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Desert-Adapted Crocs Found in Africa
The discovery of several small communities of crocodiles living on the southern edge of Africa's Sahara in desert conditions has astonished scientists.
Crocodiles rarely venture far from water. But the crocodiles in Mauritania are living in caves, burrows, and under rocks, near wetlands that dry up and disappear for months at a time.
"It's a really unusual, unexpected, and quite special find," said James Perran Ross, executive director of a crocodile specialist group of the IUCN-World Conservation Union and a research scientist at the Florida Museum of Natural History.
Adam Britton, a zoologist and crocodilian expert at Australia's Wildlife Management International, concurs. "The Sahara desert is definitely a surprising location to find a crocodile," he said. "The fact they can survive at all is testament to their remarkable ability to adapt to relatively hostile conditions."
The desert crocodiles have adapted to the changing environment in northern Africa; 8,000 to 10,000 years ago, what is now desert was probably lush savannah and grasslands. Today the Sahara is hot and arid, the land sandy, rainfall minimal, and vegetation sparse.
"The extension of range almost certainly reflects climatic changes," said Ross. "We know that even in Roman times, the Sahara was much wetter and greener than it is now. As these places slowly dried up, remnant populations became isolated from the other crocodiles on the continent. How these populations adapted to the changing conditions is most interesting."
The desert crocodiles were brought to world attention by Tara Shine, a Ph.D. student at the University of Ulster in Northern Ireland, who learned about them while working in Mauritania under the auspices of a German international aid group.
Shine was conducting a random survey of 244 ephemeral wetlands in the eastern part of the country. Ephemeral wetlands are fed by rain rather than a permanent source of water, and evaporate for several months a year during the dry season.
"The inventory collects data on the wetlands based on local knowledge, and crocodiles were reported at 19 percent of the wetlands surveyed," she said. "I kept hearing stories from the local people that there were crocodiles, but it was hard to believe. They'd never been seen this far north."
Intrigued enough to investigate more closely, Shine found crocodile populations at eight sites; anecdotal reports from local people indicate there are perhaps as many as 45 small, isolated populations living on the edge of the Sahara. "The actual number is probably nearer to half of that," she said.
The crocodile populations are small and isolated from one another, as well as from other crocodile populations on the continent. Shine found a range of populations at various sites. Some spots had as many as three dozen crocodiles, others had only a lone animal"and when that one goes, there goes that population."
Scarcity of Water
The desert crocodiles inhabit two types of wetlands: lowland wetlands and a type of wetland known as a guelta. A guelta is formed when rain, or sometimes underground springs, forms a pool of water in a depression of a rocky plateau. Lowland wetlands in Mauritania are formed when rainwater collects in clay-lined depressions in otherwise dry-land conditions.
The wetlands are surrounded by sand dunes studded with rocky outcrops, said Shine.
"There's no permanent river or other source of water where these populations are living, and there's a tremendous amount of variation in the amount of rainfall from place to place and from year to year," said Shine. "All the water creating the wetlands is runoff from the rains, so how big [the wetlands] get, how deep, and how long they last depends directly on how much it rains." They can be dry for six to eight months a year, she noted.
When the water evaporates, the crocodiles estivate, or pass the summer in a kind of torpor. "It's a kind of dry-season hibernation. They don't eat and they keep movement down to a minimum," said Shine. "They sometimes come out at night and lie on the rocks outside their burrows or caves."
Other crocodilian species estivate, but not for so long or under such dry conditions.
Although the crocodiles living in these extreme conditions are much smaller than is typical for the Nile crocodile species, scientists are currently classifying them as Nile crocodiles, the most widespread species in Africa. Nile crocodiles normally reach about 16 feet (5 meters) long; the dry-land crocodiles are about 5 feet (1.5 meters) long.
"The dwarfism exhibited by this population is typical of crocodilians in resource-poor areasthere simply isn't much food available for most of the year so they eat little, grow slowly, and reach small adult sizes," said Britton.
How Different Is Enough?
DNA analysis might help determine whether the Mauritania crocodiles are stuntedfrom not getting enough to eator are smaller as a result of genetic adaptation.
Wolfgang Böhme, a well-known herpetologist at the Koenig Museum in Bonn, is conducting DNA analysis in hopes of answering this question and to determine whether the adaptations made to live in the desert are strong enough to warrant calling them a new species.
Most experts consider it unlikely.
"There are many instances of crocodilian populationsincluding Nile crocodileswhich are isolated from each other, yet are not sufficiently dissimilar to count as new species, or even new subspecies," said Britton.
Nile crocodiles on the island of Madagascar, for example, are still similar genetically to those from mainland Africa, he noted, adding: "I would be surprised if the Saharan population was sufficiently different."
Shine and Hemo Nickel, a student at the University of Mainz in Germany, conducted some preliminary field studies under the guidance of Böhme, and hope to gain funding to further study the populations.
"We need to get hard data on population size, movementthere are some reports of the crocodiles hiking from a lowland wetland over the sand dunes to a gueltahow they compare to remnant populations found in Chad," said Shine.
"There's so much to look at," she said.
Extinction is always a threat to small, isolated populations, but the desert crocodiles may be luckier than most. They are revered by local people, who protect them from occasional poachers.
"There's a complete lack of fear of the crocodiles among the local people," said Shine. "They even swim and wade with them. There are no stories of attacks, even in times of duress."
The local people assign a noble, almost mythical value to the crocodiles, and believe if the crocodiles go, the water will disappear, Shine explained. "Water is everything in a place where people and herds rely on such limited supplies," she said, "and the crocodiles are tied to the water."
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