Women longing for a baby advised to opt for ugly men

Handsome men who are lucky in love may not be the best bet as a mate for women hoping to get pregnant, researchers have said.

Uglier men with fewer notches on their belts are likely to be more productive between the sheets, it is claimed.

Research suggests that in many species, the most desirable males restrict their "sperm load" with each mating to ensure enough to go around.

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 might be advised to avoid handsome lotharios.

The 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, and what the chances of mating with them are.

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 modelled 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, from the Centre for Mathematics and Physics in the Life sciences and Experimental biology (CoMPLEX) at University College London, said: "In some species, females mate with many different males. Each male's sperm competes with that of other males in a process known as 'sperm competition'. Since males have finite resources to allocate to breeding, they allocate them carefully to each mating to maximise their number of offspring.

"If a male puts a lot of resources into each mating he will get more offspring per mating, but at the expense of fewer matings. If, on the other hand, a male puts few resources into each mating he will secure less paternity per mating, but will be able to carry out more matings overall. Thus, there is a trade-off between number of matings and success per mating.

"How a male negotiates this trade-off depends on how easy he finds it to attract females. 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 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, including cultural preferences," said Mr Tazzyman.

"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."