Genetics of Sumatra's at-risk rhino.

In many cases, an endangered species can more accurately be described as a collection of genetically distinct endangered populations. To maximize diversity, each population needs to be managed separately-if there are enough individuals to keep the population alive.

The two-horned Sumatran rhinoceros typifies this dilemma. Only about 300 animals remain in the wild, and one population, on the island of Borneo, is down to about 50 animals. The Sumatran is considered the most ancient of rhinos. Unlike other species, the Sumatran rhino is hairy and relatively small, measuring 8 feet long and weighing less than a ton. The rhinos in the Bornean population are the smallest and have a distinct skull shape.

In an article slated for the April Conservation Biology, Columbia University evolutionary geneticist Don J. Melnick and his colleagues quantify the genetic differences that go with the obvious physical differences. Analysis of DNA from mitochondria in the animals' cells revealed little difference between rhinos in eastern and western Sumatra but a 1 percent difference between the Bornean and other Sumatran populations, indicating an evolutionary divergence.

The Bornean rhino, which inhabits tropical forests, has been separate from the other rhino populations since at least 10,000 years ago, when the local terrain became an archipelago. Disappearing forest habitat and pressure from poachers in search of rhino horns have pushed all the Sumatran rhino populations to the edge of extinction. The researchers recommend that conservationists maintain the Bornean population separately, to preserve the set of genes best adapted to the island, but "unfortunately . . . we might not have the luxury."