Asymmetrical thermal constraints on the parapatric species boundaries of two widespread generalist butterflies

1. The sibling species Papilio glaucus and Papilio canadensis meet in a narrow hybrid zone believed to be maintained by temperature thresholds acting independently on both species. The present study tests if this assertion is true for the cold-adapted species, P. canadensis, which is presumed to be limited by the effect of high temperatures in late summer and/or autumn on pupal survival. 2. Three experiments were conducted examining the effects of: (i) short periods of high temperature stress in autumn, (ii) prolonged warm temperatures in autumn, and (iii) temperatures simulating warmer winters/longer springs upon the survival of P. canadensis and P. glaucus. 3. Results indicated that short periods of high temperatures did not induce the high C mortality rates required to be the key factor limiting the range of P canadensis. However, P canadensis did exhibit a considerably lower tolerance to high temperature extremes, prolonged warm temperatures in autumn, and conditions simulating shorter/warmer winters than P glaucus. 4. Differences in temperature tolerance throughout the pupal stage are likely to be a significant factor in maintaining the southern range limit of P. canadensis. Further warming as may occur during climate change, particularly in winter and spring, will likely affect the dynamics of southerly populations of P. canadensis.