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<break></break>The Mediterranean horseshoe bat, <italic>Rhinolophus euryale</italic>, creates in the pre-hibernation period an aggregation in the Domica cave (Slovakia) and despite the harsh winter conditions displays a certain amount of flying activity. The goal of the study was to find out whether some environmental factors influence these conspicuous aggregation dynamics. We used image and statistical analysis of pictures of the aggregation taken automatically in the cave along with atmospheric pressure, humidity, outside and inside cave temperature and oxygen and carbon dioxide contents in the cave. These environmental variables were assumed to explain the observed activity. We hypothesised that the number of bats in particular time periods would correlate with some of these variables, but this was not confirmed. The only significant relations confirmed were those between bat numbers and humidity and carbon dioxide values in the cave. No trend in the spatial positioning of the bats was observed, and no environmental factors influenced the numbers, movements or groupings of bats. On the contrary, bats by their presence (breathing), activity and production of excrements may significantly influence the microclimate of the cave environment.<break></break><break></break>The Mediterranean horseshoe bat, <italic>Rhinolophus euryale</italic>, creates in the pre-hibernation period an aggregation in the Domica cave (Slovakia) and despite the harsh winter conditions displays a certain amount of flying activity. The goal of the study was to find out whether some environmental factors influence these conspicuous aggregation dynamics. We used image and statistical analysis of pictures of the aggregation taken automatically in the cave along with atmospheric pressure, humidity, outside and inside cave temperature and oxygen and carbon dioxide contents in the cave. These environmental variables were assumed to explain the observed activity. We hypothesised that the number of bats in particular time periods would correlate with some of these variables, but this was not confirmed. The only significant relations confirmed were those between bat numbers and humidity and carbon dioxide values in the cave. No trend in the spatial positioning of the bats was observed, and no environmental factors influenced the numbers, movements or groupings of bats. On the contrary, bats by their presence (breathing), activity and production of excrements may significantly influence the microclimate of the cave environment.<break></break><break></break>The Mediterranean horseshoe bat, <italic>Rhinolophus euryale</italic>, creates in the pre-hibernation period an aggregation in the Domica cave (Slovakia) and despite the harsh winter conditions displays a certain amount of flying activity. The goal of the study was to find out whether some environmental factors influence these conspicuous aggregation dynamics. We used image and statistical analysis of pictures of the aggregation taken automatically in the cave along with atmospheric pressure, humidity, outside and inside cave temperature and oxygen and carbon dioxide contents in the cave. These environmental variables were assumed to explain the observed activity. We hypothesised that the number of bats in particular time periods would correlate with some of these variables, but this was not confirmed. The only significant relations confirmed were those between bat numbers and humidity and carbon dioxide values in the cave. No trend in the spatial positioning of the bats was observed, and no environmental factors influenced the numbers, movements or groupings of bats. On the contrary, bats by their presence (breathing), activity and production of excrements may significantly influence the microclimate of the cave environment.<break></break> [ABSTRACT FROM AUTHOR]