The term 'negative foraging period' has been used to describe the time between the emergence of black bears (Ursus americanus) from their dens and the ripening of fleshy fruits in summer; the implication is that spring foods lack adequate energy for bears to gain or even maintain weight. Few biologists, however, have examined data on weight changes to determine if the concept applied to the bears they studied. We investigated age and sex-specific weight changes of black bears in northcentral Minnesota between denning and early July. Weights were obtained from bears trapped (n = 350) or handled at dens (n = 270) during 1981-89. Weights of 1- to 2-year-old males and females rose from late winter (Feb-Mar) to late spring (May-early Jun) and again from late spring to early summer (early Jun-mid-Jul), with gains averaging 8-14 kg over the period. Bones (head length) of young bears also grew during spring, although chest girth decreased, indicating loss of fat. Mean weights of 4- and 5-year-old males increased 21 and 10 kg, respectively, from winter to late spring, but decreased 20 and 25 kg between late spring and early summer. Only males ≥6 years old lost weight during both periods (x̄ = 16 kg). Behavioral and physiological changes associated with breeding, including increased testosterone, increased activity, and decreased feeding, probably explained most weight loss in breeding-age males. In females, most weight loss was associated with lactation. Lactating females either lost weight from winter to early summer (sequential weights of individuals) or showed no significant change (mean weights of pooled sample), whereas non-lactating adult females gained weight. Despite a spring diet that is generally low in carbohydrates and fats, bears apparently make efficient use of plant protein. Past studies, with small samples and data pooled among sex-age groups, did not recognize the importance of spring as a period of growth for young bears.
- Volume 10
- Issue
- Pages 521-531
- Publication Date 1 January 1998
- DOI
- File Size 368.32 KB
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