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 maritimus) during the summer ice-free period in western Hudson Bay, a period of severe nutritional restriction. The transfer of milk between adult 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 significantly more milk energy (10.9 MJ∙day−1) than females with yearlings (2.6 MJ∙day−1). Daily milk production represented a greater proportion of body mass for females with cubs than females with yearlings but milk production was not correlated with maternal mass or litter mass in either group. Milk energy consumption was correlated with body mass in cubs but not in yearlings. Cubs consumed significantly more milk energy (7.8 MJ∙day−1), and displayed a higher relative growth rate, than yearlings (1.5 MJ∙day−1). However, both age groups lost proportionally the same amount of mass during the study period, suggesting that cubs are less able to survive nutritional restrictions and are more dependent on milk for survival than yearlings.