…almost everything is food except granite…
~John Muir on what grizzlies eat
Muir was probably not far off in that observation. Grizzlies (and Black Bears as well) have a tremendously varied diet according to the season and food availability. Grizzlies are omnivores, feeding on a wide range of both plants and animals.
Grizzlies tend to use open areas more than Black Bears so are often seen out in meadows as they forage. Their long claws and musculature associated with the large hump over their shoulders, makes them well suited to dig plants, insects, and rodents from the soil. In the last post, I mentioned a Grizzly with cub seen foraging atop Junction Butte. She seemed to be digging repeatedly on a rocky slope. A week later, I hiked up to the top of Junction Butte (I had heard that a Grizzly and cub had been seen crossing the road away from Junction Butte so I hoped she was no longer in the area).
Vegetation atop the butte is sparse and much of the area is covered by loose rock the size of a basketball or smaller. It doesn’t look like a rich feeding ground so I was puzzled as to what the Grizzly had been eating. But signs of foraging by bears were everywhere.
The bear(s) had been flipping rocks, large and small. There was also some shallow digging between rocky patches. I flipped over a few rocks to see if I could find anything and saw only a few ants. I know Grizzlies are, somewhat surprisingly, fond of Army Cutworm Moths that seek shelter every summer under rocks on high elevation talus slopes. The moths migrate from the Plains up to several hundred miles away, where they feed as caterpillars, to feed on alpine flowers as adults on high slopes in Yellowstone and elsewhere in the mountains. Grizzlies have been observed eating an estimated 40,000 of these moths in a single day, so they can constitute an important dietary component. Literature suggests these massive moth aggregations occur on higher slopes than Junction Butte and are concentrated from late June to early September, so it was no surprise I found no moths. The mystery remains as to what she was digging.
Another important dietary item for Grizzlies and Black Bears is carrion – the carcasses of Elk, Bison, and other ungulates found in the park. This is especially true in spring when bears are coming out of hibernation. Many of these carcasses are from winter-killed animals or those killed by wolves. After my group arrived, we watched a Bison carcass out in Lamar Valley (there was also a Bison calf carcass nearby so maybe the cow died giving birth) as it fed various animals from Ravens to Wolves.
As is often the case, a herd of Bison stayed near the carcass for many hours and managed to run off a couple of Wolves that had been feeding. When a large Grizzly approached a little later, the Bison held their ground for awhile, but finally moved off, letting the Grizzly in to feed.
One of our last Grizzly sightings was a beautiful bear out the east entrance road near Mary Bay. Several cars had stopped to watch this bear digging in the flats near the lake. We joined the group just as the bear moved closer to the road and began to dig vigorously in one location.
It moved quickly in one spot for a couple of minutes before trotting off at a brisk pace to another clearing down the road.
At one point it looked as though the bear grabbed something and chomped on it but it is hard to tell from the photos.
One of the participants asked if we could go out and look at what the bear had been digging. After ensuring that it had left the area, I said yes, and we all walked out to examine the area. The Grizzly had dug up an area near obvious Pocket Gopher tunnels. We found these soil cores that appear after the snow melts. These are from where Pocket Gophers pack soil from their digging activities into tunnels in the snow. Pocket Gophers excavate huge amounts of topsoil (some estimates I have read said up to a ton of soil each year per gopher) as they search for plant roots and other foods.
They often create a cache of plant roots in underground chambers. Bears will raid these and steal the roots if they can find them. We found such a cache and what appeared to be a nest chamber (lined with fine plant material). I think the bear may have captured the gopher (and its young?) and left before consuming the stashed plant materials.
Just before leaving the area, the Grizzly paused, showing us all its mud-covered snout and efficient digging claws used to capture a meal. I can only wonder how many of these small mammals and their caches a Grizzly must have to eat to curb its appetite. But, as the season progresses, Grizzlies will seek out additional food sources including the young of elk and other ungulates, spawning Cutthroat Trout, Whitebark Pine nuts, and many other types of vegetation. As always, it is a privilege to witness these magnificent animals as they go about their lives.
NOTE: I have great respect for bears, especially Grizzly Bears. I try to hike in groups whenever possible and always carry bear spray. Park regulations require you to be 100 yards or more from bears. Photos of bears included in this blog are all taken with long telephoto lenses and are cropped.