The Drill Monkey varies in size from 28 to 44 pounds. They live in very large social groups that can have up to 20 members. There are a few males that are dominant but one is the leader. The Drill Monkey has a very complex hierarchy.
The diet for this monkey consists mainly of plants and fruit. The availability of the fruit is seasonal and they are able to remember where to get it during various times of the year. The Drill Monkey is very colorful and it seems that as they get older they develop more coloration. That may be part of their social status for the group.