Olm

AmphibianEurope
Written By Alexander Julius Jensen

Proteus anguinus

The olm is a blind salamander found only in the caves of the Dinaric Alps. Like the axolotl, it never metamorphoses, and its maximum lifespan has been estimated at ~102 years. Olms would occasionally wash out of caves during floods and were once believed to be baby dragons in Slovenia.

Dragons of the Dinaric Alps

Dragons lurk in the caves of the Dinaric Alps.

A 1689 book, The Glory of the Duchy of Carniola, records that locals around the Postojna cave system in southwestern Slovenia spoke of a creature “barely a span long, akin to a lizard, in short, a worm and vermin of which there are many hereabouts.”  Baby dragons, they said — although they sound more like worms than wyrms. Baby cave dragons, specifically; these creatures lived in the deep caverns carved by millions of years of waterflow, where rivers run through darkness like the Styx of the Greek underworld, and underground lakes are still as the stone around them. 

Although they were never seen, mature versions of these wormy, baby dragons were said to control the water in this cave system. They dried out springs and then flooded them, sweeping several of their odd offspring to the surface. More likely, the cause was heavy rains. Cave dragons or rainfall, the caves would flood and people would find the pale wriggling things, the baby dragons, in the surface water. 

The creatures certainly looked underdeveloped. Their skin was pale pink and squishy, their bodies sinuous and serpentine with undersized limbs spaced too far apart: a pair of two-toed legs at the back, marking the transition from body to flattened tail, and three-fingered arms at the front, just behind a set of dark-pink frills and a head like a flattened pear. And then there were their eyes. They had none; the skin across their faces was as smooth and featureless as the rest of their bodies. The longest of them were only 30 centimetres (1 foot). They were so strange-looking — not unlike stretched fetuses — so, might as well call them baby dragons. But what exactly they grew into was anyone's guess. 

The real significance of these cave creatures — later christened as olms — would not be discovered for some time yet.

A History of Cave-Dwellers

From the primordial beasts of Mesopotamia, to the coiled, antlered serpents of East Asia, and the winged monsters of medieval Europe, dragons have had a place in our writings for as long as we could write. In the 17th century, we wrote about dragons in the mountainous caves of southeastern Europe. But that account actually recorded something far more novel. It was the very first written record of a troglobite, although no one knew it at the time.

A troglobite refers to a type of animal and carries no derogatory implications. It simply means an animal that lives in caves, or more technically, a species that only lives in caves, or underground. A bat, although a cave-dweller, is not a troglobite — it is a trogloxene, a “cave guest,” that comes and goes. A troglobite never goes, at least not of its own volition. It is not to be confused with a troglodyte. To call a person a troglodyte is to insult them, insinuating that they are ignorant and old-fashioned. But what the word really means is something like “hole/cave goer” (the insult coming from an allusion to primitive cavemen).

The Scene in the Lascaux Shaft.

We often refer to our early ancestors as “cavemen,” although that association, between primitive man and cave, is likely an exaggeration. Caves are excellent environments for preserving evidence of our ancestors' comings and goings, their artefacts and art — like museum exhibits untouched by sun, rain, or wind. Meanwhile, the traces of primitive humans outside of caves have been eroded and destroyed. This preservation bias has, by and large, led to the idea of the “caveman.”

Still, there’s no denying that caves play a part in our species’ history, even if their main role was to record our ancient ancestor’s transient passings: at Blombos Cave, Lascaux, Chauvet, Altamira, etc. We’ve even dove into the dark underworld. In Lascaux, a narrow vertical shaft leads deep below the main galleries, down into a chamber completely devoid of light. On the wall is painted a massive buffalo, a bird on a stick, a human-like figure with a bird’s head, and a rhino charging away from the scene. These ancient artists clambered down through the rocky passage to paint by torchlight, watching their figures come to life as flame and shadow flickered across the walls. Did they find anything else in that dark world below?

If they did, no human wrote of it. No written record of subterranean life emerged for millennia. Indeed, for much of history, deep caves were thought too barren to support life at all — a belief that, in Western science and folk-thought at least, persisted into the 18th and 19th centuries. No one went searching for what could not exist.

The slender-necked beetle (Leptodirus hochenwartii).

Then, one day in 1831, while exploring a newly discovered inner portion of Slovenia’s Postojna Cave, a lamplighter’s assistant collected a beetle. The specimen eventually made its way into the hands of Ferdinand Jožef Schmidt, a naturalist from Ljubljana, who identified it as a new species and one so unique he created an entire genus in which to place it: Leptodirus; meaning “slender-necked,” for the beetle had a bulbous abdomen but a skinny neck and head. This slender-necked beetle would go down as the first ever recorded troglobitic species — but only because it was recorded as such. The olm, more than a century prior, was not.

The cave beetle got its hooks in Schmidt, pulling him to explore the Postojna cave system. He discovered several other subterranean species and, as he corresponded with a network of other European scientists, they too were drawn to explore the system and surrounding caves. They found more cave beetles, yes, but also spiders, pseudoscorpions, millipedes, centipedes, crustaceans and snails.* Entire subterranean ecosystems.

Biospeleology — the study of cave life — was born from the slender-necked beetle.

And the Postojna Caves, its birthplace, became the biospeleological Mecca.

The Olm

Despite initially playing second-fiddle to a beetle, Postojna’s earliest-known resident — the olm — didn’t dawdle in obscurity. Eventually, it would be recognised as the first recorded troglobite, although its history was a bit more complicated.

As specimens and drawings of the creature began to circulate throughout Europe’s scientific community, eventually, in 1768,  it was finally given a scientific name: Proteus anguinus (Proteus, for the shape-shifting Greek god, and anguinus, meaning snake-like), the same name it holds to this day. But naming isn’t the same as knowing, and the animal was still steeped in more myth than fact — indeed, Josephus Nicolaus Laurenti, the naturalist who named it, described the olm as living in lakes. 

It took a while longer, until the early 19th century, for its true nature to be uncovered. 

In a 1839 book called Curiosities of Medical Experience, one doctor speculated that "...the Proteus Anguinis is the first stage of an animal prevented from growing to perfection by inhabiting the subterraneous waters of Carniola." Not a lake creature, but a dweller of deep caves. Not a baby dragon, but a permanently underdeveloped animal.

In the late 19th century, it would be given its modern common name, as simple and mysterious as the animal itself: olm.¹

The olm, it turned out, was no dragon nor wyrm, but a salamander. And despite its uniqueness — the sole species in its genus — it shares a curious quirk with several of its relatives. The olm is neotenic. 

Forever Young

A regular and neotenic salamander lifecycle.

Neoteny is a developmental phenomenon in which adult features are delayed and juvenile ones are retained.

It is especially apparent in the olm and its relatives, given that salamanders pass through several distinct life stages: first an egg, then a limbless larva — much like a frog tadpole but with a pair of frilly external gills — then a limbed larvae with frilly gills and a flattened tail for swimming, then, finally, a terrestrial adult with a more rounded tail and no gills. The olm’s version of neoteny isn’t just the preservation of a few youthful features, it’s a complete halt in development. The olm never makes a final metamorphosis to its land-dwelling form. 

The same is true of the axolotl, perhaps the most famous example of neoteny, as well as several lesser known species like the Lake Patzcuaro salamander, and the olms closest relatives, the waterdogs and mudpuppies. These are so-called paedomorphic (“child-form”) salamanders; you won’t find their “adult” versions. At least, not in the wild. Axolotls have been induced into metamorphosing in the lab via thyroid hormones added into their water tanks or injected directly, turning them into dark-brown adults lacking their iconic axolotl frill-gills. Despite similar attempts for the olm, one has never metamorphosed into its would-be adult form. We still don’t know what a full-grown dragon looks like. But why, exactly, don’t they grow up? 

There’s a theory that suggests humans look the way we do because of neoteny. 

Many of our adult traits resemble the juvenile forms of primates like chimpanzees. And so, the theory goes, humans evolved by delaying and eventually forgoing the maturation of certain features: retaining smaller, flatter jaws and higher, more rounded heads. Perhaps this neoteny was even crucial in the evolution of our large brains and exceptional intelligence. 

The olm certainly didn’t grow a bigger brain. So what did prompt the olm to remain forever young? 

We find the answer in the deep, dark, and damp habitat it calls home. 

Deep, Dark, and Damp

In On the Origin of Species, Charles Darwin, in regards to the olm, wrote:

“Far from feeling surprise that some of the cave-animals should be very anomalous...as is the case with blind Proteus with reference to the reptiles of Europe, I am only surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the scanty inhabitants of these dark abodes will have been exposed.”

Deep caves are among the most distinctive habitats on Earth. Perhaps their most consequential characteristic is what they lack: sunlight. With no light, no harmful UV rays, there’s no reason to develop protective melanin in the skin, and so the olm — pale pink, pigmentless — does not. With no light, there’s nothing for eyes to detect, and so the olm — its eyes underdeveloped and covered by a layer of skin — allows the expensive organs to wither.²

Extra energy expenses cannot be afforded down here. With the base energy source that most life depends on absent, so too are most plants. That means food is limited,  with primary consumers forced to rely on imported energy: organic debris washed in from the surface, bat guano, or the occasional animal that wanders in and dies. 

The olm is no primary consumer, but an apex predator. It swallows whole cave crickets and snails, maggot-like worms, tiny fish, and troglodytic shrimp. Despite its blindness, the olm still has five senses. Its hearing picks up vibrations in the water, its senses of smell and taste are among the best of all amphibians, its touch is extended to a distance with a lateral line — a system that can sense pressure changes in the water — and for its fifth sense, instead of sight, it has ampullary organs on its head capable of detecting weak electric fields, the kind created by all living things when in water.

But if you’re imagining a serpentine dragon, whirling and streaming through underground streams, that is not an olm. The olm is more of a passive drifter. What’s the rush, after all? Its prey moves just as slow, if not slower, and its predators are non-existent. Slowness carries no penalty. It’s free to undulate languidly and push itself along with its tiny limbs. A lethargy that’s in line with its low energy intake.

Calling the olm an “apex predator,” while true (it does top the cave food chain), is a bit misleading. It’s not some voracious young serpent, engulfing anything that moves. Not all that much moves down here, given that the inhabitants are so few. So the olm has learned to go hungry, like a patient monk fasting for long periods, interspersed by periods of feasting. After gorging, it stores glycogen in its liver for use in its next fast — storing enough, in extreme cases, to last it for up to 10 years without any food. 

All of this sounds pretty miserable; a place truly unconducive to life, just as we once thought the underground was.

But time and again, lineages as different as fish (Mexican tetra), snails (Tumbling Creek cavesnail), shrimp (Kentucky cave shrimp), spiders (Kauaʻi cave wolf spider), beetles (slender-necked beetle), and salamanders (olm), have crept down some dark hole and evolved to become troglobitic. While caves are by no means the most biodiverse of biomes, Postojna Cave alone is inhabited by over 150 known animal species.

How does an animal benefit in becoming a troglobite, then?

There are a handful of cases where a single species is split between surface-dwelling and cave-dwelling variants. The Mexican tetra is the most striking example: the surface variants look like regular small fish, while the cave variants are translucent and eyeless. But a more relevant example is the tiger salamander. This salamander lives across most of North America in a diverse set of terrestrial habitats. Adults, patterned with splotches or stripes of yellow and black, only return to water to lay their eggs.

In 1993, a population of these salamanders was discovered in the dark and murky waters of an abandoned reservoir in Wisconsin — a habitat not unlike a deep, wet cave. The reservoir salamanders would have faced different conditions than they normally do on the surface; conditions both detrimental, such as a lack of food, and beneficial. Darwin noted his surprise that there aren’t more troglobites given the “less severe competition” in caves. That also extends to predators; the reservoir salamanders were no longer pursued by snakes, owls, and badgers. They also never metamorphosed. The reservoir population remained in their juvenile, aquatic states: lighter skin, a flattened tail and frilly gills. In other words, they become neotenic. 

The olm likely underwent the exact same kind of change, just far longer ago, when its surface-living ancestors first entered the caves of southeastern Europe. The olm spends its entire life in water. It doesn’t need stout limbs or fully developed lungs, it needs a swimming tail and gills — exactly what a salamander has prior to metamorphosis. It adapted to its environment by never growing up. 

The one adult trait the olm does develop is the ability to reproduce. Most salamanders become sexually mature some time after their metamorphosis, but since olms still obviously need to reproduce, they become sexually mature while still in their juvenile forms. The exceptional thing about this is not that it happens, but when it happens. Most salamander species reach sexual maturity between the ages of two to six. The olm reaches sexual maturity around the age of fifteen.  

Just as there’s no rush to feed or flee from predators, neither is there a rush to mature, mate, and have offspring. A deep cavern is an exceptionally stable environment: the temperature is pretty much constant (perpetually chilly), there’s no change in levels of sunlight (because there is none), and no changing seasons. While animals on the surface must conform their breeding schedules to whatever season provides the most ideal conditions, for the inhabitants of deep caves, summer, spring, autumn, winter go by with no change. It’s as if time down here is at a standstill, and the olms biology is a reflection of that.

The olm’s gross anatomical characteristics: A Ultrasonographic images of the head. From left to right: (i) diagonal section including both, eye and ear of the dead olm (assessment in live specimens is difficult); the ellipsoid shaped eyes are located laterally on the head ca. 0.4 mm beneath the skin surface and appear anechogenic; the lens is visible as a central hyperechogenic line. The ears are composed of the inner ear and a straight canal that distally slightly widens terminating directly beneath the skin surface as circular ring. (ii) Gills in transverse section, arrows indicate their proximal and distal ends; (iii) sagittal section of the body; anechogenic, approximately spherical lymph heart located dorsally of the vertebral column; and (iv) dorsal fat depots in transverse section, indicated by arrows. B Photograph of an adult olm in lateral view; head left, tail on the right; the snout is indicated by a black arrow; approximate positions of the ear, eye and gills are indicated by arrows; the position of the cloaca is denoted by a black asterisk. C Digital microscope images, from left to right: (i) frontal view of the snout, black arrows indicate the most distal row of teeth in the upper and lower jaw; (ii) black arrows indicate two premaxillary and vomerine teeth, respectively, visible in the upper jaw; (iii) dorsal skin; (iv) cloaca of a female olm; proximal part at the upper side of the image, distal one lower. (Holtze et al., 2017)

The longest lifespan for an olm, recorded in captivity, was 68.5 years. Its maximum longevity is predicted to be around 102 years. An olm might live for over a century, and yet it never really grows up.

How does the olm live for so long? Is its metabolism slower than those of its salamander relatives? Does it produce more antioxidants to combat the ageing accumulation of free radicals? Seemingly, no. The olm’s secrets to long life are still mostly mysterious. Our best bet is to look at the species’ oddities; the things that make it unique, such as its long periods of fasting and stable, predator-free environment. Perhaps longevity really is just an outcome of a slow, stress-free life.

But while the lifespan of an individual olm might be long, as a species, its years might be numbered.

Rivers of the Underworld

In ancient Greek myth, five rivers flowed through the underworld: the Acheron, the Cocytus, the Phlegethon, the Lethe, and the Styx. The Styx is perhaps the best known, for it served as the boundary between the living world and the dead, and upon its waters the doomed would be ferried into the underworld. 

The evolutionary history of the olm traces the Styx. Like the souls that travel the river into the underworld, the olm cannot leave. The same goes for most, if not all, troglobitic species; once they begin adapting to cave life — losing their skin pigment and eyes, becoming dependent on touch or smell, slowing down their biology — going back isn’t an option. Troglobites become prisoners of their niche; they float down the Lethe, the river of forgetfulness, and, eventually, they “forget” how to survive in the faster, fiercer, more competitive world above. Like an evolutionary Eurydice, the olm is stuck in the underworld.³

Adapted to such a stable environment, troglobites are very vulnerable to change. On top of that, the olm is also an amphibian: a class of animals that includes many so-called “indicator species,” because they're often the first indication of disruption to their habitats. That usually means they’re the first to suffer, the first to fall into the Cocytus, the river of lamentation — its name coming from kōkyein, meaning “to wail.” The lamentable death of amphibian species is the “wail” that draws attention to disruption in their ecosystems.

Their environmental sensitivity is the cost of a remarkable adaptation.

Via a process called cutaneous respiration, amphibians exchange gases with their environment directly through their skin (skin-breathing, essentially). And to allow oxygen and carbon dioxide to travel through freely, their skin needs to be permeable. The unintended effect is that the skin acts as a sort of sponge, absorbing whatever’s in the water, including pollutants if the particles are small enough. 

As areas around the Dinaric caves become more developed, sewage, agricultural runoff, and industrial waste leeches or overflows from disposal sites, seeping through the porous karst rocks and poisoning the water in which the olm lives. Just as caves preserve the traces of our primitive ancestors, subterranean streams and pools are slow to remove whatever may enter them. 

Like everything else, the temperature of the olm’s caverns rarely changes: always hovering between 8 and 12°C (46–54°F), depending on the particular cave and depth. That’s pretty chilly, but the olm is used to it and wouldn’t — couldn’t, really — have it any other way. But as surface temperature increases due to global warming, ground and cave temperatures very gradually rise as heat is slowly conducted downward. Even deep, stable cave systems like those in the Dinaric Karst can warm by fractions of a degree per decade, enough to matter for cold-adapted species like the olm. Warmer waters can speed up the olm’s metabolism, reduce the amount of available oxygen, and create more favourable conditions for microbial or fungal growth — including chytrid.⁴ The olms underground waterways are transformed into the Phlegethon, the mythical river of fire. Their temperatures rise and they burn with pollutants. 

Across Europe, you’ll find plenty of cave snails, spiders, and beetles. But the continent has only one cave-dwelling vertebrate: the olm. And it is restricted to a few fragmented populations, living in very specific habitats, with no opportunity for migration. How many olms are left? We’d need to fully explore their caves to find out. We know there are, at the very least, 400 of them (as that is the population of the world’s only successful olm breeding programme in Moulis, France). The olm is a protected species under Slovenian, Croatian and Italian law, with many of its sites, like Postojna Cave, located in protected areas. Still, the olm is considered globally vulnerable by the IUCN, and it has a place on the Slovenian Red List of endangered species. 

It also has its place in history and culture, especially in Slovenia. It once featured on the Slovenian 10 stotin coin — its long skinny body stretching across the reverse side and its scientific name, Proteus anguinus, engraved along the bottom edge. The oldest Slovenian popular science magazine, first published in 1933, was named Proteus, after the olm. Whereas in the past it was called a baby dragon by locals, today, the olm is more likely to be referred to as the človeška ribica (“human fish”), due to its pale, skin-like colour. 

Together, the beetle and the olm led us to discover an entire underground biology: blind wolf spiders in Hawai’i, cave angel fish in Thailand, snowy white velvet worms in South Africa, and even another cave salamander — appearing much like the olm but with a much shorter torso — all the way in Texas. 

A whole world of pale, blind creatures. Many of these troglobites are likewise threatened or endangered.

———

The olm floats towards the convergence of the underworld rivers. 

Our final river, Acheron, is the river of woe, of suffering and sorrow. It is symbolic of the grief caused by death. 

If the olm were to vanish, it would flow down the Acheron, leaving grief in the hearts of those for whom it meant something: whether a national symbol, a scientific marvel, or a mythical baby dragon.

Olms dwell in the deepest underworld, in a place once reserved only for the dead. But this remarkable and storied species — a living bridge between myths and science, a neotenic oddity, pale and eyeless, a symbol of Slovenia and of all subterranean life — is very much still alive. 

¹ We know that the name ‘olm’ is a loanword from German, but the etymology behind it is lost. Fitting, perhaps, for such an enigmatic creature.

² The deterioration or complete loss of eyes is a common occurrence among troglobitic species. You can learn more about that from the eyeless Kauaʻi cave wolf spider!

³ In an Ancient Greek myth, Eurydice — lover of a master musician named Orpheus — dies tragically from a venomous snake bite. Orpheus journeys to the Underworld to bring her back, persuading Hades to release her with a mournful song. Hades allows it on one condition: Orpheus must not look at her until they reach the surface. However, during the long journey up, he cannot hear her footsteps following him and he is overcome by doubt. He glances back, and she is lost forever to the Underworld.

⁴ Chytrid fungus is a plague upon amphibians around the world.

Highly virulent, it infects the host's skin — which amphibians use for gas exchange and regulating their electrolyte balance — leading to death by suffocation or cardiac arrest. This disease, chytridiomycosis, is implicated in the decline of over 500 amphibian species. The worst of it has occurred in tropical regions like South and Central America, where amphibian diversity is highest, but the fungus is threatening species in Europe too.

Where Does It Live?

⛰️ Subterranean, cold, freshwater streams and pools within limestone caves.

📍 Dinaric Alps; Slovenia, Italy, Croatia, and Bosnia and Herzegovina.

‘Vulnerable’ as of 03 Feb, 2023.

IUCN

  • Size // Small

    Length // 23 - 30 cm (9 - 12 in)

    Weight // N/A

  • Activity: N/A

    Lifestyle: Solitary 👤

    Lifespan: 68.5 years (max recorded in captivity, 102 years (estimated max)

    Diet: Carnivore

    Favorite Food: Small crustaceans 🦐

  • Class: Amphibia

    Order: Urodela

    Family: Proteidae

    Genus: Proteus

    Species: P. anguinus

  • A book from 1689 tells of baby dragons washing out from the Postojna cave system in southwestern Slovenia, describing one of these creatures as “barely a span long, akin to a lizard, in short, a worm and vermin.” These, in fact, were not infant worms nor wyrms, but subterranean salamanders that were swept to the surface by occasional floods. 

    Eventually christened the olm, this animal was the first recorded troglobitic species — that is, one who lives its entire life below ground — although a species of beetle from the same cave system, the slender-necked beetle, was the first species actually described as troglobitic, for the nature of the olm was still a mystery.

    In a 1839 book called Curiosities of Medical Experience, one doctor speculated that “...the Proteus Anguinis is the first stage of an animal prevented from growing to perfection by inhabiting the subterraneous waters of Carniola [a region in present-day Slovenia].” The olm was indeed prevented from growing, although the value judgement seems uncalled for. It is a neotenic salamander species, like the axolotl, meaning that it never metamorphoses into its adult form (keeping its frilly outer gills and swimming tail) yet still develops the ability to reproduce. 

    And that’s not even its most extraordinary ability. For a blind creature living in pitch black caverns, it is exceptionally aware of its surroundings: its hearing picks up vibrations in the water, its senses of smell and taste are among the best of all amphibians, its touch is extended to a distance with a lateral line — a system that can sense pressure changes in the water — and for its fifth sense, instead of sight, it has ampullary organs on its head capable of detecting weak electric fields.

    Once it finds food, it gorges and then stores plenty of glycogen in its liver, which it can draw from in order to survive up to 10 years without a single bite of food. And then there’s its lifespan: the longest lifespan for an olm, recorded in captivity, was 68.5 years. Its maximum longevity, however, is predicted to be around 102 years. How it lives so long is still not fully understood.

    The olm is currently considered a vulnerable species, although given its choice of habitat, its exact population has been difficult to survey. 

Alexander Julius Jensen
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