COS 181-6 - Consequences of a stolen lunch: Cougar kill rate in response to carcass theft by wolves and bears in northern Yellowstone

Abstract: One form of competition frequently observed in carnivores is kleptoparasitism via carcass theft and displacement by dominant con- or heterospecifics. Such displacements have the potential to drastically alter predator kill rates, and thus, their impact on prey populations. On Yellowstone’s Northern Range, cougars (Puma concolor) occur sympatrically with more dominant gray wolves (Canis lupus), grizzly bears (Ursus arctos horribilis), and American black bears (Ursus americanus) –– creating the potential for strong synergistic or antagonistic effects of displacement on cougar kill rates of important ungulates like elk (Cervus canadensis) and deer (Odocoileus spp.). Our objectives are to 1) summarize cougar kill rates and biomass acquisition rates; 2) identify how often cougars are displaced from and lose their kills to competing predators (i.e., wolves and bears); 3) determine factors that affect rates of carcass displacement; 4) evaluate how carcass displacement affects cougar kill rates and biomass acquisition rates of ungulate prey; and 5) compare findings with prior studies in northern Yellowstone. We searched GPS-location clusters of radio-collared cougars during three study periods (early winter, late winter, and spring-summer) to document predation events and displacement. Cougar kill rates were calculated using ratio estimation and summarized by total, ungulate, and elk kills/30 days. We used GLMMs to evaluate which covariates affect the probability of carcass displacement, as well as the subsequent effect of carcass displacement and other covariates on cougar kill rates. From 2016-2022, we studied 47 predation sequences on 13 unique individual cougars that averaged 32.6 days during winter study periods and 63.4 days during spring-summer. Total, ungulate, and elk kill rate by cougars was 6 kills/30 days, 5.4 kills/30 days, and 3 kills/30 days, respectively. Cougar kill rates varied significantly by study period (F2,44 = 11.5, p < 0.0001), being highest in spring-summer and lowest in early winter. Preliminary results suggest that displacement also varies by season: bears displace more in spring-summer and wolves displace more in winter. We predict that prey size, landscape openness and roughness, and wolf and bear densities will significantly influence rates of carcass displacement, while displacement itself will have a strong positive effect on cougar kill rates of prey. If predictions hold true, this work will show that competing predators may have strong synergistic effects on kill rates by subordinate predators, demonstrating that community-based approaches may be necessary to comprehensively understand even single-predator single-prey dynamics.