Few studies of interspecific mating preferences of naturally hybridizing species have been done in the field. Yet this is the only way potentially critical habitat-specific factors can be included in mating behavior evaluations. We conducted mate selection (male preference) studies using tethered pairs of heterospecific, size-matched virgin yellow females of Papilio glaucus and P. canadensis with free-flying male P. glaucus populations in south-central Florida, and with free-flying male P. canadensis populations in northern Michigan. Florida males clearly preferred the conspecific P. glaucus females of the pairs, as indicated by the touches/ attempted matings and by the actual copulations observed. In 1997, 66.7% of the total attempts ( n = 168) and 94.2% of all copulations ( n = 69) were with the conspecific. In 1998 the same pattern of conspecific male preference was observed, with 67.3% of the touches/attempts ( n = 49) and 100% of the copulations ( n = 30) being with the P. glaucus females. In contrast, the natural populations of P. canadensis males did not show the expected conspecific preference. In fact, with wild P. canadensis males in 1997 a very strong preference was observed for the heterospecific P. glaucus females of the pairs, accounting for 75.8% of all touches/attempted matings ( n = 483) and 81.7% of all copulations ( n = 476). The strong asymmetry in interspecific mating preferences among these hybridizing Papilio species may reflect an ancestral trait in P. glaucus females that elicits strong preference in P. canadensis males, due perhaps to previously possessed male sensory bias or "good genes" indicators in the females. However, reinforcement of reproductive isolation in or near the hybrid zone of species overlap (sympatry) cannot be ruled out.

No abstract