The Hungriest Month: Fall Hyperphagia and the Grizzly Bears of Glacier National Park
Right now, in the last days of September, the grizzly bears of Glacier National Park are eating with an urgency they won't match at any other point in the year. Biologists call it hyperphagia — the fall feeding frenzy when a bear's body shifts into overdrive to lay down fat before winter. During these weeks, a grizzly may take in 20,000 calories a day or more, and sleep only three or four hours between meals. Everything about the animal is tuned to one task: get fat, or don't survive the den.
A berry-sized lifeline
For the bears of Glacier, a surprising amount of that fat comes from a berry you can hold three of on a fingertip. Huckleberries make up roughly 15% of the diet of bears in Glacier and the neighboring Whitefish Range, and they matter most in exactly this window, when bears are trying to pack on weight for hibernation. In this ecosystem, a good berry year is a good bear year.
That's also what makes the science here quietly urgent. USGS researcher Tabitha Graves and her colleagues have documented "pretty big changes in the timing of when huckleberries flower and when the huckleberries get ripe" — shifts driven by warmer, drier springs. If berries ripen and drop earlier, a bear can arrive at its fall feeding season to find the shelves already bare, forcing it to travel farther or seek other foods, sometimes closer to people. The same team found that at least six different bumblebee species pollinate Glacier's huckleberries — a kind of built-in insurance, since no single bee carries the whole crop.
A recovery story
The bears themselves are a recovery story worth telling. The Northern Continental Divide Ecosystem, anchored by Glacier, is now home to more than 1,000 grizzlies — the largest population in the lower 48 — up from a genetic baseline estimate of about 765 bears in 2004. Their occupied range expanded roughly 12% between 2008 and 2022, and every bear management unit in the ecosystem now has females raising cubs. Glacier itself provides habitat for nearly a thousand bears of both species.
Hike smart this fall
All of which is a good reason to hike smart in autumn. Bears are focused, food-driven, and often working berry patches right beside the trail. The Park Service asks visitors to stay at least 100 yards (91 m) from any bear, carry bear spray and know how to use it, make noise, and hike in groups — notably, Glacier has never recorded an attack on a group of four or more.
Fall in the Rockies rewards the patient. Fewer people, low golden light, and if you're lucky, a distant grizzly turning over a hillside, doing the oldest math in the mountains: calories in, winter ahead.
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