Image caption Colourful males have poorer quality sperm than their drabber counterparts

In a study of tropical guppies, scientists have found that the less attractive males have "better sperm".

The research revealed that colourful flamboyant-looking males "invest" in their appearance at the expense of their sperm quality.

This suggests, the scientists say, that there is a trade-off between different characteristics that help the male to reproduce.

The findings are described in the Royal Society journal Proceedings B.

Professor Jonathan Evans from the Centre for Evolutionary Biology at the University of Western Australia carried out the study.

He described how his findings supported "sperm competition theory", whereby females mate with several males and the quality and swimming speed of the sperm determine which male fathers the offspring.

Guppies are a particularly useful species in which to study this, because the males engage in two types of reproductive behaviour - courtship displays and non-consensual or "sneak" mating.

"Males that predominantly performed sneak matings were less ornamented but had faster swimming sperm than those that predominantly used courtship," explained Professor Evans.

He continued in the journal article: "The reproductive advantage enjoyed by attractive males might potentially be offset by the poor performance of their ejaculates during sperm competition."