Wolves (Canis lupus) can exert top-down pressure and shape ecological communities through predation of ungulates and beavers (Castor spp.). Therefore, understanding wolf foraging is critical to estimating their ecosystem-level effects. Specifically, if wolves are consumers that optimize tradeoffs between costs and benefits of prey acquisition, changes in these factors may lead to prey switching or negative-density dependent selection with potential consequences for community stability. For wolves, factors affecting cost and benefits include prey vulnerability, risk, reward, and availability which can vary temporally. We described wolf diet by frequency of occurrence and percent biomass and characterized diet using prey remains found in wolf scats on Isle Royale National Park, Michigan, USA during May–October 2019 and 2020. We used logistic regression to estimate prey consumption over time. We predicted prey with temporal variation in cost (availability and/or vulnerability) such as adult moose (Alces alces), calf moose, and beaver (Castor canadensis) to vary in wolf diets. We analyzed 206 scats and identified 62% of remains as beaver, 26% as moose, and 12% as other species (birds, smaller mammals, and wolves). Adult moose were more likely to occur in wolf scats in May, when moose are in poor condition following winter. The occurrence of moose calves peaked during June–mid July following birth but before calf vulnerability declined as they matured. In contrast, beaver occurrence in wolf scat did not change over time, reflecting the importance of low-handling cost prey items for recently introduced lone or paired wolves. Our results demonstrate that wolf diet is responsive to temporal changes in prey costs. Temporal fluctuation in diet may influence wolves’ ecological role if prey respond to increased predation risk by altering foraging or breeding behavior.

From 5 May–5 October 2019 and 13 June–22 September 2020 , we collected all wolf scats encountered during fieldwork in IRNP. We collected fresh scats (e.g., strong smell, moist, tracks present) at wolf radio-collar GPS location clusters (Svoboda et al., 2013) and on established hiking trails (Fig. 1). We placed each scat in a plastic bag, recorded the date and location (NAD83, UTM Zone 16N), and froze them for later processing. We considered the collection date as the date of deposit, although scats may have been up to 11 days old (Sanchez et al., 2004).

We processed and identified scat contents following Chenaux–Ibrahim (2015). We placed frozen scats into nylon stockings, washed them in a washing machine to remove digestible material and then dried the contents in an oven. We spread the contents on a 21.5cm x 28cm plate, overlaid a 25-point grid, and randomly selected one hair from each point for microscopic identification (Ciucci et al., 2004). We could determine moose age class (adult or calf) until 15 September, when the first molt occurs (Muller, 2006). After 15 September, we identified all moose hair as adult.