Abstract
In most species with internal fertilization, male genitalia evolve faster than other morphological structures. This holds true for genital titillators, which are used exclusively during mating in several bushcricket subfamilies. Several theories have been proposed for the sexual selection forces driving the evolution of internal genitalia, especially sperm competition, sexually antagonistic coevolution (SAC), and cryptic female choice (CFC). However, it is unclear whether the evolution of genitalia can be described with a single hypothesis or a combination of them. The study of species‐specific genitalia action could contribute to the controversial debate about the underlying selective evolutionary forces.We studied female mating behaviors in response to experimentally modified titillators in a phylogenetically nested set of four bushcricket species: Roeseliana roeselii, Pholidoptera littoralis littoralis, Tettigonia viridissima (of the subfamily Tettigoniinae), and Letana inflata (Phaneropterinae). Bushcricket titillators have several potential functions; they stimulate females and suppress female resistance, ensure proper ampulla or spermatophore attachment, and facilitate male fixation. In R. roeselii, titillators stimulate females to accept copulations, supporting sexual selection by CFC. Conversely, titillator modification had no observable effect on the female's behavior in T. viridissima. The titillators of Ph. l. littoralis mechanically support the mating position and the spermatophore transfer, pointing to sexual selection by SAC. Mixed support was found in L. inflata, where manipulation resulted in increased female resistance (evidence for CFC) and mating failures by reduced spermatophore transfer success (evidence for SAC). Sexual selection is highly species‐specific with a mosaic support for either cryptic female choice or sexually antagonistic coevolution or a combination of both in the four species.
Highlights
In most species with internal fertilization, male genitalia evolve much faster than other morphological structures (Eberhard, 1985, 2010a; Rowe & Arnqvist, 2012; Shapiro & Porter, 1989)
Several theories have been proposed for the sexual selection forces driving the evolution of internal genitalia, especially sperm competition, sexually antagonistic coevolution (SAC), and cryptic female choice (CFC)
Based on the main hypotheses for sexual selection on genitalia, cryptic female choice, and sexually antagonistic coevolution, we developed a matrix for likely copulatory and postcopulatory responses, largely orienting on the extensive list for separating CFC from alternatives, given in Eberhard and Lehmann (2019)
Summary
In most species with internal fertilization, male genitalia evolve much faster than other morphological structures (Eberhard, 1985, 2010a; Rowe & Arnqvist, 2012; Shapiro & Porter, 1989). To test for selective forces likely to explain the evolution of titillators, we observed the responses of females mated to males of the wild type or with experimentally altered genital titillators If they are sexually selected, we hypothesize that titillator manipulations affect female behavior during or after copula. Based on the main hypotheses for sexual selection on genitalia, cryptic female choice, and sexually antagonistic coevolution, we developed a matrix for likely copulatory and postcopulatory responses (see Table 5), largely orienting on the extensive list for separating CFC from alternatives, given in Eberhard and Lehmann (2019) As we tested both symmetric and asymmetric titillator-manipulated males, we expanded the predictions to the symmetry type. Mating responses differing between species would provide support for a mosaic of forces acting, especially when there is evidence for cryptic female choice and sexually antagonistic coevolution within a species
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