1. How Much Do Polar Bear Cubs Weigh at Birth and by Spring?
According to records, the weight at birth is 454–680 g, or about 0.6 kg [2]. A 500 ml water bottle weighs about 0.5 kg, so a cub is close to that weight. From late March to April, when they leave the den, the cubs are said to weigh 10–15 kg [2]. Five 2-liter bottles weigh 10 kg. Going from about 0.6 kg to about 12 kg is a 20-fold increase. Note that 10–15 kg is the weight when leaving the den, not at age one. So, what mechanism allows this growth in the snow? First, let us look at the place where they grow.
2. How the Snow Den Stays Near 0°C Even When It Is -40°C Outside
The den is a small room made of snow. Records show a size of about 2 m long, 1.5 m wide, and 1 m high [2]. The U.S. Fish and Wildlife Service states that even if it is -40°C outside, the den stays around 0°C [5]. A study from the University of Toronto reported dens being about 20°C warmer than outside [3]. Snow contains a lot of air and does not conduct heat well, which may be one reason the den stays warm. However, 0°C is a rough guide, and temperatures vary by den. In this protected room, what does the mother eat?
3. Why the Mother Bear's Milk Is So Fatty Even Though She Barely Eats
While staying in the den, the mother bear is said to eat almost nothing but water [1]. However, this may not be true for every individual. Still, the milk is very rich. Polar bear milk has an average fat content of about 33%, which is close to the milk of marine mammals like seals [2]. A study of wild bears found about 27 g of fat per 100 ml of milk [4]. The difference in numbers may be due to different methods, timing, or individuals. The mother’s weight may drop by about half during denning [3]. This is a rough estimate with individual differences. Once the cub grows, do they leave immediately?
4. How Long Mother Bears and Cubs Stay Near the Den After Leaving It
In Svalbard, Norway, a study used cameras and satellite collars to watch mother bears with cubs leave their dens. The average first emergence was around March 9 [6]. However, they do not go far right away. They stayed near the den for an average of 12 days (range 2–31 days) [7]. On warmer days, they stayed out longer [7]. Families that stayed near the den longer were associated with better cub survival, though this does not prove the longer stay was the cause [3]. This time may help the cub get used to the outside world. During this time, the mother is still not foraging. This study was in Svalbard; dates may differ in other regions. When can the cub hunt?
5. How Cubs Learn to Hunt by Copying Their Mother for About 2.5 Years
Cubs live with their mothers for about 2.5 years (28–30 months) while drinking milk [8]. During this time, cubs watch the mother hunt and copy her to learn [10]. Until about age one, they rarely succeed. Even at 1–2 years, they spend only a small part of their time hunting [10]. Trying next to the mother, even when failing, may be part of practice. How does sea ice change affect this growth?
6. How Three Months on Land Was Linked to Lower Milk Energy in Western Hudson Bay
Mother bears hunt seals on sea ice. Less sea ice may mean more time on land. A study in Western Hudson Bay found that for mothers who spent about 3 months on land and kept nursing, milk energy dropped by about 50% for mothers with one cub, and over 75% for mothers with multiple cubs [8]. In the same region, it is said that fewer than half of cubs survive to adulthood [9]. These numbers are specific to this region. Researchers keep studying milk and behavior to understand conditions that support cub rearing, which may help conservation.
7. How to Compare a Cub's Birth Weight and Spring Weight Using Water Bottles
Hold one 500 ml water bottle in your hands. This is close to a cub’s birth weight. Then, place five 2-liter bottles on the floor. Do not lift them; just look. That is about 10 kg, the weight when leaving the den. The bottles do not move, but cubs walk around energetically. For more reading, see articles from Polar Bears International [6] [8] or USGS papers [9]. Safety Note: For safety, do not lift five 2-liter bottles together.