"Less attractive males secure fewer matings but value each of them more highly, and by allocating more sperm to each mating make the most of their meagre opportunities. This leads to the paradoxical prediction that matings with the attractive males may be less fertile than those with unattractive males," Tazzyman said.

Whether or not the same principle applied to humans and other primates was still unknown, said the researchers -- but there every possibility that it did.

"Human attractiveness is complicated and influenced by a number of factors like cultural preferences. Nonetheless ejaculate size and sperm quality are likely to have been moulded by similar forces, like attractiveness and the number of sexual partners, that are important in other species," said Tazzyman.