Choice and competition are essential parts of evolutionary biology. In an effort to pass their genes from one generation to the next, animals may carefully choose their mating partners, and/or compete for access to mates. Such choice and competition can occur both before mating through interactions between sexually mature adults (precopulatory sexual selection), and after mating (postcopulatory sexual selection), via competition among and choice of sperm (sperm competition and cryptic female choice, respectively). Choice and competition within and between the sexes have the potential to constrain or reinforce one another. How are mechanisms and episodes of sexual selection linked? Which environmental factors - external, internal, and/or social - act on specific sexually selected traits? When should an animal invest more into postcopulatory, rather than precopulatory traits? In the last decades, a number of sources of variation in sexual selection have been discovered. Yet, the sheer complexity of animal investment into traits, and the selective pressures acting onto them, prove challenging to unravel.

In this thesis, I investigated how mechanisms of sexual selection interact and how selective pressures may change in various environments and under differing conditions. I used a multitude of approaches, conducting experimental studies on Dermogenys collettei, the pygmy halfbeak, a small livebearing freshwater fish, and employing comparative tools on even-toed ungulates, a diverse group of mammals that famously invest in horns and antlers used to compete for reproductive opportunities. Using dichotomous choice assays, I identified traits relevant to mate choice in the pygmy halfbeak Dermogenys collettei, and that female mating status may influence the perceived attractiveness of these male traits (Paper I). I found no influence of male social status on female choice, with females eavesdropping on male-male competition showing no preference for either subordinate or dominant males, and male social status not influencing mating outcomes (although previous female preference did, Paper III). Lastly, I investigated the relationship between investment in male competitive traits before and after mating both in the pygmy halfbeak, as well as across ungulates. Interestingly, male halfbeaks showed a positive relationship between male competitive traits, with dominant males also possessing better ejaculate quality and producing faster swimming and more viable sperm. However, this relationship was connected to the intensity of male fighting, with differences in ejaculate quality disappearing when fighting intensity between dominant and subordinate males were experimentally relaxed (Paper II). In even-toed ungulates, a comparative analysis revealed that horns and antlers, traits that are important during precopulatory sexual selection, evolved faster than sperm morphology traits that experience postcopulatory sexual selection (Paper IV). Using novel species and approaches, the results of this thesis provide a broader understanding of interactions between mechanisms of sexual selection, highlighting sources of variation and ultimately adding towards a clearer understanding of sexual selection.