Livestock depredation frequency is negatively associated with wolf body mass

Abstract

Foraging theory predicts that behavioral decisions are state-dependent, with individuals expected to alter foraging strategies according to physiological condition. Although livestock depredation by carnivores is often attributed to seasonal changes in prey availability and livestock exposure, less attention has been given to predator state. We evaluated whether seasonal variation in wolf body mass was associated with monthly livestock depredation frequency using 17 years of monitoring data from gray wolves (Canis lupus) in Oregon, USA. Using 329 body-mass measurements from 274 wolves, we modeled age- and season-related variation in body mass and generated standardized monthly population-level predictions. Predicted body mass declined through spring and summer, reached its lowest values in late summer, and increased into winter. Monthly depredation frequency decreased as predicted population-level body mass increased; a one-standard-deviation increase in predicted mass was associated with a 36.7% decrease in expected monthly depredation frequency (95% confidence interval: 22.7-48.1% decrease). The negative association persisted across sensitivity analyses based on predicted population-level body mass. When season was modeled explicitly, the association remained negative, but uncertainty increased substantially because predicted body mass and season contained limited independent temporal information. Livestock depredations were therefore most frequent during the portion of the annual cycle when wolves were predicted to have their lowest body mass. The observed association is consistent with state-dependent foraging theory, but the available data could not determine whether body mass itself contributed to depredation frequency or whether other seasonal ecological processes drove the association. Our results identify predator state as a potentially important but understudied component of carnivore-livestock interactions. Integrating measures of predator state with established drivers of conflict, including livestock exposure, husbandry, native prey, and landscape conditions, may help determine whether predator state provides additional information for understanding and anticipating carnivore-livestock conflict.