Proximate Mechanisms of Sexual Selection in the Firefly Photinus pyralis (Coleoptera: Lampyridae)

Male competition in courtships of the firefly Photinus pyralis is intense. Morphometric analysis revealed that male size is variable, possibly exhibiting alternative body plans. We examined the outcomes of courtships to determine the proximate mechanisms of selection that may have led to male allometry. Two distinct selective events exist within mate competition: (1) searching and (2) scramble competition. Although many courtships failed, those without rivals were shorter and successful males spent less time in the mate-guarding position than when several males were in competition. Winners outflashed rivals, but flash dialogs occurred at similar frequencies across rival densities, indicating that females require some quantum of information before mating. Female flash preference was examined in choice tests to evaluate female discrimination ability. Females preferred flashes of greater intensity and precedence. This suggests that flash synchronization is a competitive display rather than a cooperative behavior to maintain female responsiveness. Selection for male size was density dependent: females preferred large males in searching, while small males were favored in scrambles. Simulated scrambles showed that small males have superior locomotor abilities, allowing them to reach the female rapidly. Comparison with a species lacking scrambles, P. macdermotti, revealed that selection in P. pyralis is disruptive, targeting traits alternately, and may have led to the evolution of higher allometric ratios in a species with direct male competition. Both male competition and female choice are important determinants of the outcome of P. pyralis courtships.