Caribou Collar Cams: What 18,000 Videos Show About How a Declining Herd Eats

Caribou — called reindeer in Europe and Asia, and closely related to them — once numbered in the millions across North America. Today they are found only in Canada and Alaska, where they are the land animal northern subsistence hunters rely on most, and an interconnected part of the Arctic environment. Many herds are declining fast. The Fortymile Herd of east-central Alaska and the Yukon Territory fell from about 80,000 animals in 2017 to about 30,000 in 2024 — partly because climate change is altering their food supply, lowering the herd's fitness and its success at raising young.

Caribou are notoriously hard to study: they live in some of the continent's most remote country and are almost always on the move. So researchers from the University of Montana and several government agencies put cameras on them.

Watching, bite by bite

Alaska Department of Fish and Game staff fitted GPS video camera collars to 30 adult female Fortymile caribou. With the help of citizen scientists and botanists, the researchers went through more than 18,000 of the resulting videos, published in the Journal of Animal Ecology.

• Each clip's behavior went into one of six categories: eating, ruminating, traveling, standing awake, napping, or other (licking soil for minerals, wading, drinking and so on).
• From the eating clips, they identified more than 7,000 food items, grouped into types such as lichens and shrubs.
• They then built statistical models of the caribou's choices of behavior and food, taking in available plants, weather, wildfires, insects, terrain and how much space the animals were using, dropped factors too closely related to each other to separate, and kept the models that predicted the choices best.

The collars let researchers see what caribou do in vast, remote country without following them by plane or on foot — and at a small, "bite-sized" scale rarely studied before.

Why crowding matters

According to density-dependent habitat theory, animals that don't defend territories — caribou among them — use the best resources first. As more crowd into the same area, they turn to poorer ones to avoid competing, and when crowding makes them compete for food, they may reproduce less well. Large-scale studies, over huge areas or based on vegetation maps, have borne this out. Much less was known about how competition within a species shapes when and where individual animals feed.

Lichens versus shrubs

Lichens — partnerships of fungi and algae that each benefit from living together — break rock down into soil, recycle nitrogen and other elements, feed animals and can signal air pollution. They are the caribou's main winter food, and a favorite in warmer months too.

Lichens: fungi and algae living in partnership. NPS / K. Joly

But in summer the collared caribou preferred shrubs, especially willows (Salix), even where lichens were more plentiful. That makes sense: shrubs are more nutritious in summer, caribou need more food then, and shrubs recover quickly from grazing. The lead author, Libby Ehlers, reported that caribou ate fewer lichens where they gathered in large numbers, possibly because summer lichens hold less biomass than shrubs.

The catch: the Fortymile Herd's summer and winter ranges overlap. Heavy summer use can wipe out lichens by grazing and trampling, and while shrubs bounce back, lichens can take 20 years or more to recover. Shrubs spreading with climate change — and the disturbance and wildfire that can come with them — could shrink lichens further. Less lichen in winter could mean poorer nutrition and fewer calves surviving.

Driven off by insects

Arctic summer insects are legendary: Alaska alone has an estimated 17 trillion mosquitoes in the warm months. Mosquitoes, warble flies and nasal bot flies can harass caribou so badly that they bunch into big groups and move to places with little to eat — in interior Alaska, Ehlers noted, often high ground with almost no forage. The study found caribou ate less when insect harassment was high. Strong winds and cooler temperatures eased the effect, but warmer temperatures and changing winds from climate change could make it worse.

Caribou in higher country in Yukon-Charley Rivers National Preserve. The animals often gather and move to alpine areas to escape biting insects. NPS / K. Joly

What it adds up to

Where many caribou gathered, they were less likely to eat lichens and more likely to eat shrubs — the same density-dependent pattern seen in large-scale studies, now visible at the level of individual bites. Crowding changes how caribou feed even at that small scale, with possible consequences for the herd's reproduction and survival. Tools like camera collars can show whether caribou risk overgrazing, how climate change may affect them, and what that could mean for the Arctic.

The research was funded by the Alaska Department of Fish and Game, the Bureau of Land Management, the National Park Service, the University of Montana and the Yukon Government.

Sources

Based on "The Great Video Reveal: Emerging Tech Tracks What Caribou Do," by Kyle Joly, NPS wildlife biologist, and Marie Lawrence, editor of Park Science, Park Science volume 39, number 1 (Winter 2024–25), National Park Service; a work of the United States government in the public domain. The lead author's remarks are paraphrased. Collar videos and an aerial photograph credited to the Bureau of Land Management and the University of Montana are left out; rewritten in hubnx's own words.