Friendship among humans could have evolved to fight off threats from predators, scientists have suggested after studying monkey behaviour.
A study found female crested macaques are more likely to respond to alarm calls made by their mates.
Researchers used a mock python, a life-size picture mounted on a metal handle, and taped the alarm call each monkey made. Their friends looked for longer towards the sound.
Dr Bridget Waller, of Portsmouth University, said: "Strong social bonds could improve the coordination and efficiency of defence against predators and therefore increase the chances of survival."
Pythons prey on macaques which make a unique prolonged call when sighting the snakes to recruit others to join in a mob-handed attack.
"This is where friendship has an important role. Mobbing poses a significant risk on those taking part and its success as a tactic depends on coordination and cooperation," primatologist Jerome Micheletta said.