Abstract and Summary
Evolutionary theories of female mate-choice tacitly assume a considerable ability of females to discriminate among potential mates on the basis of male epigamic differences. The general validity of this assumption is evaluated here through the application of comparative psychophysics. It is shown that perceptual or behavioral responses are typically logarithmic, rather than linear, functions of the stimulus intensity (the Weber-Fechner relationship). In general, then, ever-greater increments in absolute stimulus strength are required in order to elicit successive levels of discrimination. Through female choice, the mean value of a male epigamic trait increases in a population over evolutionary time. As this baseline increases, ever-greater absolute increments in the magnitude of the trait will be required in order for females to make successive discriminations among males. Thus, while male costs of producing stronger signals continue to rise, the corresponding benefits follow a diminishing returns function. This provides an inherent physiological check on the “runaway” selection process envisioned by Fisher (1930). Psychophysical theory also predicts that there should be a positive correlation between the mean value of a male epigamic trait in a species (or population) and the discriminatory acuity of females along the relevant stimulus intensity dimension. This prediction is testable via the comparative method. Sensory discrimination ability is here viewed not only as a product of natural selection but also as having important feedback influences upon the evolutionary process itself.