One of four sub-species of African black rhinoceros has been declared extinct after researchers failed to find the animal in its last known habitat.

The West African black rhino (Diceros bicornis longipes) had fallen from a population of around 3,000 to just 10 in recent years but today specialists from the World Conservation Union (IUCN) said no traces of the creature had been found during a recent survey in northern Cameroon.

"As a result this subspecies has been tentatively declared as extinct," said Dr Martin Brooks, the chairman of the African Rhino Specialist Group at the IUCN’s Species Survival Commission.

The sub-species, one of six types of rhino found in Africa, had been decimated by poaching for its horn, which is used as an aphrodisiac in Yemen and China. The long porous border between Chad and Cameroon, where the rhino enjoyed West Africa's savannah, prevented easy protection.

In 2000, Dr Brooks estimated that just ten of the rhinos were still alive and could have drifted too far apart from each other to breed.

The IUCN said today that another sub-species of rhino, the northern white, was also on the brink of extinction. Just four animals were found by ground and air surveys in its last refuge, the Garamba National Park in the Democratic Republic of Congo.

"Efforts to locate further animals continue, but we must now face the possibility that the subspecies may not recover to a viable level," said Dr Brooks.

The IUCN was able to give better news on the state of the two other rhino sub-species, the continental black and the southern white. There are now an estimated 3,725 continental black rhinos, up from 2,410 in 1995. The southern white rhino has rebounded from a population of just 50 animals a century ago to a present total of 14,540.

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