In a primate sanctuary in South Africa, several capuchin monkeys live happily alongside other animals, including tortoises.

Occasionally they act with curiosity and mischievousness towards the tortoises that live there, says Claire Hamilton of the Monkeyland Primate Sanctuary.

They slap them, hit them with rocks, and sometimes even sit on them.

The attack appeared to be driven by pure anger

But one November morning in 2012, Hamilton witnessed something entirely different. Two angry capuchins crossed her path: a male and a female. They appeared to be directing their anger towards a nearby tortoise.

The female capuchin grabbed a branch and hit the tortoise on its shell once. She then picked up a large rock and threw it at the tortoise "with such precision and force that it bounced off the tortoise's back and landed over a metre in front and to the side of it", Hamilton reports.

This is the first time I had witnessed a capuchin use an object as an actual weapon

It was all over after 15 seconds. The tortoise suffered a small chip to its shell and quickly retreated.

Hamilton, who witnessed the incident, managed to take these photos.

Hamilton says the attack appeared to be driven by pure anger, rather than mischievousness, which is very out of character for the capuchins.

I have never witnessed any capuchin using violence without reason

While they often use tools, they rarely use them as weapons.

"This is the first time I had witnessed a capuchin, or indeed any of the primates, use an object as an actual weapon," says Hamilton. "I have observed them using sticks and stones as a mode to extract or open food items and often use them for fun, but never as an act of defense or aggression."

What was even more surprising was that the attack was entirely unprovoked. But that does not mean they are a violent species, says Hamilton

"They are defensive of their own groups and will resort to physical violence if required, but I have never witnessed any capuchin using violence without reason."

The observation has been published in the journal Neotropical Primates.