Fat Bear Week: How Alaska’s brown bears nearly double in size

Every year around this time, the brown bears of Katmai National Park and Preserve in Alaska manage to double their weight in a few months. Thanks to the efforts of the park’s enterprising social media team since 2014, we now celebrate this festival of feasting as Fat Bear Week.

But how is it that the bears know when it’s time to bulk up? In late summer in Alaska, brown bears (Ursus arctos) begin to enter an extra-hungry phase called hyperphagia, triggered by shortening days, hormonal shifts and a sudden abundance of salmon. The annual sockeye salmon migration runs through the park’s rivers each summer and early autumn. A single salmon provides about 4,000 calories, and the biggest bears can eat up to 40 salmon a day, taking in a whopping 160,000 calories. Males can gain up to 230 kilograms in just a few weeks and weigh up to 540 kilograms by the time hibernation starts, usually in late October or early November.

To make the most of their efforts, brown bears in Katmai congregate around Brooks Falls, where they  snag salmon leaping upstream. But fish isn’t the only item on the menu. The bears’ voracious appetites means they will eat just about anything, including grasses and berries. Some also eat clay, a phenomenon called geophagy, which may help with digestion.

Once the bears have had their fill, longer nights, cooler temperatures and the end of the salmon migration spur them to retreat to their dens. They then enter a deep, sleep-like state in which they don’t eat, drink or defecate from November through April. Each bear’s metabolism and heart rate slows and their body temperature drops from 37.7°C to around 31°C. They take about one breath per minute. Because the bears’ body temperatures drop only slightly, and because they can wake for short periods to move around their dens, this sleepy state is called torpor rather than true hibernation.

By the time the brown bears emerge in the spring, they will have lost around a third of their body weight without losing significant muscle mass or bone density – a special adaptation that boosts their chances of survival.