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Reproduction and lifecycle
Cubs are born weighing approximately 1lb, and stay with the mother for another two years, until they are weaned, at which point they become independent of their mother. After the cubs become independent, the mother can then mate again. The gestation period for polar bears is a short few months, with cubs being born 7-8 months after fertilization. Traditionally, 2-3 cubs are born per litter, but due to decreasing food availability and added stressors related to climate change, it is becoming increasingly common for just 1 cub to be born.

Courtship and mating take place on the sea ice in April and May, when polar bears congregate in the best seal hunting areas.[106] A male may follow the tracks of a breeding female for 100 km (60 mi) or more, and after finding her engage in intense fighting with other males over mating rights, fights that often result in scars and broken teeth.[106] Partners stay together and mate repeatedly for an entire week, maximizing the probability of successful fertilization. During this time, unattached rival males may also attempt to mate with females, requiring the attached males to drive them off or fight them. If the rival males are successful, they may then also mate with the female, which in some cases leads to cubs born in the same litter having different fathers. The prolonged mating period allows for increased competition among males, with the larger and more experienced males being favoured, ultimately leading to a higher probability of cub survival. Due to the larger males having a competitive advantage, this has selected sexual dimorphism in polar bears, with males being 2-3 times larger than females.

Following mating, eggs undergo Embryonic diapause until August or September, at which time they then implant in the uterine wall. This reproductive strategy allows females an additional four months prior to embryonic growth, to accumulate and store the large amount of fat stores requires to support her pregnancy. During these four months, the pregnant female eats prodigious amounts of food, gaining at least 200 kg (440 lb) and often more than doubling her body weight. The cubs remain with their mother for approximately two years, during which they do not mate. This mothering period leads to roughly one-third of the female polar bears being available to mate every years, contributing to the intense competition among males for a mate.

With temperatures trending upwards in the arctic, the ice which the polar bears mate on is being reduced in area, resulting in a loss of habitat for the bears to mate and hunt on. Since the bears venture out onto the sea ice during their mating season and April and May, less sea ice leads to greater concentrations of mating bears, which potentially alters the population dynamics of the reproduction of the polar bears. Fragmentation of the ice that polar bears use to hunt and mate could lead to the isolation and eventual generation of distinct populations of polar bears, leading to genetic drift of the species. Since polar bears are solitary animals when they are not mating or with cubs, a reduction in available mates could also lead to a decline in polar bear numbers.