Research carried out on tropical guppies by Professor Jonathan Evans, of the Centre for Evolutionary Biology at the University of Western Australia, has found that less attractive males have a higher quality sperm.
The study has revealed that males which invest in their appearance, by becoming more colourful and developing their caudal fins, do so at the cost of having lower quality sperm, whereby their sperm swum more slowly and had fewer live sperm cells.
Poecilia reticulata - which is emerging as an important model for linking pre- and post-copulatory sexual selection - was the ideal candidate for the study, not only because of its promiscuity, but also as males of the species can switch from courting a female to a sneaking tactic – known as gonopodial thrusting. In contrast to the typical procedure of a courting male guppy, a sneaking male approaches the female from behind, and swings forward his gonopodium beyond 90° in an attempt to copulate with the female, doing so without any courtship.
The findings which are described in the Royal Society journal Proceedings B, supports what is known as "sperm competition theory", where females mate with several males both voluntarily and via coercion but, it is the viability of the sperm that determines which males will father any offspring.
This has implications in the evolution of mating techniques as Professor Evans explains: "Such trade-offs may account for negative associations between fertilisation success and mating success, and/or promote the evolution of alternative mating strategies in which males compensate for the inability to monopolise mates by investing relatively heavily in more competitive ejaculates ."
Under natural selection, or sexual selection, the continuing reliance upon male attractiveness should result in a reduced gene pool but, under sperm competition theory the gene pool remains diverse, as demonstrated by Professor Evans, who has the following to say: ".....postcopulatory mechanisms of female choice may yet provide a mechanism by which females override selection against attractive males during sperm competition."
For further information see the paper: Quantitative genetic evidence that males trade attractiveness for ejaculate quality in guppies, Jonathan P. Evans.