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Researchers Split African Elephants Into Two Species
21 December 2010 5:37 pm
It would be hard to confuse Africa's forest elephants and savanna elephants. Forest elephants, found in dense West African forests, have longer, straighter tusks and round, not pointed, ears. They're also 1 meter shorter and weigh half as much as the savanna elephants, which range from South to East Africa. Yet for years, scientists have classified the two as the same species, arguing that they were slightly different populations that mingled on the edges of the forest. A new genetic analysis, however, finds that forest and savanna elephants are as different from each other as modern Asian elephants are from ancient mammoths. The findings, which split the elephants into two species, could improve the conservation of African elephants overall, say researchers.
The study is not the first to analyze the elephants' DNA. In 2001, researchers compared the mitochondrial DNA (mtDNA) of forest and savanna elephants and reached much the same conclusion. (mtDNA is inherited only from the mother and is found in mitochondria, the cell's energy factories.) And a subsequent study of the forest and savanna elephants' nuclear DNA showed that the two had diverged more than 3 million years ago. Both studies concluded that forest and savanna elephants are separate species, but they did not sway all taxonomists, who felt that certain data suggested that some forest and savanna elephants shared a recent maternal ancestor.
Many studies use mtDNA to determine whether a species designation is valid. But mtDNA has its limitations. It represents only a small fraction of an animal's genome (the rest is nuclear DNA), and because it is transmitted only from the mother, it reveals just the genetic history of females.
To resolve the debate, an international team of scientists once again compared the animals' nuclear DNA. But this time, they analyzed large amounts of nuclear DNA sequences from one individual of each of the three existing elephant groups (Asian elephants, African forest elephants, and Africa savanna elephants), and from two elephant species that recently became extinct (a mammoth and a mastodon). It's the first time that scientists have sequenced a mastodon's nuclear genome and the first time that the five species' nuclear DNA has been compared.
"It was a big challenge to extract the DNA sequences from the fossil mammoths and mastodons and then to line these up with DNA from the modern elephants," says Nadin Rohland, an evolutionary geneticist at Harvard Medical School in Boston and the study's lead author.
The results, published online today in PLoS Biology, should finally convince the skeptics, the researchers say. They corroborate the previous nuclear DNA findings and show that savanna and forest elephants separated between 1.9 million and 6.7 million years ago. Asian elephants and wooly mammoths began diverging then, too.
The study also reveals that the species have surprisingly different amounts of genetic diversity. Savanna elephants and wooly mammoths have very low diversity, whereas the genetic diversity of forest elephants is very high; Asian elephants fall in the middle. The differences likely reflect the elephants' social behaviors, says Alfred Roca, a conservation geneticist at the University of Illinois, Urbana-Champaign, and one of the study's authors. "In savanna elephants, the large males dominate the matings. Apparently, based on what we see, the wooly mammoths had a similar type of male competition" for matings.
The team thinks this explains the previous mtDNA results, which suggested that some forest elephants shared a maternal ancestor with savanna elephants as recently as 500,000 years ago. Because the savanna males are larger, they likely outcompete the forest males in the areas where the two groups of elephants overlap, effectively erasing their deeply divergent genetic history.
If the report's conclusions are accepted by the African Specialist Group of the International Union for Conservation Nature (IUCN), the African elephant (Loxodonta africana) may be split into two species: L. africana, for those living on the savanna, and L. cyclotis, for those in the forest. Right now, the two are classified as subspecies: L. africana africana and L. africana cyclotis.
"This is wonderful work and a major step forward in our understanding of the relationships of elephants," says Robert Fleischer, geneticist at the Smithsonian Institution in Washington, D.C. "It's an absolute tour de force, with immediate consequences for the conservation of elephants," adds conservation geneticist Sergios-Orestis Kolokotronis of the American Museum of Natural History in New York City.
That's because poachers are decimating the forest elephants, says Samuel Wasser, a conservation biologist at the University of Washington, Seattle. Although African elephants are listed as an endangered species, with between 500,000 and 600,000 left in the wild, some African countries continue to push for legalizing trade in ivory tusks. If forest elephants, which number approximately 20,000 are recognized as a separate species, they may garner more protection. "[They] might have a fighting chance," he says.
Diane Skinner, a spokesperson for the African Elephant Specialist Group in Nairobi, Kenya, says that the group will review the new study. But for now, the forest elephant's status remains unchanged.