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.

Survival and reproduction in Arctic caribou are associated with summer forage and insect harassment

“…For example, our forage models were based on only six key forage species, where their average biomass, DN and DE were estimated for each pixel of the CAH summer range, with associated error (Johnson et al, 2018(Johnson et al, , 2021. Because caribou are highly selective foragers (Denryter et al, 2017) we expect that their use of different species varies across the summer (Russell et al, 1993;Ehlers et al, 2021), an aspect we had no information on and could not incorporate into our calculations. Additionally, our forage predictions were limited to those landcover types that were sampled in the field, which excluded low shrub and sparsely vegetated areas that are certainly used by caribou in summer.…”

“…During mid-summer when temperatures peak, harassment by mosquitoes (Culicidae) and oestrid flies (Oestridae) strongly influences the behavior of Arctic caribou and reindeer. Harassment has been associated with reduced time spent foraging, increased time spent walking and running, and movements to areas that are cooler, windier, and have less preferred forage (White et al, 1975;Hagemoen and Reimers, 2002;Witter et al, 2012;Ehlers et al, 2021;Johnson et al, 2021). Given these effects, summers with greater insect harassment have been correlated with reduced calf weights and adult body condition (Helle and Tarvainen, 1984;Weladji et al, 2003;Gagnon et al, 2020), presumably due to decreased foraging efficiency and increased energy expenditure.…”

Seasonal Diet Composition of Goitered Gazelle (Gazella subgutturosa) in an Arid and Semi-Arid Region of Western China

“…Fecal microhistological analysis is a commonly used method for analyzing the feces of herbivores to determine their dietary composition using histological techniques [ 38 , 39 , 40 , 41 , 42 , 43 , 44 ]. The fundamental principle involves identifying the plant types and quantities consumed by animals based on the imprinted structure of undigested phytokeratin fragments present in the feces [ 45 ].…”

Quantifying the movement, behavior, and environmental context of group-living animals using drones and computer vision

“…For example, accelerometers are commonly used to record stereotyped behavioral states such as resting, feeding, and locomotion, but generally cannot capture more complete behavioral repertoires (Brown et al, 2013). On-board cameras can record the social interactions, foraging decisions, and physical surroundings of instrumented individuals, but are restricted in their fields of view and prone to occlusion (Ehlers et al, 2021; Naganuma et al, 2021). Interpreting data from secondary sensors can also be challenging.…”