Polar Bears

Otherwise solitary creatures, breeding season is the only time when adult bears choose to interact. The reproductive season of polar bears typically begins in April and ends in June each year.

After fertilisation occurs, egg implantation doesn't happen immediately. The process waits until the mother bear's body is fully prepared to get through not only winter hibernation but also pregnancy, birthing and rearing her young.

When she has sufficient weight on her body, she's usually good to go. This generally happens at the end of September in a process known as delayed implantation. 

Mothers carry their young for around three months, giving birth in December or January. Although litters can range from one cub to four, typically the polar bear litter will consist of a set of twins weighing just under 24.5 ounces each.

The young are reared entirely by their mothers who continue to nurse them until they're 18 to 30 months old and weaning occurs. Cubs will then stay at their mother's side until they turn two or three years old and can fend for themselves without her protection. 

Polar bears are classified as marine mammals and it is only once they become pregnant that adult females will come ashore in order to den. However, as sea-ice is their principle habitat, they are quick to return to the water once their cubs are able.

The Beaufort and Chukchi Seas are notable as areas in which pregnant females choose to den and give birth on drifting pack ice.

You'll find the highest concentrations of polar bears along the shorelines, where currents and upwellings in the shallow waters provide richer food sources and prevent the build-up of solid ice during winter.

As they typically feed off marine animals, polar bear populations move with the sea-ice and are highly affected by seasonal changes in sea-ice cover.

In winter the sea-ice extends as much as 400km south of the Bering Strait, which separates Asia from North America, and polar bears extend their range to the southernmost extreme of the ice.

Polar bears are currently classified as vulnerable. Anthropogenic climate change is the primary threat to the species for several reasons. As the summer ice melts earlier and refreezes later, bears are forced onto land, or in open water, for extended periods, limiting their opportunities for hunting their primary prey of seals.

Female polar bears require large fat stores to sustain their pregnancy and produce milk. When females are underweight they give birth to fewer, smaller cubs, or even none at all.

Juvenile bears with lower fat reserves and higher energy demands are first to be affected by starvation during longer ice-free periods.

The Union for Conservation of Nature (IUCN) estimates a potential population decline of over 30% of polar bears by 2050, with studies indicating that in high-emission scenarios, nearly all high-Arctic polar bears could face functional extinction by 2100.