Multi-tasking by mammalian herbivores: Overlapping processes during foraging

Mammalian herbivores can carry out multiple tasks without interrupting food processing, but this possibility is not considered in existing foraging models. We develop a mechanistic functional response to account for herbivores' ability to search for their next food bite and walk away from competitors while chewing vegetation. We demonstrate how the possibility of multi-tasking can buffer intake rate from competition and vigilance. The functional response of herbivores can be density independent until a threshold of competitors is reached in the food patch, and only then does it become density dependent. Herbivores also should be capable of maintaining food intake rate, despite important resource depletion in the foraging patch. The possibility of animal movements during food processing offers herbivores opportunities for cost-free vigilance. When individuals find their next bite before they have finished chewing the current bite, the remaining chewing time becomes "spare time" that could be spent in vigilance without reducing food intake rate. Modeling of optimal vigilance demonstrates that such cost-free vigilance might importantly alter expected patterns of scanning by mammalian herbivores. Assuming that interference increases with competitor density, spare time available for scanning should decrease as the number of herbivores in a food patch increases. Foraging constraints on food intake thus can provide a mechanistic explanation for the commonly observed decline in herbivore vigilance with increasing group size.