Spring is a season of plenty for polar bears, a time when plump seal pups make easy prey. It’s also mating season, a period when female polar bears pair up with males on the sea ice for courtships lasting up to two weeks. After mating, the female maintains the fertilized eggs, but they don’t begin to grow until six months later, if at all. The eggs implant in the womb and begin growing into cubs only if the female bear is healthy, with a high level of fat reserves—a process called delayed implantation. 

Digging the den

The eggs implant in autumn, as the day shorten and temperatures drop. This is when pregnant female polar bears throughout the Arctic begin migrating to den locations to give birth to their cubs. While some give birth in snow dens on the sea ice, the majority den onshore, where they dig into snow drifts (or sometimes peat moss or earth along river banks) to build shelters. Some are experienced, having denned numerous times, while others partake in this activity for the first time. It’s not something they’ve ever seen or learned from their mothers; the instructions are written entirely into their DNA. This is the midway point in the reproductive cycle for a pregnant female polar bear.

Females most often dig dens into snow drifts that accumulate on the leeward side of hills and valleys near the coast. In areas like Western Hudson Bay, some dens are dug into the ground, and multiple females may reuse the same den in successive years. By analyzing the age of black spruce trees found growing in polar bear dens, scientists have been able to date some of these structures as more than 200 years old! Still, most dens are dug into the snow, even in southern populations, where snow accumulation begins later in the year. After the pregnant bear enters the den, falling snow completely covers the den opening, hiding it from view.

Born small and helpless

In January, only a couple of months after entering the den, mothers give birth to cubs weighing only 500 grams, approximately 1/400th the size of their mom. Compared to other mammals, polar bear cubs exhibit some of the smallest sizes at birth relative to their mom. The cubs are born quite helpless, unable to see or hear with only one thin layer of fur to protect them from the cold. Maternity dens provide critical shelter to the fragile new-borns, protecting them from the harsh Arctic environment. Heat generated by the mother can keep the temperature inside the den 25°C (45° F) warmer than outside. The structure of the den itself is also designed with thermoregulation in mind—a small opening and a narrow downward-sloping tunnel prevents warm air from escaping.

Over the next three months, the mother will nurse the cubs. Her milk is over 30% fat, enabling them to quickly gain approximately 10 to 15 kilograms. Around mid-March, the new family emerges from their den – the cubs seeing sunlight for the first time. For about a week, they will remain in and around the den, and through exploration and play, the cubs will develop their physical strength and abilities. These skills will be critical for their upcoming migration onto the sea ice. In some northern populations where sea ice is present year-round, females may build their dens on the ice, close to their seal prey. However, inland dens can be as far as 100 kilometers from the nearest shore.

Departing from the den

From late March to June, ringed seals and bearded seals give birth to their pups and nurse them directly on the sea ice and in subnivean lairs under the snow. During this period, the seals are most accessible, and thus vulnerable, to predation by polar bears—a welcome feast for polar bear moms who have been living off their fat reserves while in the den. The goal for these moms is to minimize the time and energy spent growing cubs while ensuring they are strong enough to survive on the sea ice in time for seal pupping.

If the bears give birth too early, they will have to sustain the cubs for longer before venturing out on the sea ice, potentially running the risk of not having enough energy to survive until the seal pupping season. On the other hand, having cubs too late risks missing the peak pupping period and having insufficient energy to survive the summer. The survival of the cubs depends not only on successful hunting on the sea ice, but also on the critical timing of egg implantation, denning, and birth. This whole process is timed perfectly to match peak seal pupping.

Increased environmental variability due to climate change may render the timing of seal pupping less predictable, potentially leading to a mismatch in den emergence and peak prey availability. Decreases or delays in snowfall may make building protective dens harder, drawing out this process later into the season. The factors associated with climate change have resulted, in part, in a significant decrease in on-ice denning in the Beaufort Sea polar bears from historical numbers. As sea ice extent continues to decline and retreat further north, the amount of suitable habitat for on-ice maternity dens will likely decrease, thereby risking the persistence of this unique ecotype. Other threats to the denning process include the expansion of industrial activity in the Arctic, putting mothers and cubs at risk during this sensitive period in their life cycle.

Ron Togunov is a PhD student in zoology at the University of British Columbia, where he is studying the foraging behavior of polar bears and narwhals in relation to environmental factors such as sea ice concentration, ice drift, and wind speeds.