Large female polar bear on ice

Photo: BJ Kirschhoffer / Polar Bears International

Is She or Isn't She?

By Dr. Thea Bechshoft, Staff Scientist



05 May 2015

Polar Bear Questions

During the summer months, I sometimes join a tourist expedition that cruises into the icy waters north of Svalbard. Whenever we meet a polar bear on these trips, there are certain questions that come up repeatedly. One of these is how to tell if a female polar bear is pregnant.

Keep in mind that polar bears are large mammals and their cubs only weigh 500-800 grams (18-28 ounces) when they are born. Because of this, a polar bear's pregnancy isn't directly visible, even in late fall when the pregnant female is about to enter her maternity den to give birth. However, especially during the fall months there is one telltale sign to look for, a sign that can be seen even from a distance—and that is how fat the bear is (that is, what is her body condition?).

Pregnant polar bears need to eat as much as possible in the summer and fall months in order to build up the needed fat reserves to survive the denning period. Doubling or even tripling in weight before denning is completely normal for a pregnant polar bear! However, when you're out scouting for pregnant bears, remember that a young male and a slightly older female bear can sometimes be hard to distinguish from each other at a distance. In other words, although a fat bear observed in the fall is likely to be a pregnant female, it could also just be a young male bear with great seal hunting skills ....

Another way of telling if a polar bear is pregnant is to measure the level of pregnancy-related hormones in the bear's urine or feces (more or less the same method used in human pregnancy tests). However, polar bear pee and poop samples are not easy to gather in the wild, and so the method has until now only been used in zoos (blood samples obtained from sedated animals have, on a few occasions, been used instead for wild polar bears). One zoo even trained a dog to be able to tell if a female polar bear was pregnant or not by simply smelling the bear's poo!

The picture below by scientist Ian Stirling, taken in the mid-1980s, gives you some idea of how female polar bears can look while pregnant. The picture shows a pregnant female in exceedingly good body condition (scientists rarely encounter bears this fat in Western Hudson Bay anymore). This particular bear was so chubby that the researchers couldn't fit the normal satellite radio collar on her. After this photo was taken, scientists had to glue a smaller radio transmitter to the top of her head instead in order to be able to track her movements.

A mother bear must live off her stored body fat for up to an incredible eight months, using the energy from it to keep not only herself alive but also to produce milk for her cubs. It cannot be emphasized enough how important it is for the survival of polar bears as a species that especially the pregnant females have good hunting conditions and the chance to be in superb body condition before entering the maternity den.

For answers to more polar bear questions, follow Thea on Facebook and Twitter @BioThea. 

Pregnant female polar bear

Photo: Ian Stirling

A pregnant female in exceedingly good body condition (bears this fat are rarely encountered anymore in Western Hudson Bay).