Alternative male phenotypes may be a source of novel adaptive traits and may evolve under strong sexual selection. We studied interpopulation differences in male mating behavior related to receptive female synchrony in the monandrous pupal-mating butterfly Heliconius charitonia. In the population in which female-receptive pupae were more synchronous, larger males were unable to monopolize mates; variance in male mating success was lower; strength of sexual selection was weak; and all males

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... ted for access to female pupae using the same strategy (pupal mating). In the population where no more than one female was receptive at a time (extreme asynchrony), only large males competed for pupae, and among these, only the largest individuals successfully mated. Thus, variance in mating success was higher, and sexual selection within pupal maters was stronger. In this population, smaller males patrolled large areas as an alternative mating behavior. When unmated females were experimentally released, small male size was associated with higher mating success. We suggest that alternative patrolling behavior may have evolved under strong sexual selection as a consequence of high asynchrony in receptive female availability in some populations.