Energy landscapes of Kodiak brown bears: A comparison of accelerometer and global positioning system derived estimates

Abstract

Within optimal foraging theory animals should maximize their net energy gain while minimizing energetic costs. Energetic expenditure in wild animals is therefore key to measure proxies of fitness. Accelerometers are an effective tool to study animal movement-based energetics but retrieval of the device is usually required and often difficult. Measuring energetic expenditures using a global positioning system (GPS) approach could provide an alternative method to study energetic ecology. We compared accelerometer and GPS methods to estimate energetic expenditures in brown bears (Ursus arctos) on the Kodiak Archipelago, Alaska, USA. We then applied the GPS method to examine how intrinsic and extrinsic factors influenced brown bear movement-based daily energetic expenditures (MDEE). We predicted that bears would have greater energetic expenditures during the high food abundance period, while females with dependent young would have lower energetic expenditures due to reduced movements. We found that while the two energetic measurements differed (Wilcoxon signed rank test: V = 2116, p < 0.001), they were positively correlated (r = 0.82, p < 0.001). The GPS method on average provided 1.6 times greater energy estimates than did the accelerometer method. Brown bears had lower MDEE during periods of high food abundance, supporting optimal foraging theory. Reproductive status and age did not influence MDEE, however movement rates had a positive linear relationship with MDEE. Energetic ecology is important for understanding drivers of animal movements. A GPS-derived estimate of energetic expenditure may be suitable when accelerometer data are unavailable, but the GPS-derived estimate should be validated for the specific taxa, ecosystem, and GPS sampling rate used. Additionally, while movement-based estimates of energy expenditure can elucidate the mechanisms driving habitat use decisions, they may not fully reflect an animal’s overall energy demands. Brown bear movement-based energetic expenditure was influenced by intrinsic and extrinsic factors which highlighted the importance of access to prime foraging sites to enhance energetic efficiency.