Grizzly Bears: The Fearsome Predator That Eats 40,000 Moths a Day

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The grizzly bear carries the scientific name Ursus arctos horribilis, yet in Yellowstone this supposed monster may spend a whole day eating 40,000 army cutworm moths. Its diet can be 80 to 90 percent plants, depending on where it lives. Nineteenth century naturalists counted 86 species of grizzly, but genetic testing shows a single species of brown bear, with coastal animals fattened on salmon to more than 850 pounds and inland bears closer to 300. A muscular shoulder hump, a dished face, and digging claws two to four inches long set the grizzly apart from the black bear.

This episode explores the physiology that carries a grizzly through winter: gaining up to 400 pounds beforehand, then spending five to seven months in a den without eating, drinking, or passing waste, recycling urea into new protein. It explains delayed implantation, the bear's role as a gardener of mountain soil and coastal forest, and its tense standoffs with wolves over carcasses. It also weighs the human side, from the Gwich'in knowledge of ice bears to problem bears trained by unsecured garbage, and a present range about half the size of the historical one.

• Meriwether Lewis wrote the name in a way that leaves historians unsure whether he meant grizzled or gruesome. George Ord chose the frightening reading when he classified the bear in 1815.
• A fertilized embryo implants only if the mother has stored enough fat. Cubs are born in the den weighing about 16 ounces.
• Spruce trees up to 500 meters from salmon streams contain marine nitrogen, carried inland in the carcasses that bears drag into the forest.
• Some Yukon grizzlies delay hibernation to keep fishing, coating their fur in frozen wet sand. Anthropologists once took these ice bears for myth.
• Admiralty Island in Alaska holds an estimated 1,600 grizzlies, while the California grizzly on the state flag has been extinct since the 1920s.