The Abanda cave system in Gabon is not a place you want to live.
In the belly of the caves, it's pitch black and hot. Fumes circulating inside tend to cause nausea.
And cavers have to wade through sludge.
"It looks like liquid mud," says cave scientist Olivier Testa, "but it's not mud."
It's bat guano. A lot of it.
Mixed with water, it becomes viscous pools of bat feces.
"It's an extremely harsh environment," says herpetologist Matthew Shirley. "When we leave the caves, we're wiped."
But Testa and Shirley are part of a scientific research team that has found an animal they think may be evolving to live in the harsh caves—crocodiles.
African dwarf crocodiles, to be exact, are found above ground throughout Gabon, but the cave-dwelling population has developed a unique genetic signature unmatched by its topside counterparts, says Shirley.
To test this, they collected blood samples from crocodiles found within the cave system. About 100 to 200 likely live in the 10 to 12 different caves in the system, says Shirley.
Distinguishing which crocodiles were unique was difficult in the darkness, says the researcher. Eventually they were able to get blood samples from 30 to 40 unique individuals. They then collected blood samples from about 200 above-ground crocodiles in the region.
Sequencing the DNA of each population, the research team found that the cave crocodiles were passing along a unique haplotype (a group of genes inherited by a single parent).
The team plans to publish their findings in a research paper that has not yet been finalized. News about the possible genetic mutation was first published in an article by the Guardian, and Shirley says it will be several more weeks before the paper comes out with more details.
"As a result of that isolation and the fact that few individuals come in or go out, they're in the process of [becoming] a new species," says Shirley. "Whether that happens soon or not is anyone's guess."
Already the scientists have observed physical and behavioral differences in the cave population, as published in a 2016 study in the African Journal of Ecology.
Unlike the forest-dwelling dwarf crocodiles, which feed on fish and crustaceans, the cave population primarily eats bats.
In fact, that might be one of the reasons the crocodiles originally migrated to the cave system, says Testa.
"There are tens of thousands of bats [in the caves]," he adds. And when the researchers analyzed the stomach content of cave crocodiles, they found bat skeletons and fur mixed with crickets.
Some of the large, male crocodiles are also orange, but this is unlikely a genetic mutation and more likely a result of soaking in bat poop.
"Bat guano is largely comprised of urea," a nitrogen-rich chemical found in urine, says Shirley. "When they're sitting in this bat guano slushie, we think the highly basic pH water is tanning their skin."
The Early Settlers
The cave crocodiles were first found in 2008 by archaeologist Richard Oslisly.
Exactly how long they've been living there is still unclear. Shirley estimates it was at least a few thousand years ago when a few individuals wandered into the cave looking for shelter or food. Several hundred generations would have been needed to develop a unique genetic signature, and dwarf crocodiles can live for 50 to 100 years.
In small populations such as this one, genetic diversity is also a concern. Inbreeding can harbor disease and birth defects.
Spelunkers rappel 300 feet (90 meters) into the Greenland ice sheet. Glacier caves such as these form when seasonal meltwater or geothermal vents cut fissures and channels through an ice sheet. Because they are made of ice, glacier caves can be quite unstable and present unique challenges to spelunkers.
Shirley theorizes that a small number of individuals from the outside finds their way into the cave in every generation. How they might be entering or exiting, however, is unclear. Some cave entrances are wide enough for a person to walk into, and other cave habitats can only be entered by squeezing through a narrow passage.
In addition to finding what may be a genetically unique population, Shirley says studying the cave crocodiles can tell scientists how the animals adapt to such an inhospitable environment.
Crocodiles are normally diurnal and depend on sunlight to regulate their metabolism, but most of these crocodiles will go without sunlight for decades of their life.