Black Bear Females Give Birth and Lactate while Hibernating

Not only can female black bears sustain themselves during hibernation, but they can also provide for up to four bear cubs when they are pregnant. While black bears actually mate during springtime, the females wait to implant the embryo in their uterus until after they enter their den to hibernate (Ramsey and Dunbrack, 1986). This typically means that females will implant the embryo in their uterus in either November or December and then give birth in January, meaning they also have a short gestation period. Reproduction has high energy costs for females, since after birth she will nurse her young until they emerge from the den, usually around April (Harlow et al., 2002). The female nurses her cubs while she hibernates, allowing them to increase from birth weights between 250 and 400g to between 4 and 5 kg upon leaving the den (Nelson,1973). This size is much smaller than would be predicted for the cubs based on size of the mother, constituting only 1/3 to 1/10 of the predicted mass for black bear young (Ramsey and Dunbrack, 1986).

Three researchers holding the bear cubs. (Image courtesy of Dr. Dorcas).

Black Bear Milk is High in Fat and Protein

Black bears are able to nourish their cubs as they nourish themselves: through fat catabolism of cholesterol, triglycerides, and other molecules formed when they break down their body’s adipose tissue. The female bear’s milk contains high concentrations of not only fat, but also protein, which aid in the cub’s rapid growth. The milk is usually 25-33% fat, and on average 6-17% protein, much higher than human’s milk, which averages 3% and 0.2 to 0.7% respectively. Thus, females provide lots of nutrients for their cubs in their milk, which gives 5 Kcal/g on average (Nelson 1973).

Reproductive Females Prepare for Hibernation by Building Larger Fat Stores

Due to the energetic pressures on reproductive females, they have to take extra measures to prepare themselves to handle the stresses of undergoing winter sleep while giving birth and nursing their young. Studies have shown that females that are pregnant will enter hibernation with 50% more body mass than non-reproductive bears. Between 31 and 45% of their body mass is constituted by fat reserves, as opposed to 28% percent for females that are not pregnant. Reproductive bears, like other hibernating individuals, try to conserve protein, and hence use 1,216 kJ/day from fat and only 214 kJ/day from protein above the average non-reproductive female (Harlow et al., 2002). In mammals, fetuses require glucose that must be produced through gluconeogenesis in the mother, which utilizes body protein. In response to this, female bears have adapted to give birth sooner to limit the gestation period where they must feed the fetus in favor of feeding an undersized cub that they can use their fat stores to nourish (Ramsey and Dunbrack, 1986). This increases loss of fat in the mother, but conserves her body protein (see Fig 1.; Harlow et al., 2002).