By Natasha Gilbert
African forest-dwelling elephants (Loxodonta cyclotis) are a separate species from those living in the African savanna (Loxodonta africana), researchers have shown.
Scientists have long debated whether African elephants belong to the same or different species. They look very different, with the savanna elephant weighing around 7 tonnes -- roughly double the weight of the forest elephant. But studies had suggested they were the same species -- DNA in mitochondria (the cell's energy factories) from African elephants found evidence of interbreeding between forest and savanna elephants around 500,000 years ago.
Now a group of scientists have taken a deeper look at the African elephants' genetic ancestry. The researchers sequenced the nuclear genomes of both types of African elephant, as well as that of the Asian elephant (Elephas maximus). They also extracted and sequenced DNA from the extinct woolly mammoth (Mammuthus primigenius) and mastodon (Mammut americanum) -- ancient elephant ancestors. By comparing all these genomes, the team found that the forest and savanna elephants diverged into separate species between 2.6 and 5.6 million years ago. The study is published online in the journal Plos Biology.
"They split about the same time as African and Asian elephants split into separate species, and much longer ago than people previously thought," says David Reich, a population geneticist at Harvard Medical School in Boston, Massachusetts, and a lead author on the study.
"You can no more call African elephants the same species as you can Asian elephants and the mammoth," he adds.
Most researchers agree that the Asian elephant and the mammoth are separate species, says Thomas Gilbert, a geneticist at the University of Copenhagen. "But this study really hammers the coffin shut on any arguments that the forest and savannah are anything but different species, or even genera," he says.
Mitochondrial DNA can only give researchers information on maternal ancestry, as this genetic material is inherited solely from the mother. Examining the nuclear genome, which is around 200,000 times larger than that contained in mitochondria, gives a broader and more accurate picture of elephants' history. "You get a different picture by looking at nuclear DNA", says Reich.
Mitochondrial DNA evidence suggesting that forest and savanna elephants interbred recently and had a recent shared female ancestor can be explained as a result of the female elephant's social behaviour, the researchers say. Females tend to stay close to their place of birth, while the males roam. Herds of female forest elephants could have repeatedly come into contact and bred with migrating male savanna elephants. Over a long period of time, the forest elephant gene pool would become diluted and displaced by that of the savanna elephants, but the forest DNA would be conserved in the mitochondrial DNA, which is passed on through the female line.
"What we see is an ancient split with a bit of gene flow more recently," he says. Hybridization happens between closely related animals and does not necessarily imply that the two are the same species, he says.
The authors suggest that the findings will help to reprioritize elephant conservation programmes. All African elephants are currently conserved as the same species. But the evidence that they are two distinct species suggests that they may be facing different pressures and require different conservation strategies. The forest elephants should become a greater conservation priority, the study says.