Male genital claspers influence female mate acceptance in the stick insect Clitarchus hookeri

Abstract

In animals with internal fertilization, male genitalia exhibit higher rates of divergence compared with other morphological trails. Recent evidence suggests sexual selection drives such as rapid and divergent trait evolution. External male genital structures which clasp or stimulate the female’s exterior are likely to be under similar selective constraints to internal genitalia; however, their function and influence on male mating success have rarely been studied in detail. Here, we modify the external genitalia of the phasmid Clitarchus hookeri (White) to assess the role of male claspers in achieving successful acceptance by females and subsequent copulation. By covering female opercular organs and abrading male claspers, we demonstrate the necessity of precise coupling between these external genitalic structures for copulation to take place. We found that modified females tolerate un-modified male clasping attempts up to four times longer than normally required for attachment. However, when un-modified females are contacted by modified male claspers, males are quickly rejected. Our results suggest that external genital structures play an important role in precopulatory mate acceptance. Here, we discuss the potential role of female choice and species, isolating hypotheses in explaining the high evolutionary rate of such structures. Many male animals possess genital structures that allow them to grip on to females before, during, and after mating. We experimentally manipulated male claspers and the corresponding female morphology that is clasped by males for the stick insect Clitarchus hookeri, resulting in clear changes in mating behavior. We show that female mate acceptance is influenced by both the structure of male claspers and the ability of females to perceive clasping. This demonstrates that external genital structures can play an important role in precopulatory mate acceptance and that female choice is likely to influence their evolution.