A mission to their last known habitat in northern Cameroon failed to find any rhinos or signs of their existence.

The sub-species has declined in recent decades due primarily to poaching, which has also brought the northern white rhino close to extinction.

In East and Southern Africa, numbers of related sub-species are rising with the use of effective protection measures.

But after two decades of warnings, the western black rhino has apparently met its final end, according to the findings of an extensive expedition by three specialists earlier this year.

"They looked for spoor, they looked for the rhino's characteristic way of feeding which has an effect like a pruning shear, but they didn't find anything to indicate a continued presence in the area," he told the BBC News website.

"They did, however, come across lots of evidence of poaching, and that's the disconcerting thing."

Bleak prospects

Even before this latest survey, prospects for the sub-species appeared bleak. In 2002, numbers were as low as 10. The animals were distributed over a wide range, making breeding more difficult.

"With small numbers, bad luck can play a much bigger role - if you just have male calves, for instance," commented Dr Emslie.

During the last 150 years, numbers of all types of rhino plummeted in all regions of Africa. The southern white rhino reached its nadir in 1895, with a single population down to about 30 individuals in one South African game park.

Since then, captive breeding and successful protection measures have brought numbers up to nearly 15,000, and groups have been re-established in other countries.

The black rhino's decline came later. The continent-wide population numbered about 100,000 in 1900, but fell to a low point of 2,400 by 1995. Again, protection measures and breeding programmes are bringing stocks back up, but only, so far, to about 3,600.

The main successes have been in Southern Africa, with some East African countries also re-introducing and maintaining populations.

It is a different story in West Africa, where poaching, often fuelled by the guns and poverty of civil conflict, has been harder to control.

The northern white rhino is down to as low as four individuals in its only remaining habitat in the Democratic Republic of Congo; and now the West African black rhino has apparently vanished entirely.

Although genetically distinct, the different sub-species may be similar enough in their food and habitat requirements that animals could be re-introduced to West Africa from other parts of the continent.

But that would require stable political and economic conditions, the resources to take on poachers, and the commitment to involve local people in the animals' conservation.

Even if this were possible at some unspecified time in Cameroon, it appears that one of Africa's great wildlife icons has now lost a valuable branch of its family.