# Thermal Niche Differentiation Shapes the Hibernating Bat Assemblages in Bulgarian Caves Across an Elevational Gradient

## TL;DR

Bats in Bulgaria's caves prefer different temperatures for hibernation, with species distribution along elevation gradients shaped by thermal preferences.

## Contribution

This study demonstrates thermal niche differentiation as a key driver of bat assemblage structure across elevational gradients in Bulgaria.

## Key findings

- Bat species show distinct temperature preferences, with warm-adapted species in low-elevation caves and cold-adapted species in high-elevation caves.
- Mid-elevation caves host mixed communities with no dominant species, acting as transition zones between thermal niches.
- Species turnover, not diversity loss, occurs with elevation, indicating thermal niche partitioning structures assemblages.

## Abstract

Bats in temperate areas enter hibernation, a survival tactic that helps them to reduce metabolic expenses during winter, when food is scarce. In this study, we investigate hibernating bat assemblages from different caves along the elevation gradient in two of Bulgaria’s largest mountain ranges: the Stara Planina Mountains and the Rhodopi Mountains. We aimed to reveal how elevation-associated temperature influences the species composition of the cave assemblages. Our results show that each bat species has distinct temperature preferences that determine where it hibernates. Species that prefer warmer conditions inhabited low-elevation caves, whereas those adapted to colder temperatures occupied high-elevation caves. Interestingly, the total number of species remained relatively stable across elevations, suggesting that species replace one another rather than diversity declining simply with elevation. Caves at mid-elevations hosted mixed communities without any single characteristic species, indicating that mid-elevations function as transition zones. Overall, our findings demonstrate that temperature preferences shape which bat species hibernate together. This has important implications: as climate warming continues, temperature-specialist bat species may be forced to shift their elevational ranges to find suitable hibernation sites.

Elevation is a strong proxy for the thermal environment because it causes a predictable drop in temperature and food availability. This restricts cave-dwelling bats to species with specific metabolic traits, such as torpor or migration to avoid cold stress. In this context, we aimed to reveal how thermal niche differentiation structures 25 cave-dwelling bat assemblages along elevation gradients in two of the largest Bulgarian mountains—Stara Planina and Rhodopi. Multivariate PERMANOVA showed significant differences in bat assemblages among elevation groups (F = 1.616, p = 0.046), with altitude and temperature explaining 32.4% of the variance (p = 0.001). A high degree of species turnover (91.12% dissimilarity), driven by temperature niches, was observed: mesophilic Rhinolophus species dominated warm, low-elevation caves, while cold-adapted Myotis species were more common at high elevations. SIMPER analysis identified R. euryale as an indicator in low-elevation caves (p = 0.012) and the M. myotis/blythii complex at high elevations (p = 0.003). Alpha diversity showed no variation across elevation groups (p = 0.293), indicating that species turnover occurs without overall changes to local diversity. Mid-elevation assemblages lacked specific indicator species and resembled high-elevation communities, forming an ecotone. Thermal niche partitioning, as a physiological filter, shapes cave-dwelling bat assemblages and affects climate change range-shift predictions.