Exposing resource allocation trade-offs in sexual selection

[Truncated] Environmental and ecological conditions shape the evolution of life history traits in many species. Among such factors, food quality or nutrition availability can play an important role in moderating an animal’s life history traits. In particular, it has been argued that sexually selected traits should exhibit heightened condition dependence, and theory predicts that only the fittest males are able to acquire and allocate the resources required for their expression. Because male condition is dependent on resource allocation, condition dependence in sexual traits is expected to underlie trade-offs between reproduction and other life-history functions. My thesis works towards determining whether such trade-offs and resource availability influence the expression of precopulatory (sexual behaviour and sexual ornamentation) and postcopulatory traits (the quality, size and number of sperm) in the guppy Poecilia reticulata, a polyandrous live bearing freshwater fish. In addition to studying condition dependence in these sexual traits, I also explore their genetic basis under experimentally manipulated dietary conditions, thus enabling me to determine whether genotype-by-environment interactions underlie the expression of pre- and postcopulatory sexual traits in this model species in sexual selection.
In my first experimental chapter (Chapter 2), I tested whether experimentally manipulating diet quality (carotenoid levels) and quantity (food levels) influences the expression of pre- and postcopulatory sexual traits. I also determined whether diet manipulation mediates relationships among these traits. The study revealed a significant effect of diet quantity on the expression of both pre- and postcopulatory male traits; diet-restricted males performed fewer sexual behaviours and exhibited significant reductions in colour ornamentation, sperm quality, sperm number, and sperm length than those fed ad libitum. However, contrary to expectation, the study revealed no significant effect of carotenoid manipulation on the expression of any of these traits, and no evidence for a trade-off in resource allocation between pre- and postcopulatory episodes of sexual selection. Thus, while this initial study failed to reveal evidence for the expected co-dependence of pre- and postcopulatory traits on dietary carotenoid levels, it nevertheless underscored the sensitivity of behavioural, ornamental, and ejaculate traits to dietary stress and corroborates prior evidence from guppies that male sexual traits are highly sensitive to levels of resource acquisition. Furthermore, this study emphasised the important role that condition dependence plays in maintaining the high variability in male sexual traits frequently reported in this species.