Foraging costs of vigilance in large mammalian herbivores

Vigilance has been assumed to reduce food intake by taking away time from food processing. Such foraging costs of vigilance have been predicted to have profound effects on the structure of communities. Recently, however, it has been argued that mammalian herbivores might be capable of maintaining their rate of food intake despite being vigilant, because of their ability to scan the environment while chewing vegetation. We conducted behavioral observations to evaluate whether vigilance decreases the bite rate of free-ranging female bison (Bison bison) in Prince Albert National Park and elk (Cervus canadensis) in Yellowstone National Park. Modeling of foraging processes indicated that chewing time exceeded the time that bison and elk spent searching for food, interacting with conspecifics, and scanning. Consequently, bison and elk might have been capable of maintaining their rate of food intake despite vigilance. The maintenance of intake rate would have required bison and elk to match scanning events closely with chewing bouts, but we did not detect a positive correlation between the duration of scanning bouts and the number of consecutive bites taken just before vigilance events. As a result, vigilance was costly, and as it increased, bite rate declined for both herbivore species. Scanning still overlapped partially with food handling. Indeed, we estimated that 31% of feeding time being vigilant decreased bite rate by 20% for bison and 26% for elk, whereas total absence of overlap between chewing and scanning should have reduced bite rate by 31%. While we observed that vigilance induced foraging costs, these costs were less important than traditionally assumed.