By Connor McCarthy

It’s completely dark. The eastern-European cave you find yourself in is silent, save the whirring of a passing aquatic invertebrate. Your undeveloped eyes can’t see it, but the vibrations on your slimy pink skin let you know as it passes, just out of reach. You know more food will come if you are patient, so you cling to your submerged rock and wait. And wait. And wait. And wait some more until seven long years have passed by your mysterious, unblinking eyes.

No, this is not a study-abroad-psychedelic-experience gone haywire or the opening to a B-list horror movie starring a Bosnian cave monster. This is the reality of the Olm salamander (Proteus anguinus), a slender pink cave-dweller from eastern Europe that spends its entire life cycle in the underground rivers that flow from the region’s karst limestone bedrock. The species is 20cm long when full grown, lives its entire life underwater, and keeps its external gills into adulthood. Like most other salamanders, the Olm eats a mix of snails and other aquatic invertebrates. What sets the Olm apart from other salamanders and other animals in general is its ability to remain effectively motionless for years at a time.

This behavior (or lack thereof), was first studied by Gergely Balázs, an accomplished cave diver and researcher at the Department of Systematic Zoology and Ecology at Eötvös Loránd University in Budapest, Hungary. Balázs and a team of other divers conducted a mark-recapture study on Olm in the eastern part of Bosnia and Herzegovina, a small country in southeastern Europe. Mark-recapture studies are one of the herpetology field’s most commonly used tools for estimating population, the idea behind them being if you capture a group of amphibians, mark them, release them into the wild, then come back later and capture roughly the same number as before, the proportion of marked individuals captured in the second survey is equal to the proportion of the total population captured in the first survey, which gives you a rough estimate of total abundance.

Due to their inaccessibly Olm are a fairly large question mark on the map of herpetology. Very few studies have been conducted on their life cycle and natural history, and most of what is known about the species has come from captive populations in zoos and aquariums. As a result, the species is shrouded in mystery. In medieval times, Olm were thought to be the offspring of cave dwelling dragons, as they would occasionally wash out of the caves during flood events. Though they cannot breathe fire, reproduce with donkeys, or do any of the other things dragons from our pop culture do, a wormlike creature with gills understandably must have seemed to be the spawn of something much more sinister.

According to Balázs, the species sedentary lifestyle makes them somewhat easy to catch. Researchers would use a flashlight to locate the Olms from a few meters away, then shut the light off (Olm are blind but can still see light and dark) as they slowly swam up to the salamanders. Once they were close, they would turn the light back on, grab the Olm, mark it with a visible implant elastomer (a liquid polymer injected under the salamanders’ skin that solidifies, allowing for re-identification later on). The Olm would then be set free, returning to the subterranean riverbed in a single burst of writhing speed.

The study itself had several layers and included data from previous expeditions into the caves. Balázs and his team conducted their first study in these caves in 2010, where they tagged 7 Olm, five of which were recaptured in the 2020 study. 19 additional individuals were tagged in 2016, 13 of which were observed in the subsequent 28-month monitoring period.

By comparing the locations of individual Olm during each of fifteen recapture expeditions, Balázs and his team attempted to discern exactly how much moving these salamanders were doing in the darkness. Like people, some Olm seemed to be travels whereas others were not. One salamander moved an impressive 38 meters over the course of 230 days, while another was found in the exact same spot after 2569 days (just over 7 years). It is unclear if this sedentary individual (along with the rest of the Olm in the cave studied) was feeding during this time or exercising its starvation resistance.

Over the entire study, no Olm traveled more than 80 meters from the site it was first captured at and on average they only moved about 5 meters a year. In the 37 total recaptures in the study, only ten animals had moved more than 10m away from their original location.

 Interestingly, all the Olm captured in the study were quite out in the open and very visible to the divers. This is in stark contrast to Olm behavior of captive Olm, which hide in cracks and crevasses of rocks. Furthermore, individuals could be found within a few meters of each other, but displayed no sign of grouping or avoidance behaviors. Essentially, Balázs and his team couldn’t make any conclusions as to why Olm spend so little time moving especially considering they have no natural predators or other competitors.

But are these individuals really not moving, or do they just move back and forth between the same places like retired old men and therefore create the illusion of a sedentary lifestyle? Balázs’ current theory is that the Olm are trying to minimize the amount of energy expended. Female Olm only reproduce about once every twelve years and it is predicted the species can live to be over 100 years old. Furthermore, they are extremely resistant to starvation and some studies have shown individuals can go ten years without eating. They do this by eating large quantities of food at once, then storing excess nutrients, glycogen, and lipids in the liver. When food is scarce, they can reduce their metabolism and in extreme cases reabsorb their own tissue until more food becomes available. They also have a high tolerance for hypoxic water, meaning their oxygen demand is quite low which allows them to survive in cave environments. Essentially, everything about the Olm, from there anatomy to their reproductive cycle, is perfectly designed for to survive in the some of the most abysmal reaches of the earth’s surface. So, will we ever really know what goes on in the caves of eastern Europe while Balázs and his expert team of divers are not there to document it? For the time being, it’s unlikely. Setting up a motion trap for a species that hardly moves is a task in itself especially when these cameras would need to be placed in subterranean rivers. What we do know is these slender, pale, blind salamanders might have the most stoic lives of any vertebrate. While it’s difficult to envy a creature that spends in entire life in darkness, hardly eating, rarely moving, and reproducing once every decade, it’s also hard to argue the evolution and lifecycle of the Olm is anything but fascinating. Hopefully future studies will give us more insight on their mysterious lives as their delicate karst environments may be at risk due to stormwater runoff, pollution, and climate change.