Bat Poop Might Be Turning Gabon’s Cave Crocodiles Orange

In 2010, scientists started exploring the Abanda cave system in Gabon after getting a tip about something strange living there: crocodiles. And not just any crocodiles. Orange crocodiles.

Crocodiles will sometimes use caves as temporary refuges during droughts, but the dwarf crocodiles (Osteolaemus tetraspis) in the Abanda caves seemed to have taken up permanent residence there. When we wrote about the expedition a few years ago, the researchers didn’t know much about the cave crocs. They knew they were there, they knew they were weird, and they knew some of them had turned orange. The scientists, led by biologist Matthew Shirley, have since gone back into the caves to study the animals. Their new paper, published in African Journal of Ecology, overturns some of their early ideas about the Abanda crocodiles, showing that life in the caves has been good to them, and offers a new explanation for their strange colors.

After seeing the crocodiles for themselves, the team began surveying the caves and capturing crocodiles by hand, taking their measurements and determining their sex. To see what the reptiles were eating, they used a method called the hose-Heimlich technique, which is exactly what it sounds like. While one scientist flushed a croc’s stomach with an improvised stomach pump, another grabbed the animal and squeezed its belly, expelling the water and the stomach contents through the animal's mouth. They did the same with a group of crocodiles living aboveground at streams in the forest around the caves.

While the forest crocs barfed up freshwater crabs, shrimp, crayfish, and a variety of insects, the cave crocodiles were eating cave crickets and bats—and little else. The difference in diets, the researchers say, suggests that cave crocodiles don’t hunt or feed aboveground and likely have very little contact with their neighbors. The animals have to come out of the caves to breed and lay their eggs because there aren’t any suitable places to build nests in there, but they apparently don’t spend much time on the surface. They’re not entirely trapped in the caves, as the researchers once thought, but they’re still very isolated.

Abanda Expedition

Even with all that time spent in the caves, the Abanda crocs don’t appear to be changing in response to life underground. Despite some physical and genetic differences between them and the forest crocs, the researchers say that the cave crocodiles “showed no signs of physical adaptation, or repercussion, of living in a hypogean environment”—such as the reduced pigmentation or smaller eyes often found in other cave-dwelling animals. The only notable physical change the team thinks is connected to their lifestyle is the crocs’ orange skin. They initially thought that the color change might have been an adaption to living in darkness or caused by their diet. But they now have a different—and grosser—idea.

With thousands of bats hanging from the cave ceilings, the floor has become covered in a caustic mixture of water and bat waste. “We hypothesize that the orange coloration in large adults is caused by ‘bleaching’ of the skin after what is presumably years of inundation in bat guano,” the researchers say. In some cases, they found, the guano isn’t just bleaching the crocs’ skin and changing its color, but eroding it to the point where the scientists could clearly see the animals’ skulls peeking out through the skin.

Those don't sound like the best living conditions, but the caves offer plenty to make up for it. After taking all the animals’ measurements, the researchers found that the cave crocodiles were in better physical condition than any of the crocs living aboveground. The team thinks this is because the cave crocs’ prey—bats and crickets—is abundant (numbering in the tens of thousands), easy to catch, and available year-round. Furthermore, the caves provide a stable microclimate and protection from the elements. The researchers also found more crocodiles in the caves than they did in the surrounding forest, where the animals are vulnerable to logging and bushmeat hunting, leading them to think that the caves also provide some safety from humans. Living in guano may have its costs, but at the end of the day, it’s not easy being green.