FEMALE-FEMALE COMPETITION IN KATYDIDS: SEXUAL SELECTION FOR INCREASED SENSITIVITY TO A MALE SIGNAL?

In contrast to studies of sex-specific weaponry and other sexually selected traits, there has been no examination of Darwin's (1871, p. 418) suggestion that elaborations or enlargements of "the organs of sense" function to enhance mating success. In certain katydids the size of thoracic spiracles, which are a main input into the hearing system, determines auditory sensitivity of females. Here we present evidence that sexual dimorphism in the spiracle size of a pollen katydid, Kawanaphila nartee, is a result of sexual selection on females competing to locate nuptial-gift giving males. In field experiments in which female K. nartee were attracted to a calling male, we show a pairing advantage to females with larger auditory spiracles. The spiracle-size advantage was not a correlated result of a larger body size or mass of winners. Finally, there was no spiracle-size advantage or body-mass advantage for mating females in a later stage of competition when experimental females struggled for access to a silent male. We suggest that research on the detection of displays has lagged behind work on the displays themselves; the focus has been on the species specificity of signal perception rather than on the fitness consequences of variation in the ability to detect cues from mates or predators.