For bears, gaining body fat is the key to their winter survival. Katmai’s brown bears often eat only the fattiest parts of the salmon—the brain, eggs, and skin—and leave behind the fleshier bits humans typically prefer.
All spring, the bears munch on plants, insects, and small mammals. But when the salmon swim up the river to spawn each summer, the bears focus on consuming as many fish as possible. Each one provides 4,500 calories.
“As they prepare for hibernation, bears are actually preparing to avoid winter famine, not cold weather,” says Fitz. As temperatures drop, food becomes scarcer. Bears survive by retreating to the safety of their dens. These winter dens can range from rocky caves to hollows under rotted tree roots. Every bear spends some of its summer scouting a safe and comfortable den location.
During hibernation, brown bears do not eat, drink, urinate, poop, or leave their den. By burning body fat while they sleep, bears stay hydrated and keep warm. Their body temperature remains fairly high, while their heart rate and breathing rate drop significantly to save energy.