Samuel Wasser of the University of Washington and his colleagues started by collecting DNA from nearly 400 elephants at 28 reference sites across Africa. They extracted some DNA from tissue samples, but they also sampled dung, which is faster and less stressful for the animals, according to Wasser. The geographic variation of specific DNA markers was then mapped separately for savanna and forest elephants, which appear to be two separate species.
Conventional tracing techniques can only show which reference sites a new DNA sample matches best, Wasser says, but chances are it didn't come from any of those locations. He and his team developed new statistical methods that generate a swarm of likely locations. For a sample taken from one of the reference sites, all of the likely origins were close by. Even when the reference DNA from that site was excluded, the possibilities clustered within a few hundred kilometers of the actual origin. Wasser hopes that the results will stimulate people to collect more reference samples, especially in the forests of central Africa, to improve the technique's precision.
Once a map is created, tracing a single ivory sample costs about $100, plus labor. Wasser's team is currently analyzing a 6.5-metric-ton cache of ivory seized two years ago in Singapore. Knowing where that ivory came from should focus both legal and political efforts to prevent further poaching in the area of origin.
More importantly, Wasser says, the technique should clarify how policy changes affect poaching. In 1989, after the number of African elephants had declined from 1.3 million to 600,000 in less than a decade, the Convention on International Trade in Endangered Species (CITES) banned all international trade in ivory. CITES has recently allowed the resumption of some trade from southern Africa, where the elephant population is relatively healthy, and may make further changes when it meets in Thailand next month. Critics worry that smugglers can bring tusks into southern Africa from other regions. When they do, the new DNA-based methods should be able to help track them down.