Why does an individual forager use a given set of resources? This is a problem often addressed with optimal foraging theory. Although optimal foraging theory has not always been successful in generating qualitative predictions of foraging behavior, it can serve as a rough tool to understand what makes an individual choose its resources. According to optimal foraging theory, an individual is expected to choose its diet from among available resources to maximize its expected fitness. This might be achieved by maximizing energy income per unit time, or by some balance between energy income and another goal such as risk avoidance. The diet of an individual then depends on a variety of factors such as the a c o a. E o o
Population niche width
Figure 2 Individual diet specialization is part of a continuum from where the WIC equals that of the total population niche width (TNW) (on the hatched diagonal) to where WIC is a small part of TNW (close to the x-axis). The three schematic diagrams represent how individuals (hatched curves) can subdivide the population's niche (thick curve). The two diagrams on the hatched diagonal represent cases where individual diet specialization is low (i.e., WIC is a large part of TNW), whereas the diagram at the bottom right represents a case where there is a high degree of individual specialization.
energetic content of different prey, handling times, search efficiencies, resource abundances, and risk such as predation. All these factors are a combination of the resources traits (e.g., size, caloric content, abundance, and defensive traits) and the forager's phenotype (e.g., search or handling behavior, morphology, physiology, and experience). Consequently, an individual's preferences for alternative resources will reflect a complex interaction between resource traits and the forager's phenotype.
Given a common environment, why would conspecific individuals specialize on different prey types? The most proximate answer is that individuals will use different resources if they have different resource use efficiencies. Different resource use efficiencies may reflect variable morphological, behavioral, or physiological capacities to handle alternative resources. This poses a second question: why does phenotypic variation result in efficiency variation? Without tradeoffs constraining efficiencies on alternative resources, phenotypic variation would not produce efficiency variation. By tradeoffs, we mean that an individual adopting one strategy that increases efficiency on one prey type will lose the ability to efficiently use alternative prey. In situations with tradeoffs, a generalist forager may be unable to perform either strategy as well as the respective specialist and may therefore be selected against. Such tradeoffs are known to occur in many aspects of foraging including search efficiency, handling time efficiency (prey capture and digestion), and prey recognition. Thus, tradeoffs remain one of most plausible mechanisms for limiting an individual's niche breadth. Consequently, food specialization within a population can occur as a result of particular age- or sex-based characteristics or by individual-level variations in phenotype. Phenotypic variation is often associated with discrete polymorphisms with distinct modes along a quantitative axis such as beak depth. However, this variation may also be simple unimodal quantitative variation, or may have a complex behavioral or biomechanical basis that is not effectively summarized by a single quantitative axis.
Not all diet variation needs to be related to variation in efficiencies. Even individuals with equal efficiencies can nevertheless have different diets, reflecting variation in social status, mating strategy, or territories. For example, competitive dominant individuals may defend and monopolize preferred resources, and suboptimal individuals will be forced to rely on lower-quality resources. In this case, diet variation arises because individuals differ in their ability to achieve their optimal diet. Such interference competition is facilitated when the optimal resource is patchily distributed and can be defended by territories. IS in several mammal species is caused by territoriality in patchy environments. For example, individuals of both bears and pine martens whose territories abut streams have more fish in their diets compared to neighbors whose territories do not include streams. However, territorial foragers with similar resource use efficiencies may also end up with similar diets due to homogeneously distributed resources, so that each territory contains the full resource spectrum in the same proportions. Furthermore, coarse-grained environmental heterogeneity may sometimes also create differences among individual diets not related to foraging efficiencies. An example of that is the moose in the boreal forest region, who are severely limited in their movement in winter due to deep snow. An individual will forage intensely in small patches of often homogeneous vegetation, while another will forage on a very different vegetation. The next winter, a given individual may or may not find itself in a similar patch.
Although most of the examples of food specialization are probably due to maximization of energy intake, other food constituents can also be important in developing specialization. For example, the musk ox in the Canadian arctic spends more time foraging for protein-rich arctic vetches on low-productivity upland tundra than in the energy-rich sedge meadows in the valleys, where in relatively little time they can gorge themselves on what is not a limiting resource in summer. Such food specialization is probably due to lack of essential nutrients in the most energy-rich food source.
However, most cases of individual diet specialization are not due to territoriality or lack of essential nutrients, but could instead be related to sexual differences, ontoge-netic niche shifts, or phenotypic variation. We will here briefly describe causes and consequences of diet specialization related to sex, age, and phenotypic variation.
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