The key role of humidity for the survival of cave beetles in a context of climate change

Climate change is altering the environmental conditions of subterranean ecosystems, leading to warmer and drier caves across many regions. Despite growing evidence of the high heat sensitivity of cave-dwelling arthropods, the role of humidity as a limiting factor remains largely unexplored. To address this knowledge gap, we experimentally assessed the combined effects of temperature and humidity on the survival of four Pyrenean cave beetles ( Euryspeonomus eloseguii , Troglocharinus impellitieri , T. hustachei , and Stygiophyes ribagorzanus ). Adult beetles were exposed to eight combinations of temperature (11, 20, 23, and 25 °C) and relative humidity (100% and 75% RH) for seven days under controlled laboratory conditions. Survival was monitored daily, and Cox proportional-hazards models were used to analyze treatment effects. Our results showed a significant interaction between temperature and humidity across all species. By the end of the experiment, no individuals survived under 75% relative humidity at any temperature, nor at 23 °C or 25 °C in any humidity condition. Particularly, E. eloseguii and T. impellitieri were highly sensitive to humidity reduction, with no survival beyond 48 hours at 75% RH regardless of temperature. E. eloseguii also exhibited the lowest heat tolerance, with complete mortality at 20 °C even under saturated humidity. Our findings highlight the extreme vulnerability of subterranean beetles to desiccation and heat. Even moderate humidity reductions can drastically reduce survival, underscoring the need to consider combined climatic stressors when assessing the conservation status of subterranean species under climate change.