Testing for adaptive explanations of bimodal genital insertion duration in the stalk-eyed seed bug

Abstract

Sexual selection leads to the evolution of various traits that increase male fitness under male–male competition including sperm competition. In many taxa, bimodal distributions of copulation and/or intromission duration are known to be the result of adaptation to sperm competition risk avoidance. However, the stalk-eyed seed bug, Chauliops fallax Scott (Heteroptera: Malcidae), exhibits completely different mating behaviour to other known examples. The male mounts the female and then inserts his aedeagus repeatedly and intermittently until the termination of copulation. Insertion duration exhibited a bimodal distribution with two peaks: a short insertion within 6 min which occurred one to three times without insemination, followed by long insertions of approximately 30 min with repeated insemination. To examine the adaptive significance of the two types of genital insertion, we investigated whether the amount of sperm transferred, female remating receptivity, number of eggs laid, hatchability and female longevity differed between the two insertion types. Sperm transfer mainly occurred during long insertions but very little occurred during short insertions. Similarly, long insertions induced mated females to resist subsequent mating attempts of other males but short insertions did not. Short insertions did not affect the number of eggs laid, hatchability or female longevity. Although our results showed that C. fallax males transferred sperm to females and inhibited female remating with long insertions, we currently cannot explain the adaptive function of short insertions without insemination.