The Temporal Scale of Foraging Decisions in Bison

Assessing the temporal scale under which gain is maximized is critical for the understanding of diet choice by animals. Classical foraging theory assumes that animals maximize long-term rates. Few studies have considered several temporal scales concurrently, however, weakening tests of rate-maximizing models. We used contingency models based on maximization of short-term vs. long-term energy intake by bison (Bison bison). Model predictions were tested against field observations conducted during six periods of 1998: two periods in the winter, one in the spring, and three in the summer. During most of the year, foraging characteristics and plant attributes suggested that intake rate of bison should be limited by ingestion time over short periods of time, and by digestive constraints over long periods of time. Diet predictions varied across temporal scales for four of the six sampling periods. Selecting Agropyron spp., rather than Carex atherodes, during these periods would result in an increase of daily energy intake by as much as 15 565 kJ (i.e., 7.4% of daily gains) but would necessitate a longer daily foraging time. We observed, instead, that bison preferred C. atherodes to Agropyron spp., suggesting that patterns of diet selection by bison were more consistent with maximization of short-term than of long-term energy intake. We thus provide some evidence that, contrary to established principles of classic optimality models, the foraging decisions of bison reduce potential long-term gains by maximizing short-term gains.