Ectoparasite-posed risk may affect the spatial organization of hibernating clusters of a social bat

Abstract

Most Palearctic bats spend the winter in hibernation, with reduced metabolic rate (including body temperature, circulation, heart-rate, etc.), while descending into a motionless stasis. They select particular sites for roosting, sites which show similar characteristics independent of bat species or geographical location: thermally stable and inaccessible for predators. Bats balance between thermal eccentricities using different techniques, like clustering, moving to colder/warmer parts of the cave or arousal and active warm-up using muscle movement. Common bent-winged bats (Miniopterus schreibersii) hibernate in the same location throughout the winter, in large tight groups independent of thermal conditions. We hypothesize that this tight clustering behaviour is interrelated with parasite avoidance, i.e., bats try to reduce the individual risk of tick-parasitism, which is present only at the edge of the cluster. Analysing photos of bat groups in deep torpor, we sought for patterns in cluster formation and compared the distribution of tick-infested individuals in smaller and larger groups of bent-winged bats. Our results showed that there are significant differences in thermal conditions inside groups, with warmer central and colder border areas. The prevalence of tick-infested individuals was larger in smaller groups, than in larger ones, moreover these individuals were positioned close to the border of the cluster. While bats positioned close to the cluster’s edge have more stable and lower temperature conditions, they bear higher risk from ectoparasites. This leads to a trade-off between maintaining energetic balance vs. parasite-infestation risk. In conclusion, bent-winged bats that are clustering in large tight-knit groups, more effectively reduce the risk of tick infestation, even if this means a higher chance of thermal instability and arousal risk in the inner part of the roosting cluster.