Why are bears so active in the fall?
By Kris Inman Explore Big Sky Contributor
Are you wondering why you might be seeing more bears around Big Sky? It is because it’s that time of the year when they become more active. Bears are preparing to enter their dens and not eat or drink for five to seven months. They consume as much as 15,000-20,000 kilocalories per day during the period of hyperphagia that runs from August to October to make it through the winter denning season. This means bears are covering a lot of area in search of food.
In August, army cutworm moths, more commonly known as millers, escape the summer heat in rock slides above timberline and bears key in on this food source. “Bears consume as many as 40,000 moths a day,” said Interagency Grizzly Bear Study Team Leader Frank van Manen. “These moths are a valuable natural food source because up to 65 percent of a moth’s body weight is fat by the time bears consume them.”
In the fall, elk carcasses and gut piles left by hunters or bull elk seriously injured during the fall mating season are sought out as a valuable source of high protein for bears.
It is easy to believe, then, that a bear traveling near Big Sky on trash pickup day might find a smorgasbord of opportunity in human trash, which provides a more consistent and easier food source than the short-duration, seasonal abundance of berries and insects.
It doesn’t take long for a bear to key in on those areas where non-bear-resistant trash cans are the norm, or where a few unaware homeowners haven’t yet realized their HOA requirement for bear-resistant trash.
Soon the cycle of trash-conditioning and habituation to humans begins. Bears then become bolder, especially during late summer and fall when they are driven to consume what is an unfathomable number of calories a day.
Open windows or garage doors are hard to pass up, “Especially for young ‘naïve’ sub-adult black and grizzly bears that are on their own for the first time, females with cubs, or older bears with worn teeth that find it more difficult to acquire enough natural food,” said Kevin Frey, the Montana Fish, Wildlife and Parks grizzly bear specialist.
In addition to consuming a high number of calories, which are converted to fat, allowing the bears to survive through the denning season, bears have a physiological adaptation that lets them survive for five to seven months: during this time, they do not eat, drink or defecate.
Bears only lose a surprising 15 to 25 percent of their body mass and they don’t get bedsores or osteoporosis. Instead, they emerge from their dens in the spring with a slowly returning metabolic rate and increased body temperature, lean muscle mass that hasn’t atrophied and normal bone density, says FWP bear research biologist Cecily Costello.
This surprising fact led researchers from the National Aeronautics and Space Administration, nearly 20 years ago, to visit radio-collared bears in their dens. I participated as a researcher in this study, where we took blood from the hibernating bears to try and understand these physiological adaptations and apply it to humans. This mystery is still unresolved and remains of key interest to medical doctors and researchers.
The period of hyperphagia gives us a better understanding for why bears are more visible in our neighborhoods in the fall. Maybe, too, it gives us another reason to want to keep bears safe.
For the cost of your morning coffee, or less than your favorite lunch sandwich, you can switch to a bear-resistant trash can and by simply keeping your windows and garage doors closed, you keep yourself and bears safe.
Remember to follow Hey Bear on social media @say_hey_bear to learn how to make Big Sky’s story a positive one for bears, people and wild places.
Kris Inman is the community partnerships coordinator for the Wildlife Conservation Society.