Investigating Africa's Mysterious Cave Crocodiles

Brady Barr with a crocodile in a cave system beneath the Ankarana nature reserve in Madagascar. The crocs that inhabit this cave may perhaps be a subspecies of the Nile Crocodile. Photograph copyright Brady Barr/NGT&F

Investigating Africa's Mysterious Cave Crocodiles
Brian Handwerk
for National Geographic News
June 20, 2003

In a vast labyrinth of caves beneath the island of Madagascar's Ankarana nature reserve off the coast of Africa, scientists are studying cave-dwelling crocodiles—perhaps the only ones in the world. Are the behemoths that inhabit these caves a new subspecies? To find out, Brady Barr led a team of researchers into the dark depths of the caves. They emerged with tantalizing clues, a scientific first, and lots of unanswered questions.
Although they dwell reclusively within the cave system, the large reptiles were certainly no secret. In fact, before they could enter the caves Barr's team had to attend a local ceremony during which a woman became possessed by the spirit of the crocodile. The spirit granted permission for them to enter—but only after a suitable offering was made of honey, tobacco, and alcohol.
"The locals had always known that crocs were living in the caves," Barr told National Geographic News, "and the scientific community had known for a long time but no scientist had put their hands on one and nothing was known about the population."
Getting his hands on one of the cave crocs is exactly what Barr set out to do, accompanied by Spanish biologist Gerardo Garcia Herrero, longtime cameraman Eric Cochran, and a local guide named Angelin. They hoped to learn if the animals represented a separate subspecies of the Nile Crocodiles (Crocodylus niloticus) that inhabit the rest of Madagascar.
Those reptiles have had a tough time in the areas near Ankarana and elsewhere on the island. Humans certainly aren't their primary prey, but enough people have been killed by crocs to instill a healthy local fear for the animals, which also prey on livestock. These factors, and Madagascar's generally poor economic conditions, mean that poaching is a real problem.
"Outside the caves, the crocs were almost wiped out by poaching," Barr said. "We saw it in action."
Inside the caves, however, populations appear to be thriving. But why here, and not elsewhere in the world, have crocs ventured into caves below the Earth? The answers to that question still await future intrepid researchers—but Barr has developed some ideas.
"I don't know why they are using the caves," he said. "Maybe it's a safe haven, maybe it's a refuge from the incredibly high daytime temperatures, and maybe they use them seasonally or as a food source because of all the bizarre animals living in the caves." The caves are home to all kinds of interesting wildlife, including blindfish, eels as thick as a mans thigh, bats, spiders, and scorpions. "Why are the crocs there?" Barr asked. "That's the million-dollar question."
A further riddle concerning the reptiles' cave existence is the fact that they are ectothermic, which means that they rely on their environment to moderate their body temperatures. That's why crocs typically bask in the sun to warm up and lie in water to cool down. The cave's internal thermometer is fairly constant and not entirely accommodating. "It's way too cool for them in the cave," Barr said, "yet they are making a living in there."
It's speculation awaiting further research, but the crocs may be adapting to the caves by adjusting their metabolism. "They can't digest prey if it's too cold," Barr explained. "It was pretty cool in there, so maybe they're not feeding in there at all. Crocs can go a year without a meal if they really have to, because they may eat 20 percent of their body mass in a single meal. Their metabolism might slow down so that they don't burn energy and they can use these caves as a refuge—[they're] not cognizant, but just in a case of survival of the fittest."
"The biggest, boldest, most intimidating crocs are always the first to be killed," Barr said, "and the survivors are the shy, reclusive animals. Maybe those left in the caves are the ones who, for whatever reason, didn't use the prime habitat. The caves weren't optimal habitat, but they might have turned out to be optimal because it's helped them to survive."
Scientific First Raises More Questions than Answers
Nile crocs can grow to 16 feet (5 meters) or more, and local tales tell of 20-foot (6-meter) crocs in the caves. Barr did not encounter any of the true giants, but he did find evidence that they exist within the 60 miles (97 kilometers) of underground rivers and passageways.
"I never saw any big crocs but there were giant footprints," he said. "There are lots of crocs using these caves and it was frustrating to know that there were a lot of them around that we couldn't see. In a lake, river or swamp they dive underwater, but here they dive underwater, and then go through a secret passage to another room on the other side of a wall. It was a 3-D environment, the ultimate maze, like an ant farm for crocs."
The difficulties and dangers, which included flooding, quicksand, and disorienting directions did not stop the team from achieving their goal—capturing the first ever cave croc and taking a tissue sample for genetic study.
"We did something that had never been done before," Barr explained to National Geographic News. "Yeah, I would have loved a 16-foot [5-meter] croc but the real achievement was that we allowed a genetic analysis to take place of the tissue samples of those crocs." While it's too early to tell, the preliminary results of those tests look intriguing.
"I'm not a geneticist," Barr said, "but we are hearing that the cave population was distinctly different genetically from other populations of Nile crocodiles in Madagascar. There's ongoing research on the tissue and DNA tests, and these results are just very preliminary, but since they appear to be different that's an extra added incentive to protect the caves where they dwell."
Barr added that he's eager to return and study the cave crocs, and that others are joining the effort as well. "What I saw has been passed on to the scientific community and I know that others are eager to do research in the caves. Hopefully someone can unravel the mystery of this bizarre subterranean population of crocs."