In 1979, two conservationists were on a field trip in a remote forest in the Udzungwa Mountains in Tanzania when they heard the call of an animal that they recognised as being that of a mangabey, a monkey that was not previously known from these mountains. On investigating further, they found that an unknown species of mangabey was involved, the first new species of primate to be discovered in East Africa for many years. The species was described in 1986 by the American primatologist Russell Mittermeier, who gave it the name Cercocebus sanjei.