A polar bear mom rests while her two cubs play

In the late fall or early winter, polar bear moms give birth to one to three tiny cubs, nursing and caring for them until they are strong enough to leave the den three to four months later. The denning period is considered the most vulnerable time in a polar bear’s life.

© Meril Darees/Polar Bears International

1/26/2021 3:57:52 PM

Top 5 Favorite Mom and Cub Facts

By Alysa McCall, Staff Scientist

As I write this, my own little cub tears around the house, demanding water and snacks and attention. She’s up and down and up and down and up … I am tired but at the same time can recognize how privileged I am. At my fingertips are all the conveniences of modern life to help me raise my daughter—from grocery stores and indoor heating to Facetiming with Nana. I can offer books or toys, keep our home safe, and turn on Baby Shark to distract for a desperately needed minute. Plus, so much coffee.

Polar bear moms have no such support. Not even close.

All moms work hard, but polar bears are undoubtedly some of the hardest working single moms. Here are five of my favorite facts about polar bear families: 

1. Polar bear moms have one of the longest fasting periods in the animal kingdom. 

Polar bear mom with young cub

Photo copyright Shannon Curtis/Polar Bears International.

  • After feeding all winter, polar bears in seasonal ice areas like Hudson Bay come ashore in the summer.
  • Females who mated in the spring and are fat enough to sustain a pregnancy build and enter a maternity den in the fall, which is soon hidden under drifts of snow. While in the den, the mother bears don’t eat or drink. Instead, they live off their body fat.
  • In the late fall or early winter, they give birth to one to three tiny and helpless cubs, nursing them until they are strong enough to leave the den three to four months later. The denning period is considered the most vulnerable time in a polar bear’s life.
  • Once the cubs are strong enough to withstand the rigors of the Arctic outside the den, the family will head to the sea ice to hunt seals right away because mama is hungry: by now she’s gone eight months without a meal!

2. Polar bear cubs grow incredibly rapidly for the first few years of life.

A tiny polar bear cub follows mom

Photo copyright Meril Darees/Polar Bears International.

  • Newborn cubs are only ~0.6 kg (1.3 lbs.) when born. By 3 months old, they may weigh ~10–12 kg (22-26 lbs.), growing approximately 20 times their original body weight in 12 weeks. If newborn humans did this, we’d need adult-size bassinets.
  • Cubs continue growing rapidly, more than doubling their weight between den emergence and their first birthday, and yet again between their first and second birthdays.
  • By 2 years old, male cubs can be as big as their moms and weigh hundreds of pounds.
  • The mother bear’s rich milk is a significant contributor to the rapid growth of cubs but comes at a significant cost to mom.

3. Polar bear milk is the fattest of any land mammal's.

A mother with two cubs in a snow den.

Photo copyright Dr. Steven Amstrup/Polar Bears International.

  • Polar bear milk is about 31% fat when cubs are born, providing enough calories to help cubs grow rapidly.
  • The mother’s milk changes fat content and composition as cubs get older and nurse less, becoming closer to 18% fat by the time the cubs are a year old.
  • Although polar bear moms may nurse cubs through their second birthday, some females wean their cubs sometime after their first birthday. It may depend on the body condition of the mother—nursing cubs is extremely costly from an energy standpoint. Either way, mom helps make sure her cubs get enough to eat! 

4. Cubs have to learn all about being a polar bear in just over two years.

A polar bear mom and cub on the sea ice
Photo copyright Kt Miller/Polar Bears International.

  • Cubs stay with their moms for just over two years, learning as much as possible including how to navigate sea ice, when and where to migrate, how to hunt seals, how to avoid danger, and how to use their innate curiosity to learn new things.
  • Young cubs listen very well to their mothers, sometimes even mimicking mom’s movements exactly. As the cubs get older, some listen better than others.
  • Moms must make decisions in the best interest of the family, balancing the best places to hunt seals with keeping cubs safe from dangers like adult males or drowning in frigid waters.
  • When cubs are weaned, they are considered subadults (from 2.3 years to 5 years old) and have to put their newly acquired skills to the test.

5. Female polar bears start a new family about every three years.

A mom and two cubs on the sea ice
Photo copyright Dick and Val Beck/Polar Bears International.

  • From about the age of 5, female polar bears mate and produce cubs approximately every three years depending on multiple factors.
  • If female bears reproduce consistently until their late 20s, that’s more than eight litters of cubs at one to three cubs each. One female polar bear could potentially produce over a dozen cubs in her lifetime, though not all cubs will make it into adulthood or reproduce themselves.
  • As soon as moms wean their cubs, their days of being solitary are short lived. Very soon it’s mating season (after which the males leave and are never seen again).
  • After mating, females must binge on seals to store as much fat as possible in order to start the cycle all over again. 

Make sure you join us for International Polar Bear Day on February 27th to celebrate even more amazing things about polar bear families! Learn how you can help protect moms and cubs by donating to help us develop a new tool for finding, and hence protecting, polar bear dens hidden under the snow.

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