Predicting bite size selection of mammalian herbivores: a test of a general model of diet optimization

The architecture of woody food plants forces mammalian herbivores to make compromises in their food choices. Rapid rates of dry matter intake can be achieved by choosing large bites. For woody plants, however, such bites are low in nutritive quality relative to small bites taken from leaves or twigs near the growing point of the plant. This trade-off between food quality and food intake rate is central to diet optimization in browsing herbivores. We developed a model that predicts a quantitative solution to 'optimal bite size' (i.e., the bite that results in the greatest daily net energy intake) based on constraints in harvesting and digesting foods. This model responds to the chemistry and morphology of plants, and the size and digestive strategy (ruminant versus hindgut fermenter) of the herbivore. We tested the model by conducting a set of experiments in which we offered six species of dormant deciduous trees common to the boreal forests of Sweden to captive roe deer (Capreolus capreolus), red deer (Cervus elaphus), and moose (Alces alces). We also tested alternative hypotheses that animals crop bites merely in response to the morphological structure of twigs, or the distribution of twig sizes on trees. Twig diameters cropped by these animals were positively correlated with the diameter at current annual growth. However, structural measures of the trees alone were not sufficient to predict differences in choices of twig diameters among animal species. In contrast, the optimal bite size model accounted for the different bite sizes selected by animals of different sizes and explained 86% of the variation in twig diameter cropped for all plant and animal species. Hence, we concluded that our model is useful for predicting, a priori, the twig diameters selected by herbivores on dormant deciduous trees. We suggest possible ways to enhance the model and how it can be used to assess forage availability, potential diet quality, and the vulnerability of trees to herbivory.