Evolution of sperm quality but not quantity in the internally fertilized fish Xiphophorus nigrensis

Sperm competition, or competition between sperm of two or more males for a set of ova, has widespread effects on the evolution of animals (Birkhead & Møller, 1998). Sperm competition’s role in evolution may be particu-larly prominent in species with alternative reproductive strategies, where suites of physiological, morphological and behavioural traits have diversified among males as a result of competition for fertilizations (Oliveira et al., 2008). Male mating strategies commonly take two forms, a ‘bourgeois ’ strategy that attempts to monopolize females or the resources they require to mate and a ‘parasitic ’ strategy that exploits the bourgeois strategy by sneaking copulations with the females they attract (Taborsky, 1998). Given a trade-off between investment in ejaculates and other traits necessary for obtainin