The researchers estimate the endangered subspecies living in the Ivory Coast has dropped in numbers by 90 percent since the last survey 18 years ago.

"The few remaining chimpanzees are now highly fragmented, with only one viable population living in Tai National Park," the researchers said. The steep decline at what was considered one of the final strongholds for West African chimps suggests the animals' status should be raised to critically endangered, said Genevieve Campbell of the Max Planck Institute for Evolutionary Anthropology.

Campbell and Christophe Boesch, also from the Max Planck Institute, said the increasing human population in the Ivory Coast is the cause of the chimpanzees' demise.

In the 1960s, the Ivory Coast chimpanzee population was estimated at about 100,000. At the end of the 1980s, the total population of chimpanzees was estimated at 8,000 to 12,000.

The detailed report appears in the journal Current Biology.