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To gain insight into how animals respond to resource patchiness at different spatial scales, we envision their responses in environments comprised of nested, self-similar patches. In these environments, all resources reside within the smallest patches, and resource density declines as a constant exponent of patch size. Accordingly, we use simple mathematical formulations to describe a self-similar environment and a null model of how animals should respond to this environment if they do not perceive resource distribution. We then argue that animals that can perceive resource distribution should partition space by reducing the relative time searching between patches as patch size increases. On an experimental landscape, we found that woolly bear caterpillars Grammia geneura could partition space in this manner, but the range of patch sizes over which they did so tended to increase with resource aggregation. Nevertheless, scaling efficiency (i.e. the scaling of search time versus the scaling or resource density) was similar in all distributions when averaged over all patch sizes. These disparate patterns with similar outcomes resulted from differences in caterpillars' abilities to discriminate spatially among patches of different sizes via their movement pathways, and differences in their use of speed to detect resource items. Our work is relevant to the characterization of resource availability from an animal's perspective, and to the linking of optimal foraging theory to the modeling of search behavior.

Energy maximization, time minimization, and linear programming models subject to various constraints have dominated foraging ecology ideas and methods for decades. However, animals must use very complex physiological processes and foraging decisions to ensure fitness that in many cases may not be adequately described by these approaches. An example of this problem occurs when brown bears, Ursus arctos, have access to both abundant salmon and fruit. Salmon are one of the most energy and nutrient dense foods available to bears. Fruits are often high in soluble carbohydrates, low to deficient in many required nutrients, and more difficult to efficiently exploit. Therefore, wild brown bears that fatten primarily on fruits without access to salmon are 50% smaller than salmon-feeding bears. Thus, we predicted based on a linear, energy-maximizing model without dietary interaction effects that wild brown bears with access to both abundant salmon and fruit would feed almost exclusively on salmon. However, wild adult females with or without accompanying offspring foraged three times longer per day on fruit than on salmon. Similarly, the relative dry matter intake of ad libitum apples and salmon by captive, adult brown bears averaged 76 ±5% fruit and 24±5% salmon. Captive brown bears consuming mixed diets with intermediate dietary protein levels had 60% lower maintenance energy costs, 37% to 139% higher efficiencies of mass gain, and 72% to 520% higher maximum rates of gain than when they consumed either salmon or fruit alone. These relationships were nonlinear functions of dietary protein content in which salmon and fruit provided complementary nutritional resources. Both wild and captive bears attempted to regulate total protein, energy, and carbohydrate intake within a multidimensional intake target that both maximized energy intake and mass gain.

Organisms respond to their heterogeneous environment in complex ways at many temporal and spatial scales. Here, I examine how the smallest scale process in foraging by mammalian herbivores, taking a bite, influences plants and herbivores over larger scales. First, because cropping bites competes with chewing them, bite size influences short-term intake rate of herbivores within plant patches. On the other hand, herbivores can chew bites while searching for new ones, thus influencing the time spent vigilant and intake rate as animals move among food patches. Therefore, bite size affects how much time herbivores must spend foraging each day. Because acquiring energy is necessary for fitness, herbivores recognize the importance of bite size and select bites, patches and diets based on tradeoffs between harvesting rates, digestion, and sheering forces. In turn, induced structural defenses of plants, such as thorns, allow plants to respond immediately to herbivory by reducing bite size and thus tissue loss. Over evolutionary time, herbivores have adapted mouth morphology that allows them to maximize bite size on their primary forage plant, whereas plants faced with large mammalian herbivores have adapted structures such as divarication that minimize bite size and protect themselves from herbivory. Finally, bite size available among plant communities can drive habitat segregation and migration of larger herbivores across landscapes.

The question of how much time a foraging herbivore should spend in a patch of food poses a central challenge in classical foraging theory. However, there remains uncertainty about the relevance of the patch paradigm to foraging decisions by large herbivores. This paper examines evidence for successfully predicting and quantifying patch departure decisions for large mammalian herbivores foraging across several spatial and temporal scales. Departure decisions at fine scales are influenced by tradeoffs between maximizing intake rate and food quality. Classical models for departure decisions at larger spatial scales, particularly the marginal value theorem, appear inadequate. We advocate exploring alternative models for predictions of residence time at the patch scale.

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.