Complex Reproductive Traits and Whole-Organism Performance.
Arnold's 1983 path-analytic paradigm, considering "morphology, performance, and fitness," has been elaborated in several ways. For example, current versions recognize the level of "behavior" (including aspects of motivation) as a filter between performance abilities (only measurable if motivation is maximal) and fitness components. Performance abilities constrain behavior, but behavioral choices may shield performance from selection. Conceptual and empirical issues remain, such as the extent to which individual variation in lower-level subordinate traits (e.g., circulating hormone concentrations) might directly affect behavior, growth rates, sexual maturation, etc., rather than having effects only through paths involving some aspect of performance. Moreover, empirical studies have yet to encompass more than a few possible paths in a given system, in part because life-history researchers rarely communicate with those focused on performance. Most life-history studies ponder trade-offs associated with reproductive effort, but studies of locomotor performance (e.g., maximal sprint speed) have rarely considered trade-offs with reproduction. This lack of connection is surprising because both life history (e.g., clutch size) and locomotor performance (e.g., locomotor stamina) traits require allocation of energy and other resources, so trade-offs between these trait types may be expected. These perspectives and cultures could be bridged by a focus on the ability of organisms to perform components of reproductive biology (e.g., lactation performance could be studied in animals maximally "motivated" by manipulation of litter size or endocrine function). Alternatively, one could study impacts of reproduction on performance, as when bats and live-bearing fishes lose maneuverability during gestation. We also consider sperm performance in the context of the paradigm and illustrate that the paradigm can easily be utilized as a frame-work within which to consider key aspects of sperm biology.