Does Heat Tolerance Explain Female Polymorphism in Damselflies?

Abstract

Morphological variation is widespread, and one case is that of female morphs in damselfly insects. Current knowledge indicates that these female morphs are maintained by a fitness balance whereby male-like, clear-colored females (i.e. andromorphs) are more likely to “escape” from male harassment given their male mimicking compared to dark females (i.e. gynomorph). However, males can learn to distinguish the more frequent morph, leading to a frequency-dependent selection. Alternative to frequency-dependent selection, we tested whether temperature plays a role in driving female morph activity and survival. We used Ischnura denticollis damselflies to test for differences between female morphs and males in: (a) daily activity; (b) time to resume activity following a chilling event (10°C); and, (c) survival at 30, 35 and 40°C. There was no difference in activity among groups after a chilling event. However, gynomorph females were more active early in the day (ca. 17°C) and less active by 1600 hrs (after reaching 32°C), and had lower thermal tolerance than andromorph females. Thus, high temperatures may act against gynomorph females. This implies a role for high temperatures in the maintenance of female polymorphism.