"They're these herbivores of the canopy that are a critical part of the ecosystem. We think they have a lot to do with processing seeds and germination in the forest."

Prof Detwiler speculates that their unusual, bright facial markings could be a visual signal to other animals, potentially attractive to a mate or that it could allow the animals to identify each other.

The monkeys also have a distinctive "roaring" call. "You often hear them, but don't see them," said Amboko.

They think the animals are rare and restricted to a part of the forest where they can find the food and habitat they need.

The animals are hunted for meat, so the researchers hope that now that Colobus congoensis is classified as a distinct species it can be officially protected.

The researchers say they still have a lot of questions about this newly described, secretive species. They plan to carry out a more detailed survey to estimate their population and study their behaviour.