Horns of a Dilemma

In 1900, millions of rhinoceroses walked the earth. In 1991, about 11,000 remain, divided among two African species and three Asian ones. They've been hunted almost to extinction because their horns are coveted for Chinese folk medicines and Yemeni dagger handles.

Rhino horns aren't particularly good for either purpose, but human desires needn't be based on reality. The craving for rhino horn is backed by money: an adult African rhino's nose ornament, which can weigh ten pounds, is worth $2000 a pound in Taiwan. The trade is illegal under international law, yet the potentially enormous profits keep sales going.

According to a recent article in Bioscience magazine, the nations of Zimbabwe, South Africa and Namibia have strived to control poaching and habitat destruction threatening their rhinoceroses. Armed guards and watchful patrols are maintaining stable or even increasing rhinoceros populations in those countries.

But the rhino-protection programs are extremely expensive. In looking for ways to pay for them, the African countries have been tempted to sell an appropriate natural resource: their stockpiles of rhinoceros horn. From confiscated poachers' hoards, from dead animals, from horns removed by causes natural (they get knocked off in fights) or unnatural (Namibia has been trimming off its rhinos' horns to make the animals less attractive targets), their supplies are growing. South Africa's alone increases by $1 million every year.

Despite the logic of having rhinos pay for their own protection, the possibility of legal rhino-horn sales worries conservationists. Keeping illegal goods out of the market could become too difficult. As a minimum first step, rhino horn would need permanent tagging so its source could be accurately identified.

Nature, it turns out, has provided an unalterable tag. It's in the isotopes, particularly the proportions between carbon 13 and the much more abundant carbon 12. (Regular readers of this column might recognize those carbon isotopes because they feature in some Alaska work noted here earlier such as Don Schell's with polar bears and Tom Kline's with salmon). The African countries turned for help to an archaeologist who uses isotopes, Nikolaas van de Merwe.

Van de Merwe came to the attention of South African officials because of his work deducing the diets of long-dead animals. Such work is possible because every chemical step in plant photosynthesis changes the normal atmospheric ratio of carbon 12 to carbon 13. The heavier carbon 13 participates in chemical reactions more slowly than does the lighter carbon 12, so every reaction lowers the ratio of carbon 13 to carbon 12. Different groups of plants have different numbers of reactions, and so amass different ratios of carbon 13. That ratio is conservative---it leaves a clear trail through everything that eats the plants or the plant-eaters.

After working with tricky fossils, van de Merwe found it easy to read the trail in recently harvested rhino horns. Using a gas mass spectrometer to analyze horn parings, he could quickly distinguish between the two African rhino species. Black rhinos browse on twigs and leaves, while white rhinos eat grasses; the two kinds of foodstuff have very different isotopic signatures, and those showed up in the rhino horns.

For more information, he turned to isotopes of other elements. The more arid a region, the higher the ratio of nitrogen 15 to nitrogen 14 in the tissues of its plants and animals. The ratios of strontium and lead isotopes reflect the age of a region's parent rock. Thus, for example, Namibia's ancient deserts leave identifying traces in rhino horn very different from those left by Asian rain forests.

The African countries think van de Merwe's work makes a strong argument for opening limited and controlled trade in rhinoceros horn. Not everyone agrees, but experts acknowledge that something will have to change soon. As the assistant director of Zimbabwe's national parks complained, "It's fine to say rhinos belong to the world and must be saved, but the world isn't paying the costs. We are."