The breeding results of Polar Bears in captivity are in general very disappointing. Unsuitable maternity dens are the main cause. These should be completely closed, very quiet without outside disturbances and should have connection with a run and thus with an outside enclosure. The female should be isolated in this den a month before birth and be provided with heavy bedding. She should not be disturbed before she comes out with her cubs in early spring. Electric heating in this den (floor or radiant) can be of advantage and in some cases means the difference between failure and success. The time of birth is November and December with a peak in the last decade of November (table 12) or one in the first decade of December (table 13). The number of young in captivity is mostly one and often two (table 12: 62 single, 47 twins born). The female is capable of giving birth at the age of 5 and can have cubs at least till 24 years of age. If the cubs are taken from the mother at about 15 months she can breed every two years, which is normal zoo practice.