Female vervet monkeys manipulate males into fighting battles by lavishing attention on brave soldiers while giving noncombatants the cold shoulder, researchers said on Wednesday.

After a skirmish with a rival gang, usually over food, females would groom males that had fought hardest, while snapping at those that abstained.

When the next battle came along, both those singled out for attention and those aggressively shunned would participate more vigorously in combat, according to a study published in the journal Proceedings of the Royal Society B.

Female grooming and aggression "both appear to function as social incentives that effectively promote male participation in intergroup fights", a research team from Switzerland and South Africa reported.

They had observed four vervet monkey groups at a game reserve in South Africa for two years.

Vervet monkeys live in mixed-gender groups and both sexes take part in frequent battles with rival troupes. Only a handful fight each time.

Males are larger than females and have longer canine teeth, making their presence valuable in the front lines.

Success in battle ensures control over territory and food sources -- a key concern for females, who take care of the young.

But why would males risk involvement in a potentially high-stakes battle just for a bit of female attention? It's all about sex, the researchers believe.

"Receiving punishment" from females for not taking part in battles "could damage the... male's social relationship(s)" either with the female in question or "other female group members", the researchers wrote.

On the other hand, being rewarded could "potentially signal to other female group members that the... male is a valuable social partner", likely boosting "male mating success".

In group animals, such as humans, a delicate balance must be maintained between participating in hunting or defence, which can be risky, and free-riding, which is less hazardous but can lead to social rejection.

The riskiest group activity of all is warfare, and few animals other than humans and monkeys engage in it.