Bat population recoveries give insight into clustering strategies during hibernation

A Palearctic view of a bat fungal disease

The fungal infection causing white-nose disease in hibernating bats in North America has resulted in dramatic population declines of affected species, since the introduction of the causative agent Pseudogymnoascus destructans. The fungus is native to the Palearctic, where it also infects several bat species, yet rarely causes severe pathology or the death of the host. Pseudogymnoascus destructans infects bats during hibernation by invading and digesting the skin tissue, resulting in the disruption of torpor patterns and consequent emaciation. Relations among pathogen, host, and environment are complex, and individuals, populations, and species respond to the fungal pathogen in different ways. For example, the Nearctic Myotis lucifugus responds to infection by mounting a robust immune response, leading to immunopathology often contributing to mortality. In contrast, the Palearctic M. myotis shows no significant immunological response to infection. This lack of a strong response, resulting from the long coevolution between the hosts and the pathogen in the pathogen's native range, likely contributes to survival in tolerant species. After more than 15 years since the initial introduction of the fungus to North America, some of the affected populations are showing signs of recovery, suggesting that the fungus, hosts, or both are undergoing processes that may eventually lead to coexistence. The suggested or implemented management methods of the disease in North America have encompassed, for example, the use of probiotics and fungicides, vaccinations, and modifying the environmental conditions of the hibernation sites to limit the growth of the pathogen, intensity of infection, or the hosts' responses to it. Based on current knowledge from Eurasia, policy makers and conservation managers should refrain from disrupting the ongoing evolutionary processes and adopt a holistic approach to managing the epizootic.

Re-examining extreme sleep duration in bats: implications for sleep phylogeny, ecology, and function

Bats, quoted as sleeping for up to 20 h a day, are an often used example of extreme sleep duration amongst mammals. Given that duration has historically been one of the primary metrics featured in comparative studies of sleep, it is important that species specific sleep durations are well founded. Here, we re-examined the evidence for the characterization of bats as extreme sleepers and discuss whether it provides a useful representation of the sleep behavior of Chiroptera. Although there are a wealth of activity data to suggest that the diurnal cycle of bats is dominated by rest, estimates of sleep time generated from electrophysiological analyses suggest considerable interspecific variation, ranging from 83% to a more moderate 61% of the 24 h day spent asleep. Temperature-dependent changes in the duration and electroencephalographic profile of sleep suggest that bats represent a unique model for investigating the relationship between sleep and torpor. Further sources of intra-specific variation in sleep duration, including the impact of artificial laboratory environments and sleep intensity, remain unexplored. Future studies conducted in naturalistic environments, using larger sample sizes and relying on a pre-determined set of defining criteria will undoubtedly provide novel insights into sleep in bats and other species.

Cold arousal - A mechanism used by hibernating bats to reduce the energetic costs of disturbance

During the season of hibernation, temperate bats alternate between prolonged bouts of torpor with reduced body temperature and short arousals with a return to normothermy. Hibernating bats are sensitive to non-tactile stimuli and arouse following changes in microclimatic conditions or disturbance from other bats, potential predators, or humans. Here, we used temperature data loggers to register the skin temperature of 38 Myotis myotis bats over two winters (between January and March), during which regular visits were made to the hibernaculum. Two kinds of arousal were observed, normothermic (T_sk > 25 °C) and cold (T_sk < 15 °C). Although bats responded to the presence of a researcher by arousals of both kinds, cold arousals were more frequent (63.8%). We found that mass loss was not affected by the number of disturbances, however it was in positive relationship with the mass at the beginning of the observation and differed between sex and age categories. Furthermore normothermic bats crawling among cluster-mates initiated arousal cascades, which mainly consisted of cold arousals. We failed to detect any effect of age or sex on the number of arousals initiated by normothermic individuals. Warming by only a few degrees requires less energy than a normothermic arousal and we propose it is sufficient to activate the sensory system in order to assess the relevance of external stimuli. Our results indicate that cold arousals reflect a physiological and behavioural adaptation aimed at avoiding the energetic costs of disturbance that can lead to depletion of fat reserves.

Heterothermy and antifungal responses in bats

Hibernation, a period where bats have suppressed immunity and low body temperatures, provides the psychrophilic fungus Pseudogymnoascus destructans the opportunity to colonise bat skin, leading to severe disease in susceptible species. Innate immunity, which requires less energy and may remain more active during torpor, can control infections with local inflammation in some bat species that are resistant to infection. If infection is not controlled before emergence from hibernation, ineffective adaptive immune mechanisms are activated, including incomplete Th1, ineffective Th2, and variable Th17 responses. The Th17 and neutrophil responses, normally beneficial antifungal mechanisms, appear to be sources of immunopathology for susceptible bat species, because they are hyperactivated after return to homeothermy. Non-susceptible species show both well-balanced and suppressed immune responses both during and after hibernation.

Temperature driven hibernation site use in the Western barbastelle Barbastella barbastellus (Schreber, 1774)

In temperate regions, winter is characterized by cold temperatures and low food availability. Heterothermic animals can bridge this period by entering a state of torpor characterized by decreased body temperature and reduced metabolic rate. Hibernation site choice is crucial since temperature conditions in the hibernaculum will impact torpor. We analysed temperature-dependent hibernation site use of Barbastella barbastellus. Bats and temperature were monitored in an underground system (1999–2019) and standalone bunkers (2007–2019) in Western Poland. During the winter of 2017–2018 we analysed the thermal variability of the hibernacula. Seasonal variation is higher in bunkers and thus temperatures get colder in winter than in the underground system. On the other hand, short-term variability (thermal variability index) in the bunkers was lower than in the underground system. This makes bunkers a more stable environment to hibernate for cold dwelling bats in warm winters, when temperatures in the bunkers do not get below freezing. Bats use both the warm underground system and the colder bunkers. During the last decade, a continuous series of warm winters occurred and the population of barbastelle bats partly moved from the underground system to the bunkers. These present temperature increases broadened the range of potential hibernation sites for barbastelles. Our study indicates that long-term trends, seasonal variation and short-term variability in temperatures are all important and should be analysed to investigate hibernaculum use by bats. Our study shows that small hibernation sites may become more important in the future.