Stressed, aggressive, anxious? 

Crested macaques can recognise all of these facial expressions in their own species, a new study has found.

These monkeys, only found in the Indonesian island of Sulawesi, are an extremely social bunch with a limited hierarchical structure between group members.

That means these macaques tend to treat each other, for the most part, as equals. 

And an important part of their social lives, we now know, is being able to tell the emotion of close companions. That's the findings of a new study published in the journal Animal Cognition

This ability to recognise facial expressions is key for any social group of non-human primates.

That way they can infer intentions, emotional states and adjust their behaviour accordingly.

We are trying to argue that social tolerance is one of the important factors for social complexity

Researchers therefore wanted to confirm that the highly social world of crested macaques meant they too could easily categorise several emotions.

After learning how to use touch screens, the monkeys were able to correctly identify two comparable facial expression in a computerised game, played a bit like the human card game "snap".

Researchers presented three captive macaques with pictures and videos of various facial expressions such as anger or playfulness.

The macaques then had to select which expression matched the original picture or video.

The facial expressions used in the test are commonly observed in the wild, and included:

  • The yawn  - an indicator of anxiety or stress
  • The scream face - produced when the target of aggression
  • Silent bared teeth - equivalent of human smiles and often used to signal a peaceful intention such as grooming or an embrace
  • A mild expression of threat indicated by a half open mouth

Crested macaques now follow close in the footsteps of their more hierarchical cousins, the rhesus monkey. Others which have shown similar skills includes capuchin monkeys and chimpanzees.

In that respect, the discovery they can recognise the emotions of friends and foes may not be surprising.

This species, though, was a particularly interesting group to study, says lead author Jerome Micheletta of Portsmouth University in the UK, because of their so-called "social tolerance".

The more complex an animal's social world, the greater the evolutionary pressure to develop sophisticated ways of communicating.

"For us that would be language. We are trying to argue that social tolerance is one of the important factors for social complexity, " Dr Micheletta told BBC Earth.

"For a more socially tolerant species we would expect more complex communication," he adds.  

Understanding how non-human primates communicate could therefore provide insights into how our own communication system evolved.

The next step, says Dr Micheletta, will be to understand how this rare macaque species combines facial expressions with vocal signals and body postures.