Nile crocodile is two species

Discovery has implications for conservation and management of the iconic species.

                       

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.

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