Shedding Light on Life Underground
Since the early days of modern biology, the study of animals adapted to cave environments has captivated researchers. Subterranean habitats—often isolated, nutrient-poor, and dark—have served as natural laboratories for examining evolutionary processes. Organisms that live in such settings typically evolve a suite of traits known as troglomorphisms, including loss of eyesight, depigmentation, slowed metabolism, and reduced circadian rhythms. The degree of these traits reflects an organism’s specialization to life underground.
Species exhibiting the highest degree of such adaptations are often referred to as troglobionts (terrestrial) or stygobionts (aquatic), with the latter term rooted in Greek mythology’s river Styx. These animals are commonly thought to be evolutionary cul-de-sacs, confined to their isolated, lightless niches—unable to survive outside them.The Olm: Icon of Subterranean Specialization
The olm (Proteus anguinus) is perhaps the most iconic example of a stygobiont. Found in karstic groundwater systems of the Dinaric Alps, this blind, pigmentless salamander represents the only stygobiont amphibian in the Palearctic and the largest known troglobiont vertebrate. With its ancient lineage—believed to have diverged from surface ancestors 8–20 million years ago—the olm exhibits extraordinary cave-adapted features. These include magnetic navigation capabilities, resistance to hypoxia, and a metabolic rate so low it can survive years without food. Notably, it is also an obligate paedomorph, retaining juvenile traits into adulthood.
Olms reproduce infrequently—females lay eggs roughly every 12 years—and have been regarded as apex predators within their subterranean ecosystems. However, most existing knowledge comes from lab studies, with relatively few direct field observations due to the animal’s cryptic habitat.Challenging Assumptions: Are Olms Really Strictly Subterranean?
Conventional cave biology assumes a rigid boundary between subterranean and surface environments. While surface-dwelling species are known to occasionally utilize caves (for refuge or during specific life stages), it has long been thought that cave specialists like the olm are strictly restricted to the underground. Sightings of troglobionts outside caves have generally been attributed to accidents—usually flood washouts or rare displacements.This perspective, however, is increasingly being questioned. Some surface species like the Pyrenean newt (Calotriton asper) are known to exploit groundwater. But could highly specialized troglomorphic species actively venture above ground under certain conditions?Research Objectives and Study Area
To investigate this question, researchers set out to explore whether the olm could actively utilize surface environments, and if so, under what conditions. The study aimed to document these behaviors and determine what surface resources, if any, olms exploit.
From June 2020 to May 2023, researchers conducted systematic surveys in the eastern sector of the Classical Karst region in northeastern Italy. This area—rich in caves, springs, sinkholes, and estavelles—offered a dynamic hydrogeological setting ideal for examining transitions between subterranean and surface environments. Ten caves and 69 springs were monitored across the municipalities of Doberdò del Lago and Monfalcone.Methodology: Eyes on the Springs
The research team employed visual encounter surveys (VES) lasting 20 minutes at each spring site, conducted both during the day (10 a.m. to 4 p.m.) and at night (9 p.m. to 2 a.m.). From September 2022 onward, captured olms were tagged for a long-term capture–mark–recapture (CMR) study. Each spring was surveyed 11–25 times to ensure comprehensive data collection.Results: Surface Activity Confirmed
Olms were recorded in all 10 caves and in 15 of the 69 springs surveyed. Crucially, these were not isolated or accidental sightings. Observations occurred during both day and night. Daytime sightings—previously considered highly unlikely—occurred in seven springs and were observed multiple times (average of three sightings per spring), in some cases accounting for over 60% of diurnal surveys.Camera traps also confirmed daytime activity. Notably, olms were observed displaying behaviors such as feeding and movement rather than passive drifting.A Rare Discovery: Larvae on the Surface
Perhaps the most unexpected finding came in January 2022, when a young olm larva—measuring just 3.56 cm—was spotted during the day in a spring. The larva, which bore a visible injury, actively sought shelter in substrate crevices, suggesting familiarity with its environment. According to captive-breeding data, this larva was approximately 90–110 days old. Importantly, there had been no flooding in the area during that time, suggesting it had not been washed from a cave, but rather had emerged or even been born in the spring environment.
Given that olm larvae retain functional eyes for a period post-hatching, this surface zone may provide an intermediate developmental habitat, albeit a risky one due to exposure to predators and environmental fluctuations.Feeding on the Surface: Earthworms in the Diet
Another compelling piece of evidence came from handling adult olms. In multiple instances, individuals regurgitated earthworms—species that do not inhabit caves, strongly suggesting the olms had foraged above ground. These observations confirm active feeding in surface habitats and hint at a more flexible and opportunistic feeding strategy than previously believed.Future work involving fecal DNA metabarcoding or stomach content analysis could quantify the extent of surface-derived nutrition in olm diets.Rethinking Cave Biology: A Call for Integration
This study provides the first empirical evidence that the olm, a species long thought to be strictly subterranean, can actively and repeatedly exploit surface habitats. These behaviors were observed during both night and day, and even involved sensitive life stages like larval development.The findings challenge traditional dichotomies in cave biology and emphasize the need to study transitional environments—like springs and estavelles—that link groundwater and surface ecosystems. Springs, with their crevices and variable conditions, may play a larger role in the ecology and evolution of cave specialists than previously assumed.Comparable studies on Nearctic stygobiont salamanders or subterranean invertebrates could further illuminate how widespread such surface usage is among other troglomorphic species.A Flexible Life in a Rigid World
Olms are far from being prisoners of their subterranean realms. This study shows that they can adapt to variable conditions and utilize surface habitats for feeding—and potentially even reproduction. Such behavioral plasticity reveals a new dimension of this enigmatic species and underscores the importance of adopting a more nuanced, integrated approach to studying subterranean life.
By shining light on the surface habits of the olm, researchers not only dispel myths about its ecological rigidity but also open new doors for understanding the evolutionary biology of life in the dark.
Since the early days of modern biology, the study of animals adapted to cave environments has captivated researchers. Subterranean habitats—often isolated, nutrient-poor, and dark—have served as natural laboratories for examining evolutionary processes. Organisms that live in such settings typically evolve a suite of traits known as troglomorphisms, including loss of eyesight, depigmentation, slowed metabolism, and reduced circadian rhythms. The degree of these traits reflects an organism’s specialization to life underground.
Species exhibiting the highest degree of such adaptations are often referred to as troglobionts (terrestrial) or stygobionts (aquatic), with the latter term rooted in Greek mythology’s river Styx. These animals are commonly thought to be evolutionary cul-de-sacs, confined to their isolated, lightless niches—unable to survive outside them.The Olm: Icon of Subterranean SpecializationThe olm (Proteus anguinus) is perhaps the most iconic example of a stygobiont. Found in karstic groundwater systems of the Dinaric Alps, this blind, pigmentless salamander represents the only stygobiont amphibian in the Palearctic and the largest known troglobiont vertebrate. With its ancient lineage—believed to have diverged from surface ancestors 8–20 million years ago—the olm exhibits extraordinary cave-adapted features. These include magnetic navigation capabilities, resistance to hypoxia, and a metabolic rate so low it can survive years without food. Notably, it is also an obligate paedomorph, retaining juvenile traits into adulthood.
Olms reproduce infrequently—females lay eggs roughly every 12 years—and have been regarded as apex predators within their subterranean ecosystems. However, most existing knowledge comes from lab studies, with relatively few direct field observations due to the animal’s cryptic habitat.Challenging Assumptions: Are Olms Really Strictly Subterranean?Conventional cave biology assumes a rigid boundary between subterranean and surface environments. While surface-dwelling species are known to occasionally utilize caves (for refuge or during specific life stages), it has long been thought that cave specialists like the olm are strictly restricted to the underground. Sightings of troglobionts outside caves have generally been attributed to accidents—usually flood washouts or rare displacements.This perspective, however, is increasingly being questioned. Some surface species like the Pyrenean newt (Calotriton asper) are known to exploit groundwater. But could highly specialized troglomorphic species actively venture above ground under certain conditions?Research Objectives and Study Area
To investigate this question, researchers set out to explore whether the olm could actively utilize surface environments, and if so, under what conditions. The study aimed to document these behaviors and determine what surface resources, if any, olms exploit.
From June 2020 to May 2023, researchers conducted systematic surveys in the eastern sector of the Classical Karst region in northeastern Italy. This area—rich in caves, springs, sinkholes, and estavelles—offered a dynamic hydrogeological setting ideal for examining transitions between subterranean and surface environments. Ten caves and 69 springs were monitored across the municipalities of Doberdò del Lago and Monfalcone.Methodology: Eyes on the SpringsThe research team employed visual encounter surveys (VES) lasting 20 minutes at each spring site, conducted both during the day (10 a.m. to 4 p.m.) and at night (9 p.m. to 2 a.m.). From September 2022 onward, captured olms were tagged for a long-term capture–mark–recapture (CMR) study. Each spring was surveyed 11–25 times to ensure comprehensive data collection.Results: Surface Activity Confirmed
Olms were recorded in all 10 caves and in 15 of the 69 springs surveyed. Crucially, these were not isolated or accidental sightings. Observations occurred during both day and night. Daytime sightings—previously considered highly unlikely—occurred in seven springs and were observed multiple times (average of three sightings per spring), in some cases accounting for over 60% of diurnal surveys.Camera traps also confirmed daytime activity. Notably, olms were observed displaying behaviors such as feeding and movement rather than passive drifting.A Rare Discovery: Larvae on the Surface
Perhaps the most unexpected finding came in January 2022, when a young olm larva—measuring just 3.56 cm—was spotted during the day in a spring. The larva, which bore a visible injury, actively sought shelter in substrate crevices, suggesting familiarity with its environment. According to captive-breeding data, this larva was approximately 90–110 days old. Importantly, there had been no flooding in the area during that time, suggesting it had not been washed from a cave, but rather had emerged or even been born in the spring environment.
Given that olm larvae retain functional eyes for a period post-hatching, this surface zone may provide an intermediate developmental habitat, albeit a risky one due to exposure to predators and environmental fluctuations.Feeding on the Surface: Earthworms in the DietAnother compelling piece of evidence came from handling adult olms. In multiple instances, individuals regurgitated earthworms—species that do not inhabit caves, strongly suggesting the olms had foraged above ground. These observations confirm active feeding in surface habitats and hint at a more flexible and opportunistic feeding strategy than previously believed.Future work involving fecal DNA metabarcoding or stomach content analysis could quantify the extent of surface-derived nutrition in olm diets.Rethinking Cave Biology: A Call for Integration
This study provides the first empirical evidence that the olm, a species long thought to be strictly subterranean, can actively and repeatedly exploit surface habitats. These behaviors were observed during both night and day, and even involved sensitive life stages like larval development.The findings challenge traditional dichotomies in cave biology and emphasize the need to study transitional environments—like springs and estavelles—that link groundwater and surface ecosystems. Springs, with their crevices and variable conditions, may play a larger role in the ecology and evolution of cave specialists than previously assumed.Comparable studies on Nearctic stygobiont salamanders or subterranean invertebrates could further illuminate how widespread such surface usage is among other troglomorphic species.A Flexible Life in a Rigid World
Olms are far from being prisoners of their subterranean realms. This study shows that they can adapt to variable conditions and utilize surface habitats for feeding—and potentially even reproduction. Such behavioral plasticity reveals a new dimension of this enigmatic species and underscores the importance of adopting a more nuanced, integrated approach to studying subterranean life.
By shining light on the surface habits of the olm, researchers not only dispel myths about its ecological rigidity but also open new doors for understanding the evolutionary biology of life in the dark.
