Women keen to get pregnant might want to consider the least attractive man around (I see a lot of you, ugly men, nodding!)

Ugly men are apparently the most fertile. Good-looking men with lots of lovers, on the other hand, are likely to be less successful at siring offspring.

The reason is all to do with 'sperm load' says scientists.

Research suggests that in many animal species, the most desirable males restrict their sperm load with each mating to ensure there is enough to go round.

The same could be true of humans and other primates, say scientists. If they are right, women looking for the best chance of getting pregnant should steer clear of the handsome lotharios.

The scientists' theory proposes that males have evolved to look for the optimum 'sperm load' per mating.

This varies depending on how many available females there are to mate with.

Males with the opportunity to mate with a lot of females would be likely to produce less sperm on each occasion than those making fewer sexual conquests.

A smaller 'sperm load' reduces the chances of any individual female getting pregnant. However, this is outweighed by the fact that many different females can be impregnated.

Such a trade-off is seen in the wild and has been observed in chickens and fish.

Researchers examined the concept of 'spreading sperm' mathematically in a paper to be published in the journal American Naturalist.

One member of the team, Phd student Sam Tazzyman, of University College London, said: 'In some species, females mate with many different males.

'Since males have finite resources to allocate to breeding, they allocate them carefully to each mating to maximise their number of offspring.

'The more attractive a male is, the more females will be willing to mate with him, reducing the value of each mating to him.

'This means it is optimal for him to contribute-fewer sperm per mating. Although this reduces fertility per mating, it maximises the number of offspring he sires overall.

'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 rather paradoxical prediction that matings with attractive males may be less fertile than those with unattractive males.'

Whether or not the same principle applied to humans was unknown, said the researchers, but, despite complicating factors such as cultural preferences, the same forces such as the number of partners and attractiveness were involved.