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Large reptiles living along the almost-dry Boteti River have evolved a unique method of survival in a hostile environment.

We have a set idea about crocodiles (Crocodylus niloticus), do we not? We know them in the water, stalking prey from underneath without a ripple – perfectly adapted to these surroundings.

But here, along the Boteti River in Botswana, things are different. This river hasn't flowed in 15 years. There are a few very small, shallow pools remaining, yet there are plenty of crocodiles. Here they live in caves and have a completely different set of hunting rules.

We had to see this and so set off to where it was said there were plenty of small caves set in the cliffs. There we found the massive creatures, coloured a milky white from the dust, lying almost immobile in the heat of the afternoon.

At first we found two sharing a cave quite low down on the face of the slope, one with just its head peering out, the other stretched fully in the sun, warming its reptilian blood.

They made barely a movement – just the occasional blink of an eye. By slowing down their heart rate dramatically they can sit immobile all day, a case of 'eat no energy, burn no energy'. Any meal they have, even a small one like a bird, will sustain them for days, and if they're lucky enough to get something larger, they would be set up for months.

The sunning crocodile slowly opened its mouth, showing off its impressive teeth, and lay like that for minutes. This acts as a coolant for them. Being cold-blooded is tricky; they're sluggish in the cold and need warmth to function, but overheating can be a problem too. Eventually the 40 degrees C (104 degrees F) heat proved too much and the reptile retreated into its cave, only the head extended out into the shade.

We moved on and found another crocodile. Only its snout was visible in its cave, which was located a good 20m (66ft) up a sheer cliff face above a small pool. Judging from the size of its head, it was at least 3m (10ft) long and how it managed to traverse the steep, sandy pathway down to the riverbed and again up to its cave is hard to imagine. It is most likely a journey it would not undertake too often.

At the base of the cliff lay the clean bones of a zebra. It is possible that these crocodiles can still make a decent kill; perhaps they manage to take some meat from lion kills, but the most likely scenario is that they snap up unwitting birds.

There was some activity in the pool below. A bachelor herd of elephants drank there and a monitor lizard emerged from the water, feathers stuck on either side of its mouth. This reptile at least managed to find food here today.

We left in the late afternoon and on our way back saw two young crocodiles – most likely between two and three years old. One disappeared down its hole immediately but the other lay content in the evening sun.

These crocodiles seem to be doing well here if they're breeding and the youngsters are surviving. But then, this is a scenario that has not deviated in millions of years. If an animal can live well and reproduce in a variety of circumstances and ecosystems, why evolve any differently?

– by Vanessa Stephen, Earth-Touch © Earth-Touch

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