The undeveloped, immature cubs — usually three but sometimes as many as four to a litter — arrive to the world early by necessity. Pregnant female bears in hibernation have only their own body fat to use as nourishment for the placenta that feeds their unborn young. If bear cubs were carried to what humans consider "full-term," the mother would emerge from hibernation completely depleted — a state that would serve neither her nor the cubs well. Instead, bears experience a shortened gestation period and give birth to premature young.
Fortunately, the protective environment of the den, coupled with the mother's rich, nourishing milk (it boasts a full 22 to 24 percent fat content!) will help the cubs to grow and mature to the point where they will be prepared to leave the den when spring arrives. They feed every few hours, guided to their food source by the heat radiating from their mother's nipples. The sounds of their contented humming fill the den — it is the sound of well-fed, well-cared for bear cubs.
This regimen of feeding will help the cubs develop a full coat of fur and the boisterous energy that will help them keep up with their mother once they leave the den. The mother bear will be in dire need of food come spring, after several long months hibernating and caring for her young without food — simply relying on her stores of fat from the fall.