Without Fur at -40ºC: The Hard Life of a Polar Bear Cub

Spanish version here

Consider donating to Polar Bears International on this World Polar Bear Day


Weighing just over half a kilo, Inuka was born with hardly any hair, a condition that makes him extremely delicate. From the nest, his eyes will remain shut for the first month of life, ignoring the beauty of the Northern Lights that illuminates the permanent night of the Arctic winter. Inuka will not be able to move on his own during this period, making him easy prey for any predator walking by if it wasn't for Ursula, his mother.

Figure 1. The Northern Lights illuminate the permanent night of the Arctic winter. ©Madison Stevens / Polar Bears International

Ursula is a female polar bear and a very conscientious mom. Months before, Ursula built a large nest in a safe environment with the right conditions to raise her baby. The nest is a complex construction; a tunnel two meters long gives access to a chamber of three-square meters that connects with two rooms. The main room has been built on a level above the entrance, allowing to retain the heat inside. The construction is surprisingly efficient, maintaining a constant temperature around -5ºC, more bearable than the -40º outside. However, this temperature is still too low for the cub, so Ursula must keep Inuka on his lap for the next few months to provide him with the necessary heat. Inuka must multiply his weight by ten before venturing outside. He will put on that weight in just three months thanks to a tranquil lifestyle based on sleeping and ingesting his mother's milk which has a high-fat content.

Figure 2. Inside a polar bear's nest. ©Ian Stirling/ Polar Bears International

During nesting, Ursula will face the longest fasting period in the animal kingdom, up to eight months without ingesting any food, while offering warmth and protection to his cub until spring comes. But her duty is not over yet; once Inuka is ready to withstand the outdoor conditions, Ursula will leave the nest and dedicate the two following years to teaching Inuka to cope with one of the most hostile environments on the planet. She will show him to locate prey, hunt, and seek refuge. Ursula will guide the cub through the unstable ice terrain forming and melting under their feet and won't hesitate to defend him against any threat, including adult males crossing their path.

Figure 3. Female polar bear defending her cubs against an adult male. ©Mike Lockhart/ Polar Bears International
Female polar bears undergo the longest fasting period in the animal kingdom, up to 8 months without a single meal

After nearly three years of care and training for the harsh conditions, Inuka and Ursula are ready to say goodbye and follow their own paths. Inuka already masters the art of hunting and will become a large male of more than 500 kg and future head of his family. On the other hand, Ursula will start again the delicate breeding process that will only complete if the conditions allow.

Figure 4. Polar bear with her cubs. ©Shannon Curtis/ Polar Bears International

This true story is based on data taken from Polar Bears International scientists (PBI). This organization is dedicated to studying the ecology and threats of polar bears, which has closely followed Ursula for the past four years. This polar bear represents resilience and dedication. Polar bear females are extremely vulnerable during the breeding process, and not all manage to complete it successfully like Ursula. Experts warn that the high breeding mortality of up to 50% is causing a decline in polar bear populations linked to two key factors. On the one hand, the lack of appropriate physical conditions to complete the breeding cycle, on the other the scarcity of safe areas for maternal nesting.

Figure 5. The cubs will remain with their mother learning to survive until they become independent at the age of three. ©Simon Gee/ Polar Bears International

Fit to breed

The physical condition is linked to access to enough food sources. Polar bears depend on sea ice to find and hunt their prey. They have to maximize the season when ice covers enough areas of the Arctic to hunt seals and eat as much as possible before the ice melts in spring when they have to return to the mainland and will not be able to find food for several months again. The alteration in ice formation caused by climate change is forcing Ursula to wait a month longer for favorable conditions than her grandparents did, which means that Ursula's fasting period lasts 30 days more than her ancestors. Increasing fasting periods are causing a progressive loss of body condition and fitness in all populations. In the extreme life conditions of the Arctic, the reproductive success of polar bears is related to their ability to enter burrows with sufficient fat reserves to perform their functions. Therefore, the current trend in body condition loss will reduce the chances of successful reproduction in polar bears. We already see these consequences.

Figure 6. Polar bear with her cubs. ©Steven C Amstrup/ Polar Bears International

Protective nests

Nests are the focus of interest on this International Polar Bear Day. Experts warn that the reduction of Arctic ice combined with the expansion of human activities threatens key denning sites for polar bears. During the sensitive nesting period, any disturbance usually causes the nest's abandonment and the death of the cubs. Mothers and cubs are particularly at risk during the maturation period due to the industry's expansion into the Arctic. New studies have shown that the tool to detect polar bear nests used by the oil and gas industry misses 55% of them, putting a large percentage of mothers and offspring at serious risk. Dr. Steven C. Amstrup is the Research Director of PBI. He emphasizes the magnitude of these findings: "The survival of every single cub is vitally important and is why we must protect denning sites." Dr. Amstrup warns that most polar bear populations will disappear in the next 80 years if greenhouse gas emissions and pressure on breeding areas do not change. Krista Wright, Executive Director of PBI, adds that, while improvements are being made to mitigate climate change, denning moms and cubs must be protected if we want our next generations to know this iconic animal, "A future that's good for people is good for polar bears too." Says Krista, since the threats for polar bears also threaten the health of our planet.

Figure 7. Cubs with mom polar bear. ©Simon Gee/ Polar Bears International
"A future that's good for people is good for polar bears too." Krista Wright
Consider donating to Polar Bears International on this World Polar Bear Day


1. Blix AS, Lentfer JW. Modes of thermal protection in polar bear cubs--at birth and on emergence from the den. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. 1979 Jan 1;236(1):R67–74.

2. Pagano AM, Williams TM. Physiological consequences of Arctic sea ice loss on large marine carnivores: unique responses by polar bears and narwhals. Journal of Experimental Biology [Internet]. 2021 Feb 15 [cited 2021 Feb 25];224(Suppl 1). Available from: https://jeb.biologists.org/content/224/Suppl_1/jeb228049

3. Hunter CM, Caswell H, Runge MC, Regehr EV, Amstrup SC, Stirling I. Climate change threatens polar bear populations: a stochastic demographic analysis. Ecology. 2010;91(10):2883–97.

4. Smith PA, Elliott KH, Gaston AJ, Gilchrist HG. Has early ice clearance increased predation on breeding birds by polar bears? Polar Biology. 2010;33(8):1149–53.

Ursula is the real name of the tracked female by PBI. However the name Inuka has been made up for the story. In loving memory of Inuka the beloved polar bear from Singapore Zoo

Cover picture ©Steven C Amstrup/ Polar Bears International

#PolarBearDay #SaveOurSeaIce #ProtectMomsAndCubs #TalkAboutIt #EnergyShift #ClimateHope