In a shaded rock pool in north-eastern Chad, about a thousand kilometres from the nearest reliable river, a handful of crocodiles are still hanging on. They live at the Guelta d’Archei, a permanent pool on the Ennedi Plateau.
As the reptile specialist Klaas de Smet reported in a review of the Saharan record, only a few survive in pools in a handful of river canyons on the Ennedi plateau, where they face extinction. This is not a healthy population. It is a leftover, stranded in one of the driest places on Earth.
The obvious question is how a big, water-loving reptile ends up living in the middle of a desert. The answer is that the crocodiles didn’t move to the Sahara. The Sahara moved.
What is actually living in that pool
For a long time these desert crocodiles were treated as the same animal as the big, famous Nile crocodile. That turns out to be wrong. They are West African crocodiles, Crocodylus suchus, a separate species mistaken for the Nile crocodile for over a century.
Research on West African crocodiles notes that C. suchus has a smaller size and non-aggressive behaviour than its larger, more aggressive cousin, which is part of why it can hang on in tight, harsh spots where a bigger animal could not.
The pool is not the palm-fringed oasis you might picture. It sits in a narrow canyon with high walls that keep the water shaded through the hottest part of the day. Seasonal rain fills it, and the rock around it keeps it from drying out.
Fish live there, algae grows there, and the crocodiles find just enough to get by. This is survival at the very edge of what a crocodile can take.
How they got there: when the Sahara was green
Between roughly 11,000 and 5,000 years ago, the Sahara was not a desert at all. During what scientists call the African Humid Period, monsoon rains reached far to the north and filled the region with lakes, rivers and grassland. Rock art from that time shows elephants, giraffes, rhinos and hippos. This was a well-watered landscape across much of what is now sand.
Two lines of evidence show the crocodiles once lived across this whole area, and they agree. De Smet wrote that “Middle Holocene remains and rock paintings show that the Nile crocodile” once lived across the whole Sahara. Fossil bones prove the animals were physically there. The rock paintings prove they were common enough that people bothered to paint them.
What makes the story sharper is how fast the green landscape disappeared. Dust records from the seabed suggest the humid period started and ended abruptly, each within just one to two centuries, a blink in geological time. The rains failed, the lakes shrank, and the water retreated into a few permanent rock pools. The crocodiles that survived were the ones that happened to be sitting in the right pools when the water ran out.
How they got trapped, and why so few are left
Once the region turned to extreme desert, there was nowhere for the crocodiles to go. Each surviving pool became an island, cut off from the next.
Drying was only half of it. The team led by José C. Brito found that crocodiles were once widespread across the Sahara, and that a drier climate combined with human persecution wiped most of them out locally. This is one detailed set of surveys rather than the final word on every colony, but the pattern holds: as the habitat shrank, people finished off what the climate had already weakened.
What a colony of a few crocodiles actually means
Mauritania is where you see just how broken up these groups are.
Brito and colleagues reported that “Relict populations of Crocodylus niloticus persist in Chad, Egypt and Mauritania.” Their fieldwork found crocodiles at 78 locations across 10 river basins, but at most of them they counted fewer than five animals. Some survive the dry season by hiding in caves and burrows and waiting the drought out, which sounds almost impossible if you only know crocodiles from a riverbank.
The same surveys listed the threats. Several pools are drained by herders and their livestock, causing water shortages in the dry season. Climate forecasts for the region predict more heat and less rain, which are expected to increase population isolation and local extinction. A group of five or ten animals in a single pool has almost no margin. One bad drought, one poisoned pool, or a run of years with no surviving young, and the colony is gone.
These animals are, in a real sense, a living record of a climate that no longer exists. The crocodiles at Archei are not thriving in the desert. They are the last few who never left a place that dried out from under them, still holding ground their species held when the Sahara was full of water. That they are there at all is surprising. That they are there in such small numbers is, on the evidence, a situation no amount of admiration for their toughness can fix.