Protandry and postandry in two related butterflies: conflicting evidence about sex-specific trade-offs between adult size and emergence time

Abstract

Natural selection acting on timing of metamorphosis can be sex-specific, resulting in differences in timing between males and females. Insects with discrete generations frequently show protandry: males usually mature before females. Both Euphydryas editha and E. aurinia butterflies followed this trend. The present study was motivated by the unusual observation of consistent postandry in addition to protandry. In a single E. editha population observed over 20 years the emergence period of males was longer than that of females, both the first and last emerging individuals being males. Variance of timing among individual E. editha larvae is imposed by spatial patchiness of the snowmelt that releases them from winter diapause. If individual larvae released late from diapause were to compensate for their lateness by shortening their development times, they would be small at maturity. If such compensation were only partial, they would be both late and small. Size and timing would become associated. If females were more prone to such partial compensation than males, the observations of postandry could be explained and the prediction made that any tendency for late individuals to be small should be stronger in females than in males. This was the case: in 1 year late males were the same size as early males, in a second year they were larger. Late females were significantly smaller than early females in both years. In E. aurinia, results were opposite both to theoretical prediction and to the observations from E. editha: although the male emergence period was longer than that of females exactly as in E. editha, late males were smaller than early ones, while late females were not small. The data from E. editha support the hypothesis of a sex-specific trade-off between size and emergence time, the data from E. aurinia do not.