Caribou foraging changes with the seasons and insect harassment

An image from a Fortymile Caribou Herd camera collar of caribou foraging in the summer in east-central Alaska.

Researchers from the University of Montana collaborated with wildlife biologists from the Bureau of Land Management, Alaska Department of Fish and Game, and the National Park Service on a study analyzing the summer (May-September) diets of caribou from the Fortymile Caribou Herd in east-central Alaska. The data were collected while watching over 18,000 videos recorded using camera collars fitted on caribou. Caribou spent the most time (44% of their day) eating. Lichens were consumed more than any other food item during May, August, and September, when green vegetation is limited, but shrubs were used the most in June and July. The leaves of shrubs contain much more protein than lichens and are selected preferentially during the short, green, Alaskan summer so the caribou can grow, gain weight, and meet the demands of raising their calves that were born in May. The researchers showed that despite the strong urge to spend as much time foraging on shrubs during the summer, insects harass caribou enough during the warm summer months that their intake is reduced when harassment is high (often when it is warm and the winds calm). As the herd is declining, understanding its foraging ecology is particularly important for management decisions.

Learn more

What scientists learned about caribou from a new collar camera study, Arctic Today, January 6, 2022

Critical summer foraging tradeoffs in a subarctic ungulate

Abstract

Summer diets are crucial for large herbivores in the subarctic and are affected by weather, harassment from insects and a variety of environmental changes linked to climate. Yet, understanding foraging behavior and diet of large herbivores is challenging in the subarctic because of their remote ranges. We used GPS video-camera collars to observe behaviors and summer diets of the migratory Fortymile Caribou Herd (Rangifer tarandus granti) across Alaska, USA and the Yukon, Canada. First, we characterized caribou behavior. Second, we tested if videos could be used to quantify changes in the probability of eating events. Third, we estimated summer diets at the finest taxonomic resolution possible through videos. Finally, we compared summer diet estimates from video collars to microhistological analysis of fecal pellets. We classified 18,134 videos from 30 female caribou over two summers (2018 and 2019). Caribou behaviors included eating (mean = 43.5%), ruminating (25.6%), travelling (14.0%), stationary awake (11.3%) and napping (5.1%). Eating was restricted by insect harassment. We classified forage(s) consumed in 5,549 videos where diet composition (monthly) highlighted a strong tradeoff between lichens and shrubs; shrubs dominated diets in June and July when lichen use declined. We identified 63 species, 70 genus and 33 family groups of summer forages from videos. After adjusting for digestibility, monthly estimates of diet composition were strongly correlated at the scale of the forage functional type (i.e., forage groups composed of forbs, graminoids, mosses, shrubs and lichens; r = 0.79, p < .01). Using video collars, we identified (1) a pronounced tradeoff in summer foraging between lichens and shrubs and (2) the costs of insect harassment on eating. Understanding caribou foraging ecology is needed to plan for their long-term conservation across the circumpolar north, and video collars can provide a powerful approach across remote regions.

Ehlers, L., G. Coulombe, J. Herriges, T. Bentzen, M. Suitor, K. Joly, and M. Hebblewhite. 2021. Critical summer foraging tradeoffs in a subarctic ungulate. Ecology and Evolution 11(24): 17835-17872.

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