Directional Patterns of Female Mate Choice and the Role of Sensory Biases

A review of the literature reveals that, if females prefer traits that deviate from the population mean, they usually prefer traits of greater quantity. In cases in which the sensory bases of these preferences are identified, females prefer traits of greater quantity because these traits elicit greater sensory stimulation. However, two caveats apply. First, the studies surveyed might not represent an unbiased sample of mate choice, because researchers usually study systems characterized by exaggerated traits. Second, a preference for traits of greater quantity does not suggest that preference for average traits is unimportant; it might be more usual than preference for exaggerated traits. Phylogenetic comparisons sometimes allow one to distinguish among competing hypotheses for the evolution of female mating preferences. Two hypotheses, Fisher's theory of "runaway" sexual selection and the "good genes" hypothesis, predict that traits and preferences coevolve, whereas the "sensory exploitation" hypothesis predicts that males evolve traits to exploit preexisting female biases. Some studies of frogs and fish support the sensory exploitation hypothesis, although this does not exclude the role of other factors in establishing the preexisting bias or in the subsequent elaboration of the preference. It is suggested that studies of mate choice will benefit by a more integrative approach, especially one that combines knowledge of sensory mechanisms with appropriate phylogenetic comparisons.