Photo from the study of a bear digging for moths. Fleeing moths are circled in yellow.

Each summer, a peculiar feast takes place high in the mountains of Glacier National Park. You usually hear it before you see it—the clinking of small rocks as heavy paws dig into loose talus. This extraordinary banquet is one of the keys to the survival of an iconic species—the grizzly bear.

For how large these animals are, the main dish of this feast is startlingly small. The army cutworm moth, about three-quarters of an inch long, is one of the best and most important food sources available to grizzly bears. New research, led by Erik Peterson and made possible with your support, gives us the most detailed glimpse yet into the fascinating, and surprising, interplay between these species.

The army cutworm moth, also called the miller moth, is a nondescript insect that migrates from the Great Plains to spend its summers in the talus slopes above treeline in the Rocky Mountains. (Talus is made up of smallish fragments of broken rock—think large, flaky gravel.) When the moths arrive in the alpine, they spend their days resting in little pockets of air under the rocks. At night, they feed on the nectar of alpine plants.

A hand holding a small rock with a moth on it.

An army cutworm moth clinging to a rock. Photo: Erik Peterson

Moths that arrive in Glacier and find some home high up on Going-to-the-Sun Mountain, Mount Siyeh, or some other iconic palisade, have around 15% body fat. During the course of the summer, something remarkable happens. This lifestyle of resting in the day and feeding on nectar at night allows army cutworm moths to build up some of the largest lipid (fat) reserves found anywhere in the animal kingdom.

By the time grizzlies come to feed, these moths often have over 70% of their body weight in lipids, making them far richer in calories than goat or elk or anything a Glacier National Park grizzly might hunt. They are, as Erik Peterson likes to say, bear butter.

To succeed as a grizzly, you need to be good at math. The life of a bear is all about making sure that your fat reserves are enough to make it through hibernation, to survive to another year, to breed. The equation is simple: energy for hibernation = calories consumed – calories burned. Bears need to eat as much as possible, while expending as little energy as possible doing so. This means that while an adult moose may contain a motherlode of calories, the energy required to hunt it means it’s rarely worthwhile. Moths, on the other hand, don’t fight back.

A grizzly bear overturning a large rock.

Grizzlies use their powerful shoulders to dig in the rocks.
Photo from the study

Grizzly bears are omnivorous generalists—they will eat just about anything that makes sense mathematically (i.e. it’s not too much work to catch it). Because their food requirements are so broad, they could theoretically use many habitats. Their range is limited by human disturbance and the amount of food available. In other words, the best grizzly habitat has high food abundance and limited overlap with people.

Thus, we return to moths. Army cutworm moths make an ideal food source for grizzly bears: they are calorically dense, easy to catch, live at high elevations where humans are only fleetingly present, and are available in summer when grizzlies are active. 

The difficulty in accessing moth habitat, a benefit for the bears, has made studying this relationship difficult. Peterson’s approach included a wide range of methodologies to overcome these difficulties. Aerial surveys, using high-definition infrared video cameras on helicopters flown by Two Bear Air, were used to identify sites where grizzlies fed on army cutworm moths. Ground surveys were conducted to identify additional moth dig sites, which are distinguished from bedding sites because they are clustered, and foraging bears don’t dig down to soil like an animal creating a wallow would. Over 100 moth foraging sites were positively identified in Glacier in this way.

A grizzly bear foraging in a vast talus field.

A grizzly forages for moths. Photo: Andrew Smith/GNPC

Army cutworm moth sites are incredibly important habitat. In a single survey, Peterson and his team once observed about 10% of the park’s entire grizzly population feeding on moths in an area that was less than 0.5% of the park’s acreage. The known foraging sites, identified from aerial and ground surveys, were used as the basis of a model that could predict additional moth and grizzly locations.

This model gives park management extremely powerful data. The future of Glacier’s grizzly population depends on a resource (moths) concentrated in a small, rugged, and remote portion of the park. Understanding where moth foraging happens, and why it is so important, allows managers to monitor army cutworm moth gatherings and take action to protect grizzly bear use in these places. Donor support makes science like this possible, and gives park managers the tools they need to ensure that Glacier is home to a thriving grizzly population for generations to come.

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This project and many other critical projects would not be possible without your donations to the Glacier Conservancy.

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