A group of rangers had collected piles of plastic bottles for recycling from a beach at the edge of the Tangkoko Batuangus Nature Reserve on the Indonesian island of Sulawesi. Most of the bottles had washed up from the sea. It wasn’t long until a Celebes crested macaque came to investigate.

The local Celebes crested macaques have learnt that these plastic bottles contain liquid and are often seen chewing off the caps to reach the contents. They have even worked out that the coloured bottles often contain sweet liquids so pick these ones out. Some even carry bottles away into the forest, frustrating the rangers’ efforts.