Summer wolf diet in northwestern Montana

Wolf (Canis lupus) diet can be estimated from undigested remains of prey in scats or through stable isotope analysis (SIA) of wolf hair when distinct δ13C and δ15N values of potential diet sources are known. Our objectives were to compare diet analysis methods, to estimate intra-population diet variability, and to determine proportions of prey consumed by wolves. We collected scats of 4 wolf packs in northwestern Montana from June to August 2008, and guard hairs of 45 wolves from 12 packs, May to August 2009. We calculated percent biomass consumed of deer (Odocoileus spp.), elk (Cervus canadensis), moose (Alces alces), and other items from scats, and used Pearson’s chi-squared tests of proportions to measure differences among packs. We used hierarchical Bayesian stable isotope mixing models to determine diet and scales of diet variation from δ13C and δ15N values of wolves and prey. We used bootstrapped scat data, and Markov Chain Monte Carlo simulation data from stable isotopes to estimate confidence intervals of difference between results from each technique for 4 packs with matched samples. Diet results were not consistent between techniques. Deer was the most common prey item based on scats, and moose the most common based on SIA. Wolf diet was significantly different among packs based on scats, and differences among packs explained most variability in diet based on stable isotopes. We sampled 3 times as many packs for less than half the cost with SIA compared to scat analysis. Experimental data on wolf hair growth period and wolf-specific δ13C and δ15N fractionation values would provide important information for recommending the better technique