Male genital titillators and the intensity of post-copulatory sexual selection across bushcrickets

Abstract

Animal genitalia are diverse and a growing body of evidence suggests that they evolve rapidly under post-copulatory sexual selection. This process is predicted to be more intense in polyandrous species, although there have been very few comparative studies of the relationship between the complexity of genital structures in males and measures of the degree of polyandry. In some bushcricket families, males possess sclerotised copulatory structures known as titillators, which are inserted into the female's genital chamber and moved rhythmically. Like other genital structures, bushcricket titillators are widely used as important taxonomic characters and show considerable variation across species in structure, shape and the extent to which they are spined. Here, we examine relationships between the presence/absence of titillators, titillator complexity and both mating frequency and the degree of polyandry in bushcrickets, using phylogenetic comparative analyses. Using published sources combined with original observations, data were obtained for the mean level of polyandry, the duration of the male and female sexual refractory periods and the level of complexity of titillators. To analyse data, we fitted phylogenetic generalised least squares models. No significant relationships were found between titillator presence or complexity and either the level of polyandry, duration of the male's sexual refractory period or the ratio of the female and male sexual refractory periods. The duration of the female's refractory period, however, was positively associated with titillator presence and negatively associated with titillator complexity. The data therefore partially support the hypothesis that post-copulatory sexual selection drives genital evolution in this taxon.

Demonstrating sexual selection by cryptic female choice on male genitalia: What is enough?

“…; Lehmann et al. ). Sperm are transferred from the ampulla to the female's spermatheca during several hours, after the pair has separated.…”

“…) and have more extended female refractory periods (Lehmann et al. ). Because the titillators are moved rhythmically during copulation, it was suggested long ago (Gerhardt , ; Boldyrev ) that they are used to stimulate females (the Latin word titillare means stimulated).…”

Sexual selection on bushcricket genitalia operates in a mosaic pattern

“…The males of the three Tettigoniinae species bear paired titillators with several spines on the tips (Harz, ; Lehmann et al, ; Vahed et al, ), whereas L. inflata males possess a single titillator with several spines, which is merged with the surrounding tissue of the phallobasis (Heller & Liu, ) (Figure ).…”

“…Males of several bushcricket subfamilies possess spiny genital organs which are part of the male's phallus (Chamorro‐Rengifo & Lopes‐Andrade, ). These sclerotized "titillators" exist in various quantities, structures, and shapes, ranging from simple fields of small tubercles up to a double pair of long and spine‐bearing titillators, depending on the Tettigoniidae subfamily (Lehmann, Gilbert, Vahed, & Lehmann, ; Vahed, Lehmann, Gilbert, & Lehmann, ). The morphological features of the genital appendages are well described and used for taxonomic purposes (Harz, ; Rentz, , , ).…”

“…The morphological features of the genital appendages are well described and used for taxonomic purposes (Harz, ; Rentz, , , ). However, information on the titillators' function in the mating process is still limited to experimental studies on a single species (Wulff, Kamp, Santos Rolo, Baumbach, & Lehmann, ; Wulff, Lehmann, Hipsley, & Lehmann, ; Wulff & Lehmann, , ; Wulff, Schöneich, & Lehmann, ) or comparative, nonexperimental, data from a larger number of species (Lehmann et al, ; Vahed et al, ). These investigations showed that the males' titillators are used during copulation to tap rhythmically on the surface of the female's flap‐like genital fold, which covers the opening of the genital chamber (Wulff et al, , , ).…”