Search efficiency of free-ranging plains bison for optimal food items

Optimal decisions in classical foraging models depend on the encounter rate with food items that are often distributed heterogeneously at one or multiple spatial scales. Only a few studies, however, provide a detailed assessment of the determinants of foraging efficiency by free-ranging animals across spatial scales. We evaluated multilevel search efficiency of free-ranging plains bison, Bison bison bison, and whether it varies with social factors, sex and season. We assessed search efficiency by contrasting encounters with the most profitable plant species, Carex atherodes, from random expectations within meadows or along foraging paths. We found that bison selectively occupied areas of meadows rich in C. atherodes. Once in these areas, females took shorter steps in locations of high C. atherodes biomass in summer and winter, as expected from an area-restricted search tactic. In winter, the presence of conspecifics further increased the chances of finding locations of high C. atherodes biomass, presumably due to social information. Unlike females, once males were in areas generally rich in C. atherodes during summer, they moved towards locations with lower C. atherodes biomass more than expected by chance alone. The fine-scale movement strategy of males does not appear to maximize instantaneous energy intake rate, but might reflect a tactic for efficient guarding of females during the rut. Overall, our study shows that the efficiency with which free-ranging herbivores find high-quality food during social foraging strongly differs between sexes, and depends on conspecific distribution and season. (C) 2012 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.