The Tana River Primate Reserve was formed to protect two endangered species of monkey; the crested Mangabey and the Tana River red Colobus.

It is also one of the last remaining relics of the once massive Central African Lowland Rainforest, but became isolated from the rest of the forest during the seismic eruptions that caused the formation of the Rift Valley.
As a result, much of the reserve's flora and fauna are unusual to East Africa and bear traces of ancient links to the Congo Basin forests of the Miocene period.