How Do Female Polar Bears Know When to Den?

October is upon us, and a select group of polar bears—pregnant females—have started to look for a place to create their maternity dens. These bears will spend the winter inside their snowy lair, giving birth to their cubs(s) around New Year’s, and emerging as a new family group into the Arctic world around March. Meanwhile, all other polar bears will be out on the Arctic sea ice, in the 24-hour darkness of the polar night, spotting seals and trying to pack on the pounds necessary to sustain themselves during the summer melt season. Wintertime is hunting time for polar bears—unless they’re pregnant females!

In discussing this on my Polar Bear Questions page, I received a follow-up question from Virginia: “Do all female polar bears (who don't already have cubs with them) go into a den in the fall? Does a female polar bear know if she is pregnant or not?”

I love this thoughtful question—thank you, Virginia! As the female polar bear does not eat during her time in the maternity den, overwintering inside the isolated lair means that she is in effect spending her winter fasting instead of hunting. This makes the decision to den a serious one. If she dens, she will lose a substantial amount of body weight but hopefully have healthy cubs to show for it. If she doesn’t den, she will have no cubs but instead time to hunt seals and hopefully build up that essential layer of body fat she must rely on during the leaner time that comes with the summer sea ice breakup.

So how does she decide? The fact is, she doesn’t—at least not in the way that we humans tend to understand the concept of choice. Instead, it is her body condition and her body’s hormones that make the decision for her.

Whether a lone female polar bear digs a maternity den and holes up to give birth is determined by her hormonal status, which is dependent on a number of factors. Was the springtime mating successful? Did the fertilized egg implant and start growing? And, importantly, has she been a successful enough hunter to maintain a good layer of body fat. even through the preceding lean summer months? This is essential, as hormones related to reproduction, such as progesterone, are closely tied to body condition or weight in many mammals. Thus, her weight is related to whether the pregnancy will be successful, while also being likely to determine whether she can produce enough fatty milk for her offspring, as well as how long she can stay in the den with her newborn cubs before hunger drives her to go in search of seals. In polar bears, studies in the Western Hudson Bay population indicate that a body weight of 189 kilograms in the fall, just prior to the start of the denning period, may approximate the threshold below which the females are no longer able to successfully reproduce.

A recent paper by Dr. John Whiteman, chief research scientist at Polar Bears International and assistant professor of evolutionary biology at Old Dominion University, further explores the question. His research on polar bears in the Southern Beaufort Sea population shows that raising cubs is a big commitment, and that females with reduced energy reserves may skip denning or abandon cubs—choices that could potentially influence the reproductive success of a population.

But back to the core of your question, Virginia!

What happens if a female polar bear’s pregnancy fails at any time between conception and birth? Her hormones will change back gradually to their non-pregnant state and so will her behavior. In other words, I wouldn’t say that a polar bear would “know” whether she was pregnant, but she would follow the physiological behavioral processes set in motion by her hormones. And if her hormones are telling her to dig a maternity den and hole up to give birth, she will. If not, she will instead spend the winter season on the sea ice like the other polar bears, hunting for seals.