Autumn is an important time for bears

By Jill Mundy

As summer draws to a close in the northern hemisphere and there is a whiff of autumn in the air, bears are getting active. This is the time when they need to consume a lot of calories in preparation for the long, cold months ahead, even if they will not be retiring to a den for the whole winter. 

As evenings become longer and darker, a bear’s hormones send a signal to the brain to stock up on food. Often this will be high protein items such as nuts and acorns, plus fruits that are hopefully in abundance - blackberries, blueberries and apples are a just a few they may find.

An adult bear can put on weight by around 14 ounces each day (approximately 400g). In terms of calories, bears generally consume around 5000 calories per day in summer, but in September/October this may increase to a whopping 20,000 calories per day. They can also spend as many as 20 hours each day foraging. 

The amount of food available will determine how many months a bear will hibernate for (usually between five and seven months). For example, bears in North America may find food less easily available than their cousins in the South and will enter their dens once food supplies are scarce.

When not foraging and eating during this time of year, adult bears spend their time building dens, and most bears will keep to the same area as in previous years. Then, when November arrives, the bears will make these dens their home until the spring.

Bears may choose a variety of locations to make a den, be that at ground level or at a higher level. A hollow tree can be a good place for a den, keeping a bear snug so its body heat is contained. Some bears will find a large dip nestled within tree roots to call home if they feel safe and protected.

Mother bears and those that are mothers-to-be are more likely to dig a den that will be more enclosed as they will be there for several months with their cubs. The dens are often lined with leaves to make them more comfortable. 

A porch is a common outdoor space on American houses and bears have been known to make a den underneath a porch, often unknown by the residents of the home. Bears have also been known to build dens where humans have been carrying out excavation work or under abandoned machinery.

Younger bears spend a lot of time play-fighting during fall, perhaps making the most of outdoor opportunities before they retreat to their dens for the colder months.

In South America, food is generally available all year round so Spectacled bears have no need to hibernate. In Asia the climate is quite different and some Asiatic Brown Bears (Moon bears) will hibernate from late autumn, depending on their location. Others choose to spend the warmer months at high altitude and then migrate to lower levels when the temperatures drop.

Polar Bears

One bear that thrives in the colder months is, of course, the polar bear. During fall they are biding their time until their main hunting season begins. Many bears spend their time sparring with each other, sharpening skills and honing instincts in readiness for the big freeze and their pursuit of prey.

However, climate change means that in recent years fall has lasted longer, forcing a delay to the bears’ hunting season. Arctic ice is melting earlier too, reducing the length of time polar bears have to hunt and eat their catch, stocking up on much needed calories.

Some polar bear colonies, such as those in Svalbard, Norway, are desperately waiting for sea ice to form again, having melted in the spring and forcing them to remain on land through the summer. Similarly, those bears in Canada’s Hudson Bay. All are looking forward to venturing onto the ice again and hunting for seals and other food sources.

Pregnant polar bears retreat to dens built in snowy hillsides during these autumn months, taking shelter from the extreme cold and preparing for giving birth over the winter. The shorter feeding season due to late formation of sea ice means many females are unable to eat sufficient to sustain them through their pregnancy. The decline in pregnancies and therefore births is a huge threat to the survival of polar bears as a species and is a worrying trend.