Inbreeding, immunity and male sexual signaling in the black field cricket Teleogryllus commodus
Male sexual traits might be honest signals of male quality because they are costly to produce and/or maintain. Only high quality males can bear the costs of extravagant sexual display, creating a positive relationship between quality and trait expression. I conducted a series of experiments to test for the honesty of male sexual signals in the cricket Teleogryllus commodus. Males produce two acoustic sexual signals prior to mating: advertisement and courtship calls. first focused on the role of immunity in sexual selection. The immunocompetence handicap hypothesis (ICHH) proposes that sexual signals reliably indicate male immunocompetence because of a trade off between sexual trait expression and immunity. I first tested for the predicted trade off between male sexual attractiveness and immunity (Chapter 1). Males were injected with bacterial lipopolysaccharides (LPS) to induce an immune response and their subsequent attractiveness to females was assessed. There was no effect of LPS injections on male attractiveness. The ICHH predicts that the expression of sexual traits should be positively correlated with immunocompetence. I tested this by measuring courtship and advertisement call structure, advertisement calling effort, and immune function (Chapter 5). I also calculated the multivariate attractiveness of calls. Immune function was not related to call structure or multivariate attractiveness. There was a weak, positive relationship between advertisement calling effort and lysozyme-like activity. Therefore a female preference for higher calling effort might select for males with higher lysozyme activity. Finally, the assays used to measure insect immunity presumably reflect disease resistance. If disease resistance enhances fitness, we might expect a positive relationship between fitness and immune function. I tested this prediction by comparing the immune function of inbred (which should have reduced fitness) and outbred males (Chapter 2). Inbreeding did not affect lysozyme activity, but caused a significant increase in haemocyte counts. Therefore it is not always the fittest individuals that have the highest measured immune function. I then investigated the effect of inbreeding on sexual traits. If sexual traits are reliable signals of male fitness (e.g. male condition, heterozygosity), then they should show reduced expression with inbreeding (because inbreeding reduces fitness). I first tested for inbreeding depression on advertisement calling (Chapter 3). Inbreeding reduced calling effort but had no effect on call structure. I compared the number of females that were attracted to the calls of inbred and outbred males in the field. There was no difference. I also compared the multivariate attractiveness of inbred and outbred males. Again, there was no difference. Therefore, inbreeding does not affect the attractiveness of the actual call but, because inbred males call less, they will attract fewer females. Finally, I tested for inbreeding depression on courtship call structure and multivariate attractiveness (Chapter 4). Inbreeding resulted in a small but significant change in call structure, but did not affect multivariate attractiveness. The results suggest that the courtship call of T. commodus is not a reliable signal of aspects of male quality that are affected by inbreeding (which generally reduces fitness-enhancing traits).
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