By Ed Yong of Nature magazine

The iconic Nile crocodile actually comprises two different species -- and they are only distantly related. The large east African Nile crocodile (Crocodylus niloticus) is in fact more closely related to four species of Caribbean crocodile than to its small west African neighbour, which has been named Crocodylus suchus.

Evon Hekkala of Fordham University in New York and her colleagues revealed evidence for the existence of the second species by sequencing the genes of 123 living Nile crocodiles and 57 museum specimens, including several 2,000-year-old crocodile mummies.

The results resolve a centuries-old debate about the classification of the Nile crocodile, and have important implications for the conservation of both species. "The paper has generated a great deal of interest and support," says Grahame Webb, head of the specialist crocodile group of the International Union for Conservation of Nature.

Croc-infested waters

Hekkala's work began when she received a sample from herpetologist Michael Klemens of the Wildlife Conservation Society. In Chad, Klemens had stumbled across six crocodiles in a small oasis and, at his guide's recommendation, jumped in with them. Puzzled by their docile behaviour, Klemens took tissue from a dead one. He sent the sample to the American Museum of Natural History in New York, where Hekkala and her co-workers sequenced it.

"I kept on sequencing it because I was convinced I was 100% wrong," says Hekkala. "It wasn't even remotely related to the Nile crocodile samples I had been working on."

Hekkala's group collected as many Nile crocodile samples as they could find, including several from ancient mummified animals. All of the mummies were of C. suchus, indicating that the ancient Egyptians had recognized the differences between the two reptiles. Indeed, the ancient Greek historian Herodotus wrote that the Egyptians selectively used a smaller, tamer crocodile in ceremonies and regarded it as sacred.

The name C. suchus was coined in 1807 by the French naturalist Geoffroy Saint-Hilaire, who singled it out as a subspecies of Nile crocodile. However, his ideas were not widely accepted. "He called it the sacred crocodile in one of his papers," says Hekkala. "We've talked about proposing that as a common name."

Hekkala is now working to formally describe the new species. "Crocodiles are generally very hard to tell apart from their exterior features," she says. Even so, unpublished preliminary work suggests that C. suchus and C. niloticus have distinct skulls, and research in the 1970s by the leather industry suggested that they have different scale patterns.

Hekkala says that her results refute the idea of crocodiles as "living fossils" that have remained unchanged for millions of years. "Our paper shows that the true Nile crocs in east Africa are as young as humans are. C. suchus is only slightly older."

Shrinking range

Analysis of the mummified DNA suggests that both species used to overlap throughout Africa. This is no longer the case. " C. suchus seems to have recently gone extinct from suitable habitats in the Nile, perhaps because C. niloticus grows bigger and is far more aggressive," says Dietrich Jelden at the German Federal Agency for Nature Conservation in Bonn.

While C. suchus was retreating into the dry interior of western Africa, C. niloticus spread across the Atlantic to the Caribbean. From eastern Africa, the crocodiles could have ridden currents that flow around the cape and into the Caribbean. "Traders used these during the slave trade, so transoceanic dispersal isn't unrealistic," says Hekkala, "especially since crocodiles can store sperm and go without eating for up to 10 months."

The shrinking size of C. suchus ' range has immediate implications for conservation. The newly confirmed species is already declining and is under threat from industries such as oil extraction, as well as from unregulated trading of skin and bushmeat.

Meanwhile, the group's finding halves the range of the traditional Nile crocodile. "It's going to shake things up," says Hekkala. The Nile crocodile has long been viewed as a model for the sustainable use of wildlife and several nations had plans to increase their harvest of the species for its skin. Hekkala's discovery could put paid to that. "Some people are going to be very unhappy," she says.

This article is reproduced with permission from the magazine Nature. The article was first published on September 14, 2011.