Polar Bears: What Little We Know

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Chinook: Is she or isn't she?

Chinook: Is she or isn't she?

We are anxiously awaiting the sounds of polar bear cubs squawking, humming, and crying in Chinook’s den. But we really have very little idea of when that sound will reach us because we still have much to learn when it comes to polar bear reproduction. Like other bears, the polar bear exhibits reproductive characteristics that are perfectly tailored to their environment. Without these adaptations, reproduction in the extreme, often harsh, climate of the Arctic, would not be possible.

Delayed implantation is just one of the adaptations polar bears have developed to cope with the challenges of life in the Arctic. This adaptation make predicting just when to expect newborn cubs quite a challenge.

Delayed implantation is a phenomenon common among all bears—at least we think! It serves its most obvious function in bear species that inhabit highly seasonal environments—that is to say, environments where food and good weather are not available year round. The more extreme the environment, the longer the delay: polar bears in the high Arctic are thought to delay implantation for about five months, whereas polar bears in more southerly latitudes may delay implantation by about three months. During this delay, regardless of how long it is, females go through an intensive feeding period during which they will gorge themselves on ringed and bearded seals for as long as possible before denning up in the fall. This summer feast allows them to deposit a thick layer of blubber that provides the essential nutritional support needed during the extended fast associated with pregnancy and denning.

True gestation takes about 60 days and typically begins in the fall, soon after pregnant females excavate and enter their subnivean (undersnow) dens. Females will remain in their dens until spring emergence, cubs in tow, ready to face the elements and begin the neverending search for food.

Fall will soon arrive in San Diego. Although it will be months before a chill wind is in the air, the change in day length (the first day of fall is that day when days begin to be shorter than nights) may cue hormonal and behavioral changes that prepare the polar bear for implantation, pregnancy, and impending motherhood. We are all watching Chinook with great anticipation: Is she or isn’t she? And when will she?

For a bear like Chinook, living at the San Diego Zoo where the weather is warm and food is always available, a layer of blubber is probably not necessary for successful reproduction. However, she has put on weight, and we are hoping that this is a sign that we will hear those hums and squawks from the den in the next couple of months. Stay tuned!

Megan Owen is a conservation program specialist at the San Diego Zoo. Read her previous post, Speaking of Polar Bears in Beijing.

Watch the bears daily on Polar Cam.