What Collar Cameras Reveal About How Caribou Feed

Fortymile Herd caribou feeding in the alpine of Yukon-Charley Rivers National Preserve. Photo: NPS/Kyle Joly.

Researchers from the University of Montana and several government agencies, the National Park Service among them, studied how caribou of the Fortymile Herd choose what to eat in summer. They fitted caribou in east-central Alaska with collars that carry both GPS and a video camera, so the animals' own view showed what they ate and when.

Why it matters

The Fortymile Herd ranges widely across subarctic Alaska and Canada, moving almost constantly in search of good food. A research blog post about the project stresses caribou's importance to Indigenous and Native peoples over thousands of years, for food, clothing and spiritual life. Caribou also do a great deal in their ecosystem: their movements and waste cycle nutrients, they eat and spread plants, and many predators, humans included, eat them. The post adds that their resilience is being tested by climate change, which it says is running five times faster in Arctic and subarctic regions than anywhere else on Earth.

The question

Competition for food and space can shape what large plant-eaters eat, from a single bite up to a whole landscape. Crowding effects, known as density-dependent effects, had been seen at broad scales. Few studies had checked whether the same effects show up bite by bite.

The team used how intensively caribou used an area as a stand-in for how crowded it was over space and time. Behavior and food choices recorded by the collars of caribou (Rangifer tarandus granti) were analyzed with a multivariate discrete-choice model.

What the cameras showed

• Availability drove choice. The more of a food type there was, measured as percent cover, the more likely caribou were to pick it.
• Crowding shifted diets. Where caribou used an area more heavily, they were more likely to eat shrubs and less likely to eat lichen. Shrubs were also eaten more where Salix shrubs covered more ground.
• Insects cut feeding. Insect harassment lowered the chance that caribou were eating at all.
• Wind helped. Strong eastward winds offset the insects' effect and raised the chance of eating lichen.

The drop in lichen may come from caribou competing with one another and from grazing depleting it. The rise in shrubs may reflect the greater amount of food shrubs offer, or their ability to stand up to heavier feeding than lichens can.

What it means

The crowding signals at the scale of single bites match density-dependent patterns seen at larger scales, and match recent evidence of falling reproductive rates in this same herd. The findings point to:

1. the risk that caribou overgraze sensitive foods such as lichen;
2. a better grasp of how many caribou a range can support;
3. how climate change may alter the way caribou feed.

Watching animals remotely like this could also feed spatial models that pick out high-quality habitat for conservation.

Sources

• Ehlers L, Palm E, Herriges J, Bentzen T, Suitor M, Joly K, Millspaugh J, Donnelly P, Gross J, Wells J, Larue B, Hebblewhite M. 2024. "A taste of space: Remote animal observations and discrete-choice models provide new insights into foraging and density dynamics for a large subarctic herbivore." Journal of Animal Ecology 93(7): 891–905. https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2656.14109
• "The CariView Experience: Migratory caribou teach researchers where critical summer foods and habitats are located – all from the animal's perspective," Animal Ecology in Focus: https://animalecologyinfocus.com/2024/05/30/the-cariview-experience-migratory-caribou-teach-researchers-where-critical-summer-foods-and-habitats-are-located-all-from-the-animals-perspective/
• National Park Service, "Influences on caribou foraging dynamics": https://www.nps.gov/articles/000/influences-on-caribou-foraging.htm
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.