Georgia’s Caves Reveal a Hidden World of Twenty Bat Species

Deep beneath the karst landscapes of the Caucasus, the country of Georgia harbors a remarkable but poorly documented assembly of bats. A new synthesis published in the journal Subterranean Biology has for the first time pulled together more than a century of scattered records and two decades of intensive fieldwork to map which bat species use the nation’s caves, mines and other underground sites. The result is the most complete picture yet of subterranean bat diversity in a country that sits at the crossroads of Europe and Asia, within both the Caucasus and the Irano-Anatolian biodiversity hotspots. Out of thirty bat species known from Georgia, twenty have now been documented using underground habitats, a figure that far exceeds the thirteen species recorded from such sites by the end of the Soviet era.

The scale of the knowledge gap that the research team confronted is striking. Georgia contains roughly 2,000 known underground sites, including more than 1,500 karst and conglomerate caves, yet only about 15 percent of them had ever been surveyed for bats. Historical accounts, written largely in Russian and Georgian and scattered across obscure journals, museum archives and gray literature, documented just 13 species from 71 sites between 1912 and 2010. To recover this fragmented record, the authors employed a snowball survey method, starting from the works of influential early bat biologists and tracing every cited reference backwards through the literature. They searched the libraries of Georgian and Russian research institutions, including the Zoological Institute of the Russian Academy of Sciences in Saint Petersburg and the Zoological Museum of Moscow State University, and combed through European databases such as GBIF and the EUROBATS underground habitats database.

On top of the historical record, the team added their own data: more than 300 surveys conducted between 1999 and 2024 across 44 previously unreported underground locations, in every season of the year. Surveys combined visual searches inside caves with mist-netting at entrances during the swarming period, when bats gather to mate and socialize. Captured animals were measured using standard morphological criteria for species identification and released at the point of capture, with all handling conducted under permits issued by Georgia’s Ministry of Environmental Protection and Agriculture. Where colonies were encountered, the researchers estimated sizes through direct counts of small visible groups, subgroup extrapolation for larger aggregations, and photographic counting for the very largest congregations, techniques that acknowledge the well-known difficulty of obtaining reliable abundance estimates for roosting bats.

The resulting dataset comprises 331 records of 20 bat species from 115 underground sites, spanning three families and ten genera. The haul includes one genuine discovery: Blasius’s horseshoe bat, Rhinolophus blasii, recorded in Georgia for the first time in 2006 and subsequently found in three additional caves. It also includes a subtle loss. Mehely’s horseshoe bat, Rhinolophus mehelyi, which had been documented as recently as 1964, could not be confirmed during any of the modern surveys, although the authors caution that its continued presence in the country cannot be definitively excluded. Six species that do not typically use underground shelters at all, including Bechstein’s bat and Leisler’s bat, two largely forest-dwelling species, turned up unexpectedly in caves and cave entrances, underscoring how much remains to be learned about the roosting flexibility of even familiar European species.

To make sense of this diversity, the researchers classified each species into six lifestyle categories based on the strength of its association with subterranean habitats, ranging from exclusively cave-dwelling species through partial cave users and forest species to generalists with no clear shelter preference. Only Rhinolophus blasii qualified as an exclusive cave dweller. Six species, including the greater horseshoe bat Rhinolophus ferrumequinum, the lesser horseshoe bat R. hipposideros, R. euryale, Myotis blythii, M. emarginatus and Schreibers’s bent-winged bat Miniopterus schreibersii, were classed as mainly or primarily cave dwelling, while seven others, among them the whiskered, Brandt’s, Daubenton’s, Natterer’s, barbastelle and two long-eared bats, use caves mainly for wintering, mating or swarming while roosting in trees during summer.

The most frequently recorded species across the two decades of surveys were Rhinolophus ferrumequinum, R. hipposideros, Miniopterus schreibersii, Myotis blythii and R. euryale, and it is these five that form the large, persistent colonies on which much of Georgia’s cave bat conservation hinges. All five are listed as priority species under the Emerald Network, the pan-European system of areas of conservation interest, and two carry formal extinction risk assessments: R. euryale is classified as Vulnerable on the Georgian Red List, and M. schreibersii is listed as Vulnerable by the International Union for Conservation of Nature. Three of the twenty cave-using species in total hold Georgian Red List status, giving the subterranean fauna a clear conservation weighting.

Mapping the records onto a grid and binning richness by elevation revealed a pronounced spatial structure. Bat diversity in underground habitats peaks at the lowest elevations, between sea level and 300 meters, and again between 1,200 and 1,500 meters above sea level, with a secondary peak at 600 to 900 meters. Geographically, the richest assemblages cluster in western Georgia, where karstic outcrops cover about 6.4 percent of the national territory, some 4,475 square kilometers, concentrated on the peripheral zone of the southern slope of the Greater Caucasus and the hills north of the Colchis lowland. Eastern Georgia, by contrast, has only weakly developed karst, and its underground roosts are largely man-made: sandstone caves along the gorges of the Mtkvari, Iori, Khrami, Algeti and Mashavera rivers, plus volcanic caverns widened by human hands and the abandoned manganese mines of the Chiatura region, many of which are too dangerous to enter.

Some of the individual colonies documented are of international significance. Ghliana Cave hosts one of Georgia’s largest nursery colonies, a mixed maternity assemblage of 4,000 to 8,000 individuals that peaks in late July and early August. Myotis blythii and Miniopterus schreibersii are present every year, joined in different seasons and years by Rhinolophus ferrumequinum, R. euryale and occasionally M. emarginatus. Samertskhle Klde Cave holds another of the country’s biggest maternity colonies, also reaching 8,000 or more individuals in favorable years, again dominated by the same two species. For wintering bats, Sakishore Cave is the standout site, sheltering more than 1,000 hibernating Schreibers’s bent-winged bats, the largest known winter aggregation in the country.

Threats loom over these strongholds. The team scored three categories of human pressure at key colony sites, excessive disturbance, habitat destruction or change of use, and light pollution, on a three-point scale, and combined these with a parallel score of cave accessibility to produce a composite vulnerability index. Ghliana Cave emerged as the most threatened site in the country, with high scores for disturbance and habitat degradation combined with easy access, a worrying combination given that a government conservation action plan was drawn up for the cave as recently as 2019. The remaining colony caves, including Melouri, Samertskhle Klde, Vardigora, Sakishore, Becho and Matkhoji, face moderate threat levels, driven mainly by human visitation.

Yet the overall message is one of cautious hope. No Georgian bat species has been lost, and several species of concern remain locally abundant, a rare positive trajectory for cave faunas in a region of accelerating tourism, quarrying and light pollution. Protection, however, lags far behind need: only 18 caves or cave complexes enjoy legal protection, just two are listed by the Emerald Network, and of all the protected sites only Ghliana and Melouri actually harbor large colonies. The authors recommend formal conservation status for four more caves, Samertskhle Klde, Vardigora, Becho and Sakishore, and single out the Imereti, Samegrelo and Apkhazeti regions as the country’s most important bat conservation landscapes, where remoteness and altitude currently provide only a fragile de facto refuge. Crucially, they argue that protecting cave mouths alone is not enough: commuting routes and foraging habitats in the surrounding landscape must be included within any effective safeguard. Sustained long-term monitoring, they conclude, is the foundation on which the future of Georgia’s underground bats will depend.

Subject of Research: Distribution and conservation of bat species in the underground habitats of Georgia

Article Title: The distribution of bat species across the underground sites of Georgia

Article References: Natradze, I., Bukhnikashvili, A., Gazaryan, S., Sheklashvili, G., & Mumladze, L. (2024). The distribution of bat species across the underground sites of Georgia. Subterranean Biology, 50, 65-77. https://doi.org/10.3897/subtbiol.50.134613

Image Credits: AI Generated

DOI: 10.3897/subtbiol.50.134613

Keywords: bats, caves, Georgia, subterranean habitats, karst, chiroptera, conservation, biodiversity hotspot, hibernation, nursery colonies, Red List, Caucasus