Mona monkeys live in groups, called 'troops', of about 12 to 30 monkeys. The troop moves together through the trees, eating fruit, leaves and the occasional insect. Each monkey has its own job - some watch for danger, some look after the babies, some lead the way to the next fruit tree.
Their cheek-puffs are not just decoration. Mona monkeys can stuff their cheeks full of food - like little squirrel-pouches - and carry the food away to eat somewhere safer. A monkey with stuffed cheeks looks a bit like a child with too many sweets in their mouth.
In one famous Ghanaian village called Boabeng-Fiema, the mona monkeys are treated as friends. The villagers protect them, and the monkeys are so used to people that they walk through the village and even into people's gardens. When a mona monkey dies in Boabeng-Fiema, the villagers give it a proper burial.
Mona monkeys talk to each other with lots of calls. There are different sounds for 'food over here', 'danger from a snake', 'danger from a bird of prey', and 'just saying hello'. A scientist who knows the calls can tell exactly what is happening in the forest without seeing the monkeys.