"A Celebes crested macaque investigates the contents of a plastic bottle from a pile ready for recycling on a beach at the edge of Tangkoko Batuangus Nature Reserve, Indonesia.

Most of them had washed up from the sea, and despite the sizeable pile, more would soon be added. The Celebes crested macaques have learnt that these bottles contain liquid and are often seen chewing off the caps to reach the contents. They’ve also worked out that the coloured bottles are more likely to contain sweet liquid and so pick these. Some even carried bottles away into the forest, frustrating the rangers’ efforts."