Effects of supplemental food on weights and reproductive success of black bears in northeastern Minnesota

Mansfield, Susan A.
Jon Atwood, PhD
Department of Environmental Studies
This study examines growth and maturation of wild black bears (Ursus americanus) with and without access to supplemental food in northeastern Minnesota. In the study area with supplemental food, homeowners have fed bears since 1968. Despite the availability of supplemental foods, bears continued to spend most of their time foraging for wild foods, caring for cubs, maintaining territories, exploring new areas, and searching for mates. Use of supplemental foods was influenced by competing activities (searching for mates, establishing and defending territories, etc.), social factors, and scarcity of wild foods. Bears foraged preferentially on wild foods, and no bears used supplemental foods to the exclusion of wild foods. Bears with access to supplemental food provided multiple weights per individual, revealing significant seasonal changes in weight gain and loss throughout the non-denning period. Despite having access to supplemental food, mature males, young males, and nulliparous females lost weight during the mating season, whith mature males losing, on average, 25% of their weight in 45 days. Weights increased rapidly after mating season with the ripening of wild berries and the onset of hyperphagia. Grouping weights into 2-week periods showed that subadults and adults reached their highest rate of weight gain during 6-19 Aug, while cubs and yearlings gained weight most rapidly during 20 Aug-2 Sep. Comparing weights of bears with access to supplemental food during 2003-2006 with weights of bears without such access in a nearby study area during 1969-1983 revealed that supplemental food extended weight gains beyond the period when natural food was abundant, leading to heavier body weight, earlier maturation, and increased cub survival. Cubs with access to supplemental food were twice as heavy by mid-summer as their entirely wild-fed counterparts, and this difference persisted or increased through at least 3 years of age. Females with access to supplemental food produced first litters, on average, at 3.4 yrs, compared to 6.3 yrs for entirely weld-fed females, but litter size was not significantly larger-2.5 cubs per litter with supplemental food and 2.4 cubs per litter without supplemental food. Cub survival was significantly higher among bears with access to supplemental food (91%) than those without (75%). Several bears with access to supplemental food allowed behavioral observations that provided unprecedented insights into courtship and mating behavior, seasonal food choices, daily activity patterns, and family dynamics.