Pre- and post-copulatory sexual selection in two species of lygaeid seed bug
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Sexual selection arises via competition for access to mates, and is thus intimately tied to the social environment. For example, individual mating success may depend strongly on how many rivals or mating partners are available. Studies of mate choice and sexual selection may vary the number of mates a subject is presented with during mating experiments, yet it is not clear how this influences the strength and shape of sexual selection acting on traits in either sex. In this thesis I investigate the effect of social environment on sexual selection acting in two closely-related species of lygaeid seed bug: Lygaeus equestris and Lygaeus simulans. Males in both species possess an extremely elongate intromittent organ, which is over two-thirds average male body length. I show that the strength of pre-copulatory selection acting on male processus length in Lygaeus equestris and genital clasper shape in Lygaeus simulans is significantly influenced by the social context. However, selection on male and female body size in Lygaeus equestris is not. Additionally, I use a meta-analysis of 38 published studies to show that mating preferences are significantly stronger when more than one mate option is available, compared to when only a single option is available. I also investigate the functional morphology of male genital traits in Lygaeus simulans, and use formal selection analysis to quantify the strength of selection acting on these traits before, during and after mating. Finally, I use experimental manipulations in Lygaeus simulans to confirm that male processus length directly influences sperm transfer, and that intact genital claspers are required for successful intromission. Overall, my results illustrate that sexual selection in the wild may vary both spatially and temporally depending on the social environment. It is thus especially important that experiments are performed under ecologically relevant conditions.
Thesis, PhD Doctor of Philosophy
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