Male drills are much larger than females and can weigh up to 30 kilograms. Their colourful face markings are used to communicate - a brighter face is a sign of a stronger, healthier male, and females tend to prefer males with the most vivid colours. The males also use expressions and postures to show dominance, calm other group members, or signal danger.
Drills are social animals that live in large groups called hordes, sometimes containing 100 or more individuals. Inside a horde, smaller family units of one male and several females stick close together. The groups travel together through the forest, foraging for fruit, seeds, fungi, insects and small animals. Their calls - booming and surprisingly loud - help keep the group in contact.
Drill monkeys are an important species for the forest because they spread seeds as they move around eating fruit. They are also what scientists call an 'indicator species' - if drills are thriving in a forest, it usually means the forest itself is in good health. When their numbers drop, it often signals a wider problem with the ecosystem.
Drills are listed as endangered and are found in few places in the world. The Limbe Wildlife Centre in Cameroon cares for rescued drills and runs a breeding programme to help the species recover. Visitors to the centre can watch drills in large forested enclosures and learn about the conservation work being done to protect them.