Then oil prices soared, resulting in a seven-fold increase in income per head in Yemen, where elaborately carved rhino-horn dagger handles are prized as a sign of status and wealth.

Demand for the horn has risen along with prices, since it is now also peddled as a cure for cancer.

"Horns have been appearing in London at these natural history sales," says Martin, "and before, the price used to be quite low.And now, they're paying up to a $100,000 for a pair of black rhino horns.