Polar bears do not lay eggs; they are mammals that give birth to live young. This reproductive method is fundamentally different from oviparous animals, such as birds or reptiles. The offspring develops internally within the mother’s womb. This strategy is linked directly to the extended parental care necessary for survival in the harsh Arctic environment, timing the birth to the most survivable conditions.
Mammalian Identity
Polar bears (Ursus maritimus) belong to the class Mammalia, which defines their reproductive characteristics. Mammals are distinguished by features like having fur, being warm-blooded, and feeding their young with milk. Unlike egg-laying (oviparous) species, polar bears are viviparous, giving birth to live offspring. Fetal development occurs entirely within the mother, requiring significant physiological investment.
Sustaining this internal development requires substantial energy, especially since the mother must fast during late pregnancy and the initial nursing period. This prolonged internal gestation and subsequent maternal care ensures the cubs are born with a higher degree of initial development. Though still extremely vulnerable, this preparation helps them face the icy environment.
The Reproductive Process
Mating typically occurs on the sea ice between late March and May. A male tracks a female using scent trails and may stay with her for several days. Following fertilization, the egg does not immediately begin to develop in the uterus, a phenomenon known as delayed implantation or embryonic diapause.
This delay allows the female to continue hunting and accumulating the massive fat reserves necessary to survive the denning period. The early embryo remains in suspended animation for several months. Implantation occurs only if the female has gained sufficient body mass, usually around September or October. Once implanted, gestation lasts approximately four months. Pregnant females enter a maternity den in late October or November to give birth in a protected environment.
Rearing the Cubs
Cubs are born inside the maternity den between November and January, when the mother is in a state of dormancy. Newborn polar bears are tiny and extremely helpless, weighing less than a kilogram and measuring about 30 centimeters. They are born blind, deaf, and covered only in sparse fur, making them entirely dependent on the mother’s body heat and milk.
The mother fasts throughout the entire denning period, relying on stored fat reserves to produce extremely rich milk, which can be over 30% fat. A typical litter consists of one or two cubs, and the family remains inside the insulated den for about four to five months. During this time, the cubs grow rapidly, opening their eyes within the first month and developing a thicker coat of fur. By the time they emerge from the den in late March or April, they weigh around 10 to 15 kilograms. The cubs stay with their mother for an extended period, typically two to three years, learning essential survival and hunting skills before achieving independence.