What won’t grizzlies eat?
In the Greater Yellowstone Great Ecosystem, grizzlies will eat just about every animal that lives in the region, if given the chance. In a surprising moment on Monday’s episode of Yellowstone Live, on the National Geographic Channel, a female called Mini Mom was seen digging up a clump of tasty grass as her two cubs looked on. She clawed at the ground, scooping up the Earth like a badger. Nearby, her two little cubs also tore into the ground, imitating her. It looked almost as if they were playing. The cubs didn’t succeed at snagging a plant, but moments later one of them found an earthworm and gobbled it down.
By following their mother’s example over two or three years, the cubs will learn how to survive. That grizzly bears eat worms and grasses may surprise people, but the bears actually eat a huge range of things, from insects to berries and roots to mammals that seem far too big for them to take down. The bears are always in search of food and will gladly eat just about anything with a decent number of calories.
Though members of the order Carnivora, grizzlies mostly consume plants. In the summer, for example, grasses make up a major part of their food intake. They also eat massive quantities of fruits like huckleberries and serviceberries. In the fall, before hibernation, whitebark pine seeds are a favorite meal.
In spring and early summer they’ll also go after larger prey, like elk and even bison.
In another striking sighting on the show, a mother grizzly charged a massive herd of elk, with her two cubs trailing behind. “She’s nursing her young and hasn’t eaten in a long time,” Rae Wynn-Grant, a large carnivore ecologist and National Geographic explorer. “She just recently came out of hibernation.” Female grizzlies give birth in January, and for the first several months of the cubs life, they nurse the young without eating. This bear was clearly ready for a meal.
Grizzly bears usually won’t take down a healthy adult elk because they’re too fast, but they’ll go after calves. At four weeks or younger, elk calves aren’t yet fast enough to outrun grizzlies. Injured elk, too, may be a target, Grant says. And this charge was an attempt to scare up or startle a young or injured animal, she says.
Grizzlies will also take down bison calves, as shown in a segment of the show using an infrared camera to film at night. The bison calf was swept down a river while attempting to follow its mother across. A grizzly came across the calf before the mother could catch up, and the bison calf was no more.
One of the stranger animals that the bears eat must be army cutworm moths, also known as miller moths. These insects fly to boulder fields in the Rocky Mountains in the summer to find flowers and escape the heat. High in the mountains of Yellowstone and elsewhere in the region, grizzlies will sometimes voyage to eat the insects in massive quantities. The bears may sometimes down up to 40,000 moths in one day, according to researcher Don White, Jr., a wildlife ecology professor at the University of Arkansas.
But bears also seem to waste food at times. In the late summer, when they’re putting on weight for hibernation, they’ll take bites out of the fattiest bits of fish—the bellies and eyes—and toss the rest of the prey aside, Wynn-Grant says, in order to focus on fattening up.
“To me that’s the most surprising thing,” she says, to watch them appearing to pass up a chance to eat more.
This is another example of optimal foraging strategy, she says. It’s the theory that explains that grizzlies and other large animals will exert the minimum energy necessary to eat the maximum quantity—and quality—of calories. It also explains why they can get hooked on human food. But the animals are better off, of course, going for natural sources, which are abundant in Yellowstone.