Researchers have been using simple morphological measures as indicators of ecological features for some time. The utility of morphological variables as ecological indicators depends upon our understanding of how the variable affects the ability of the organism to perform a particular task. Functional morphological analyses identify those features that can be directly related to behavioral performance and help to distinguish causal functional relationships from spurious correlations. The behavioral abilities of the individual, in turn, shape patterns of resource use and fitness by placing limits on the range of resources that can be utilized and by shaping the cost/benefit curve for resource choices. Examples from research on the feeding biology of North American sunfishes are discussed to illustrate how functional morphology can be used to provide explanations for differences between species in patterns of prey use, patterns of habitat use, ontogenetic diet switches, and population size. Trends from analyses of the evolution of fish feeding mechanisms suggest specific functional features that are most likely to vary among taxa and underlie differences in feeding performance and diet. Included in this group of predictive variables are the organization of lever arms in the jaw opening and closing systems, the size of the mouth, and the size of muscles used in prey-crushing behaviors. The link between morphology and ecology will be made most firmly when variables are chosen that clearly reflect the ability of the organism to perform relevant behaviors.
