MILK-PRODUCTION AND MILK CONSUMPTION IN POLAR BEARS DURING THE ICE-FREE PERIOD IN WESTERN HUDSON-BAY

Milk yield and milk consumption were measured in polar bears (Ursus ma ritimus) during the summer ice-free period in western Hudson Bay, a pe riod of severe nutritional restriction. The transfer of milk between a dult and dependent offspring was measured for five females with cubs ( aged 8 months) and four with yearlings (aged 20 months) by a hydrogen isotope dilution-transfer method. Females with cubs produced significa ntly more mill; energy (10.9 MJ.day(-1)) than females with yearlings ( 2.6 MJ.day(-1)). Daily milk production represented a greater proportio n of body mass for females with cubs than females with yearlings but m ilk production was not correlated with maternal mass or litter mass in either group. Milk energy consumption was correlated with body mass i n cubs but not in yearlings. Cubs consumed significantly more milk ene rgy (7.8 MJ.day(-1)), and displayed a higher relative growth rate, tha n yearlings (1.5 MJ.day(-1)). However, both age groups lost proportion ally the same amount of mass during the study period, suggesting that cubs are less able to survive nutritional restrictions and are more de pendent on milk for survival than yearlings.