota are identified.

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