Across most of their range, pregnant female polar bears excavate dens in snow and ice in early winter (Harington 1968; Lentfer and Hensel 1980; Ramsay and Stirling 1990; Amstrup and Gardner 1994). They give birth in those dens during midwinter (Kostyan 1954; Harington 1968; Ramsay and Dunbrack 1986) (see section on reproduction), and emerge from dens when cubs are approximately 3 months old. Because neonates are so altricial, the period of denning is essential to their early survival. Recognizing it as a critical phase in the polar bear life cycle, scientists have devoted much attention to aspects of maternal denning.