Quantity versus quality: the sperm war

T evolution of sperm traits manifests itself prolifically across species, and postcopulatory sexual selection (PSS), as executed by the female, accompanies this process. The adaptive significance of some sperm traits (for example, the shape and number of sperms) is well understood. However, the evolution of germ size has not been fully exploited. The most recent study by Lüpold et al.1 reveals that the evolution of longer sperm is driven by the female seminal receptacle and mating frequency in Drosophila, which, in turn, increases the benefits to females. These findings provide a comprehensive interpretation regarding the evolution of sperm size. Male sexual traits have evolved more rapidly than other types of characteristics.2,3 For example, it is well known that antlers, horns, tail feathers, mandibles, and dewlaps are exaggerated ornaments related to reproduction and that natural selection is the driving force behind this rapid evolution.4,5 Sexually antagonistic (precopulatory sexual selection) and postcopulatory sexual selection are the two main reasons responsible for this evolutionary pattern.6 Sperms are highly specialized cells for transferring genetic information from the male to female egg. Although the process of spermatogenesis (i.e., meiosis and the sperm maturation process) is highly conserved across different taxa, sperm morphology shows great variation.7,8 Variation in sperm tail length is of particular interest, and a positive relationship between sperm size and sperm competition has been established for different organisms such as birds, butterflies, and fruit flies.2,3 A coevolution of the female reproductive tract Quantity versus quality: the sperm war