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.
Highlights
Environmental Sustainability Research Centre, Department of Natural Sciences, Humboldt-University Berlin, Department of Biology, Behavioral Physiology, University of Derby, Derby DE22 1GB, UK
Strong support for the role of sexual selection in genital evolution comes from comparative studies that have found a positive relationship between indices of the degree of polyandry and the degree of evolutionary divergence or elaboration of genital structures in mammals (Ramm 2007; Orr and Brennan 2016) and insects (Arnqvist 1998, Rowe and Arnqvist 2012)
We examine the relationship between titillator complexity and two related measures of the intensity of sexual selection: the lifetime degree of polyandry and the relative potential reproductive rates of males and females
Summary
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, 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. Bushcrickets (Orthoptera: Tettigoniidae) are an excellent model taxon in which to test hypotheses relating to the evolution of copulatory structures They are a diverse and relatively well studied family of insects, which exhibit varying degrees of polyandry (Gwynne 2001; Vahed 2006, 2007). Bushcricket titillators are widely used as important taxonomic characters (Chamorro-Rengifo and Lopes-Andrade 2014) and show considerable variation across species in structure, shape and the extent to which they are spined (Vahed et al 2011)
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