We have an intuitive sense that what we say – if we say anything at all – largely depends on whether somebody is listening.

It's a trait shared by our closest cousin, the chimpanzee.

When we eavesdrop on chimps, it's clear that much of what they say is for the benefit of their friends and enemies alike. They consider who their audience is before they make calls, for example.

They also make meaningful grunts for different types of food, and can be identified by their unique screams when they are fighting.

This happens frequently. Chimpanzees can be an aggressive bunch, especially the males.

But victims of aggression do not take it lying down. They tend to scream for help in the hope their companions will come to their rescue.

They will even exaggerate their screams when high-ranking individuals are nearby.

Researchers now believe they have decoded the differences between some of their screams, giving new insights into how chimps try to influence each other. 

Their screams serve different social functions depending on their audience. These include: signalling submission, signalling the readiness to retaliate, or recruiting aid from a nearby onlooker.

But occasionally, a chimpanzee that is being picked on by an enemy will follow up a scream with a bark-like sound, known as a "waa bark". Nobody knew what these waa barks were for.

To find out, Pawel Fedurek of the University of Neuchâtel in Switzerland and his colleagues listened to screaming chimpanzees among a group of 54 in Budongo Forest, Uganda. They noted every time a chimp barked. Listen carefully to the sound file above and you can hear two waa barks: they come after three screams.

The chimps that made the "waa" sounds were more likely to retaliate against their aggressors, rather than reconcile with them,

"This behaviour may express a victim's unwillingness to behave submissively towards his aggressor, which can contribute to his social status in the long term," says Fedurek.

While reconciliation is an important component of chimpanzee society, social status is too. The waa barks might play a role in empowering a chimpanzee to get ahead.

However, victims of aggression were more likely to scream (not waa bark) when the aggressor was a high-ranking, dominant male. That suggests they could judge whether or not to submit.

These slightly different screams may therefore play a key role in influencing the outcome of the aggressive event, says Fedurek.

The study has been published in the journal Animal Behaviour.

"Screams are primarily directed at bystanders to attract help," says Fedurek. "Waa barks, on the other hand, are directed at the aggressor to repel him."

Chimpanzees' vocal repertoire is somewhat limited: they can't make nearly as many sounds as we can. But this finding does suggest that their communication is more complex than previously believed.

Just like us, chimpanzees can use simple sounds to convey more than one message.

"The evolutionary basis of human language seems to be deeply rooted in the primate lineage," says Fedurek.