The theory emerged following the latest research into why men have larger brains than women.

Researchers now plan to test their working hypothesis - that the extra brain mass is used for visuospatial skills.

They point to evidence that men can read maps better than women and have a better awareness of their surroundings.

These essential tools in the search for sexual partners would mean that large-brained men would be more likely to make it through the perils of natural selection.

More grey matter

The researchers found that humans are not the only primates where males have larger brains than females.

They conclude this means men's larger brains did not come about through development of the higher cognitive abilities that distinguish humans from other primates.

Instead, they say, it could be because they have better visuospatial skills.

Men's brains are on average one tenth larger than women's, with a mean weight of 1347g compared to 1223g.

Professor Dean Falk, of Albany University New York, wanted to see if the difference existed in other primates too.

She and colleagues examined rhesus monkeys and discovered a similar disparity in brain size between the sexes.

The significance of comparing rhesus monkeys to humans lies in earlier studies of voles.

Primates leave their birth groups to mate

Some species of vole are polygynous - meaning they go in search of sexual partners during mating season - while others are not.

Scientists who have studied voles have found differences between the sexes in such species.

Professor Falk said: "They have shown that in those species and only in those species you have extensive differences between the sexes in their mapping skills and their visuospatial skills with males outperforming females.

"We were wondering if the difference we see in humans - with men have bigger brains than women - and an equivalent difference we see in rhesus monkeys had something to do with the visuospatial skills."

Men consistently outperform women at such visuospatial tasks such as the mental rotation of figures and map reading. This could be what they use the additional brain mass for.

Sex search

Usually in primates other than humans, one sex leaves the birth group in search of a mate to prevent inbreeding, Professor Falk said.

"When rhesus monkeys reach puberty it's the males who change birth groups and travel away.

"It's possible that in terms of our own evolution many millions of years ago that in our own species it was men who did the wandering."

The researchers plan to test their theory by examining rhesus monkeys' visuospatial skills.

They will also look at macaque monkeys, where it is the female who wanders away from the birth group, Professor Falk said.

"Whatever they wander for," she added, it was likely to help them survive natural selection because they would be "perpetuating their genes into the future".