Dian Fossey ( January 16 , 1932 -- December 26 , 1985 ) was an American zoologist . Dian Fossey ( daÉ Ë Ã n Ë fÉ 's i , January 16 , 1932 â `` December 26 , 1985 ) was an American zoologist who undertook an extensive study of gorilla groups over a period of 18 years .