Lactation, hibernation, and mass dynamics of American black bears and grizzly bears

Milk composition and intake, cub growth, and mass loss of hibernating lactating and nonlactating American black bears (Ursus americanus) and grizzly bears (Ursus arctos horribilis) were investigated. Except for ash content, milk composition was similar between species. Lipid content varied only slightly throughout lactation, whereas carbohydrate content increased from 1 to 3% during hibernation before decreasing to less than 0.5% at the end of lactation. Protein and dry matter content increased from 6.6 +/- 0.4 and 29.8 +/- 3.9% during hibernation to 13.7 +/- 1.1 and 34.4 +/- 3.7% post hibernation, respectively. The ash content of black bear milk increased from 1% during hibernation to 2% after den emergence, but the ash content of grizzly bear milk (1.3 +/- 0.1%) did not fluctuate. Mean milk intake and growth during hibernation were 185 +/- 89 and 49 +/- 9 g/day for black bear cubs and 353 +/- 54 and 98 +/- 22 g/day for grizzly bear cubs, respectively, which accounted for about 9% of the estimated yearly milk consumption by the cubs. Milk intake peaked during the summer at levels approximately 4 times higher than those occurring in the winter den. The mass lost by older, hibernating, nonlactating bears was proportional to their metabolic body mass and was almost exclusively lipid. The rate of mass loss by denning, lactating females relative to nonlactating bears was 45% higher for black bears and 95% higher for grizzly bears. The less costly black bear cub may be at an important competitive advantage when both species occupy nutritionally limited habitat.