Costs of sexual interactions to females in Rambur's forktail damselfly, Ischnura ramburi (Zygoptera: Coenagrionidae)

Abstract

Several species of damselflies, dragonflies and butterflies are characterized by a female-limited polymorphism in which one type of female, the andromorph, looks and behaves like males whereas the other type of female, the gynomorph, looks and behaves differently. Sexual conflict has been hypothesized to play a role in the maintenance of this polymorphism in that andromorphs may have an advantage over gynomorphs by avoiding costly sexual interactions through male mimicry. We tested for costs of sexual interactions to female Ischnura ramburi damselflies by comparing the success of singly mated females maintained with no males to the success of females maintained continuously with males at a 3:1 (male to female) operational sex ratio (OSR) and a 1:1 OSR. Our findings suggest that sexual interactions affect the two morphs differently. The time spent feeding, number of eggs laid and egg-laying rate of andromorphs were lower in the 3:1 OSR treatment than in the treatment with no males. Time spent feeding and number and rate of eggs laid by gynomorphs did not differ among treatments. Sexual conflict may be occurring between males and mated andromorphs because sexual interactions are associated with net costs to mated andromorphs whereas sexual interactions with mated andromorphs are beneficial to males because there is high last-male sperm precedence. Based on this experiment, andromorphs cannot be said to have an advantage over gynomorphs by avoiding costly sexual interactions because sexual interactions were not associated with net costs to gynomorphs.