Striking variation is observed in sexually selected traits. These traits influence the outcome of competition and choice before and after mating. Before mating, animals compete for the access to mating partners and choose who to mate with between several potential mates. After mating, sperm from several males can compete for the fertilization of the same eggs, and there can be cryptic choice, since the female may be able to influence which sperm gets to fertilize her eggs. Animals are expected to invest in traits that increase their success in competition and choice. However, external and internal factors shape which sexually selected traits are most beneficial and can cause animals to trade investment in some traits over others. Environmental conditions and availability of resources can influence how animals balance their time and energy to different traits, and males and females can respond to these factors in different ways. However, understanding these complex selective pressures remain challenging. 

In this thesis, I combine field and experimental studies to examine how environmental conditions and resource availability influence how males and females invest in sexually selected traits. The thesis uses the pygmy halfbeak (Dermogenys collettei) as a study system, a small livebearing fish which exhibit a range of sexually selected traits in both males and females. Using observations of natural populations of halfbeaks in Singapore, I found that male and female halfbeaks show considerable behavioral variation in varying degree of canopy cover and water flow (Paper I). Next, I experimentally manipulated resource quantity in a series of laboratory studies to examine resource-dependence in sexual traits in male and female halfbeaks. In males, the expression of some, but not all, pre- and postcopulatory sexually selected traits were reduced following resource restriction. These patterns were surprisingly consistent between when the resource restriction was applied on adults and throughout development (Paper II). In females, the size and color reflectance of an ornamental trait (orange belly spot) was resource dependent, and males preferred females with larger belly spots (Paper III).  Lastly, I examined if female reproductive fluids are resource-dependent in their effect on sperm, as female reproductive fluids may be a key mechanism for cryptic female choice. While female reproductive fluid increased sperm swimming speed and viability, there was no evidence that female reproductive fluid effects on sperm are resource-dependent (Paper IV). The results of this thesis highlight how environmental conditions and variation in resources can influence how males and females invest in different traits and emphasize the need to study both males and females in a sexual selection framework.