Stable isotope analysis of diet has become a common tool in conservation research. However, the multiple sources of uncertainty inherent in this analysis framework involve consequences that have not been thoroughly addressed. Uncertainty arises from the choice of trophic discrimination factors, and for Bayesian stable isotope mixing models (SIMMs), the specification of prior information; the combined effect of these aspects has not been explicitly tested. We used a captive feeding study of gray wolves (Canis lupus) to determine the first experimentally-derived trophic discrimination factors of C and N for this large carnivore of broad conservation interest. Using the estimated diet in our controlled system and data from a published study on wild wolves and their prey in Montana, USA, we then investigated the simultaneous effect of discrimination factors and prior information on diet reconstruction with Bayesian SIMMs. Discrimination factors for gray wolves and their prey were 1.97‰ for δ13C and 3.04‰ for δ15N. Specifying wolf discrimination factors, as opposed to the commonly used red fox (Vulpes vulpes) factors, made little practical difference to estimates of wolf diet, but prior information had a strong effect on bias, precision, and accuracy of posterior estimates. Without specifying prior information in our Bayesian SIMM, it was not possible to produce SIMM posteriors statistically similar to the estimated diet in our controlled study or the diet of wild wolves. Our study demonstrates the critical effect of prior information on estimates of animal diets using Bayesian SIMMs, and suggests species-specific trophic discrimination factors are of secondary importance. When using stable isotope analysis to inform conservation decisions researchers should understand the limits of their data. It may be difficult to obtain useful information from SIMMs if informative priors are omitted and species-specific discrimination factors are unavailable.

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Lycaon pictus (Temminck, 1820), the African wild dog, is a moderately sized carnivore with dog-like appearance and irregularly mottled black, yellow-brown, and white pelage. It has a head–body length of 76–112 cm, tail length of 30–41 cm, shoulder height of 61–78 cm, and body weight of 17–36 kg. Lycaon pictus has four toes on each foot, differentiating it from other canids; is the only extant species within the genus with no subspecies; and is unlikely to be confused with any other canid. Lycaon pictus was once widespread throughout sub-Saharan Africa inhabiting nearly all environments and now inhabits grasslands, montane savanna, and open woodlands. Lycaon pictus is recognized as “Endangered” (EN) by the U.S. Fish and Wildlife Service and the International Union for Conservation of Nature.

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