 The Bioko drill is of a similar appearance to the mainland drill, with a green-brown coat and white fur on the underbelly.It is distinguished by a yellowish crown of fur with black tips that lines the drill’s face, while the mainland drills have a crown that is mostly white.The drills are also extremely sexually dimorphic with the males being much larger, weighing 20kg on average in adulthood, whereas females typically reach 8.5kg.