Title: How sexual selection can drive the evolution of costly sperm ornamentation
Abstract: The 'big-sperm paradox', the observed production of few, gigantic sperm by some fruit flies (seemingly at odds with fundamental theory addressing how sexual selection works) is shown to be a result of co-evolution driven by genetic and functional relationships between sperm length, design of the female reproductive tract and features of the mating system. Male fruit flies have perhaps the most extreme sexual ornaments known. Although the flies are a mere 3 mm or so long, their sperm can exceed 5 cm in length. This presents a paradox, as under sexual selection one would expect males to produce large quantities of small sperm, rather than investing precious resources in making a small number of very large sperm. Here Scott Pitnick and colleagues show that both 'good' genes and 'runaway' processes contribute to the solution of the paradox. First, sperm production is related to male condition. Only high-quality males have resources sufficient to produce expensive, giant sperm. But that wouldn't work without some female connivance: the sperm have to fit inside a giant seminal receptacle in which the big sperm have a competitive advantage, as they can displace smaller sperm from other mating. Sperm length co-evolves with the length of the seminal receptacle with what looks like a runaway dynamic. Post-copulatory sexual selection (PSS), fuelled by female promiscuity, is credited with the rapid evolution of sperm quality traits across diverse taxa1. Yet, our understanding of the adaptive significance of sperm ornaments and the cryptic female preferences driving their evolution is extremely limited1,2. Here we review the evolutionary allometry of exaggerated sexual traits (for example, antlers, horns, tail feathers, mandibles and dewlaps), show that the giant sperm of some Drosophila species are possibly the most extreme ornaments3,4 in all of nature and demonstrate how their existence challenges theories explaining the intensity of sexual selection, mating-system evolution and the fundamental nature of sex differences5,6,7,8,9. We also combine quantitative genetic analyses of interacting sex-specific traits in D. melanogaster with comparative analyses of the condition dependence of male and female reproductive potential across species with varying ornament size to reveal complex dynamics that may underlie sperm-length evolution. Our results suggest that producing few gigantic sperm evolved by (1) Fisherian runaway selection mediated by genetic correlations between sperm length, the female preference for long sperm and female mating frequency, and (2) longer sperm increasing the indirect benefits to females. Our results also suggest that the developmental integration of sperm quality and quantity renders post-copulatory sexual selection on ejaculates unlikely to treat male–male competition and female choice as discrete processes.