REPRODUCTIVE COMPETITION AND SELECTION ON MALE TRAITS AT VARYING SEX-RATIOS IN THE FIELD CRICKET, GRYLLUS-PENNSYLVANICUS

Intensity of male-male competition and sexual selection were studied i n the field cricket, Gryllus pennsylvanicus, in an outdoor arena at fo ur sex ratios, male-only (5:0), male-biased (5:2), unity (5:5) and fem ale-biased (5:10). Fighting frequency was highest at male-biased and u nity sex ratios. Fighting success was correlated with male body weight at all sex ratios. Calling duration and searching distance decreased with increased female numbers. Opportunity for selection was highest a t the male-biased and lowest at the female-biased sex ratio. Selection gradients and differentials were calculated and demonstrated that dir ect and total selection was highly variable and often relaxed. Direct selection favored male weight at the male-biased sex ratio and total s election for weight occurred at unity. There was no selection on male weight at the female-biased sex ratio. Total and direct selection for increased calling duration occurred at the male-biased sex ratio. Only total selection for calling duration was found at unity, whereas dire ct selection acted against calling duration at the female-biased sex r atio. Selection did not act on searching at any sex ratio. Natural var iations in sex ratios occur and fluctuations in selection on correlate d male traits may maintain additive genetic variation for traits impor tant in male-male competition.