The world’s largest bear starts life as a fluffy piece of butter

Polar bears are the world’s largest land carnivore, growing up to 10 feet long and weighing more than 800 pounds, but did you know they start out life no bigger than a stick of butter? With a birth weight of about 500 grams (17 ounces), polar bear cubs hatch about 1/400e as tall as a mother, but luckily the magic of polar parenting was caught on camera.

The den visualization images were recorded in captivity and provided by Polar Bears International zoo partners at Toledo Zoo & Aquarium. It shows the newborn polar bear cubs resting against their mother’s relatively massive snout as they rest for the big task of strengthening her little polar bear babies.

To learn more about how these little butter stick babies survive and – for the lucky ones – live long enough to grow to around 400 times their birth weight, IFLScience International marked Polar Bear Day by speaking with Alysa McCall, Polar Bears International Director of Conservation Outreach and staff scientist. It turns out that growing from a blind, toothless ball of fine fluff to the world’s largest land predator means growing up fat and fast.

Where do polar bear cubs come from?

Alysa McCall: A polar bear’s pregnancy only lasts about a few months. After a female successfully mates on the sea ice, the fertilized egg undergoes a small number of divisions and then enters a state known as delayed implantation.

The fertilized egg does not develop until the fall, around October, when the female’s body “decides” whether she has stored enough fat to produce cubs. If the answer is no, the fertilized egg is rejected or reabsorbed. If the answer is yes, the fertilized eggs will implant and cub growth begins, with the birth of the little cubs taking place just a few months later, around December.

How quickly do polar bear cubs develop?

BEN: The cubs grow quickly on their mother’s rich milk, which contains about 31 percent fat at birth – the fattiest milk found on land. When the young are three months old, they can weigh about 10 to 12 kilos, which means that in 12 weeks they have grown about 20 times their original body weight. If newborn humans did this, we would need adult cribs.

polar bear cubs with mother

“By the time male cubs are two years old, they can be as big as their mothers.”

Image credit: KT Miller / Polar Bears International

Cubs continue to grow rapidly, more than doubling their weight between leaving the den and their first birthday, and again between their first and second birthdays. By the age of two, male cubs can be as large as their mothers and weigh hundreds of pounds. The mother bear’s rich milk makes an important contribution to the rapid growth of cubs, but comes at a significant cost to the mother.

What are some of the toughest challenges polar bear cubs and mothers face?

BEN: Polar bear mothers have an incredibly tough job. They only have about 2.3 years to teach their cubs how to be successful polar bears, including how to find and hunt prey, how to navigate their changing sea ice habitat, how to avoid dangers like adult male polar bears avoidance, and seasonal migration patterns.

This becomes even more difficult when you consider how quickly their sea ice habitat is already changing in some parts of their range. Not to mention that mothers are constantly breastfeeding; Although their milk fat content decreases as the cubs grow older, breastfeeding is incredibly energetically expensive for the mother.

What questions does Polar Bears International hope to answer with their den research?

BEN: Cub survival rates can vary widely depending on region and annual sea ice conditions, but on average only about 50 percent of cubs survive the first year. In parts of the Arctic where more sea ice is being lost, cub survival rates are even lower.

It is crucial that we work to protect vulnerable mothers and cubs, which we do through various research projects. Better understanding the needs of sheltering families is critical, especially as human activities increase in the Arctic. Such knowledge can make a real difference to polar bear families – providing a scientific framework to inform policy and help ensure crucial protection.

Our long-term studies of den formation in Alaska and Svalbard contribute to our understanding of mothers and cubs during this sensitive period and provide data on their needs and vulnerabilities. One of our studies focused on how Denning families respond to disturbances – data directly related to oil and gas development and other human activities. The findings underscore the importance of adequate buffer zones to protect mothers and cubs from harm.

Another study found that current den surveys conducted by the oil and gas industry to locate and thus protect polar bear dens are missing more than half of known dens. The high failure rate has implications for policy decisions in sensitive denning areas such as the Arctic National Wildlife Refuge.

Our team is now investigating a promising new den detection method that could significantly improve our ability to locate dens under the snow, allowing shelters to be identified and protected to ensure the safety of mothers and cubs.

How can people support polar bear cubs on International Polar Bear Day?

BEN: Tune in to our events. Watch a video broadcast from our study team in Spitsbergen, Norway, and participate in a live question and answer session with staff in Churchill, Manitoba, Canada. Find the program and details here.

Donate to protect mothers and cubs. From better understanding the denning period to protecting mothers and cubs from disturbances, and tackling the overarching threat of climate change, your gift will help polar bear families.

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