**3. The susceptibility to** *Pseudogymnoascus destructans*

Field studies on hibernating bats demonstrated that *P. destructans* is found throughout both Europe and Asia, appearing on the skin of these bat species with no apparent increases in over-winter mortality or WNS [32, 33]. The greater mouse-eared bat (*Myotis myotis*) of Europe has been shown to be highly resistant to cutaneous *Pd* infections in both field [34] and laboratory studies [35]. Examination of hibernation sites in Europe revealed *Pd* growing on the muzzles of 5 different European species during torpor: pond (*Myotis dasycneme*), greater mouse-eared bat (*M. myotis*), Daubenton's (*Myotis duabentonii*), Brandt's (*Myotis brandtii*), and lesser mouse-eared (*Myotis oxygnathus*) bats. Mass deaths were not observed at these sites, however, and there were no apparent disruptions in torpor bout duration [34]. Histological analyses of infected *M. myotis* revealed that the hyphae of *Pd* do not extend beyond the epidermis of this bat species, even after several months of hibernation [36]. Some bat species are thus more resistant to *Pd* infections than others, thereby avoiding WNS. Cutaneous *Pd* infections and some associated skin lesions have subsequently been observed in 11 different European and 2 Asia bat species during hibernation [32, 33], with no apparent disruptions in torpor bout length or mortality. These studies indicate that European and Asian species of hibernating bats have evolved a resistance to *Pd* that greatly reduces the extent to which this fungus can infect the skin, thereby reducing the number of lesions that appear during hibernation to the point where torpor bout length is not significantly affected.

A similar resistance to both *Pd* infections and subsequent WNS is displayed by 4 species of North American bats as well. Field studies demonstrated that big brown bats (*Eptesicus fuscus*) hibernating where *Pd* occurs have torpor bouts of normal duration, and usually survive the winter with depot fat remaining [37]. Laboratory hibernation experiments with *E. fuscus* also revealed that *Pd* does not extensively grow in the skin of this species during hibernation [38]. The Eastern small-footed bat (*Myotis leibii*) is also highly resistant to cutaneous infections with *Pd*. A survey of 42 bat hibernation sites in the USA revealed that the number of *M. leibii* at these locations declined on average by only 12% during the first several years since the first appearance of *Pd*, whereas the number of *M. lucifugus*, *M. septentrionalis*, and *P. subflavus* at these sites decreased by 75–98% during this same period [39]. The Southeastern myotis (*Myotis austroriparius*) is a hibernating species found in Southern USA. Examinations of hibernation sites for this species in Alabama reveal although *P. destructans* first appeared in this area during 2011, no increases in the over-winter mortality of *M. austroriparius* have been observed. The skin of 99 hibernating *M. austroriparius* was examined for both the presence *P. destructans* DNA on it, and the UV-florescent skin lesions characteristic of *Pd* infections. Although 77% of the bats tested had *Pd* DNA on their skin, none of them had *Pd* skin lesions [40]. These findings indicate that although *M. austroriparius* was exposed to propagules (spores) of *Pd*, this fungus did not invade the skin of this bat species during hibernation, thus WNS does not develop. The gray bat (*Myotis grisescens*) is listed as an endangered species by the U.S. Fish & Wildlife Service, and some populations hibernate in caves located in the U.S. state of Tennessee. *Pseudogymnoascus destructans* first appeared in these caves during 2013, and some of the *M. grisescens* hibernating in these areas were found to have *Pd* skin lesions. No mass mortality has been observed for this species during hibernation, however, and the number of *M. grisescens* hibernating in these caves has been increasing since 2013. The total number of *M. grisescens* hibernating at 3 Tennessee caves increased by 15.4% during between 2016–2017 and 2018–2019, and it increased by 2.9-fold

*The Physiological Ecology of White-Nose Syndrome (WNS) in North American Bats DOI: http://dx.doi.org/10.5772/intechopen.100369*

at another cave during this same period [41]. It therefore appears that the degree of *Pd* infection that occurs during hibernation by *M. grisescens* is not sufficient to cause WNS.

There is ample evidence that some populations of *M. lucifugus* in the Northeastern United States have evolved a greater resistance to cutaneous infections with *Pd*, thereby avoiding WNS. A mark/recapture study conducted by Reichard et al. [42] revealed that the over-winter survival rate of *M. lucifugus* at 8 hibernation sites where *Pd* occurs in the Northeastern USA was at least 5.4%, with some individuals surviving 4 consecutive winters. Another mark and recapture study with *M. lucifugus* hibernating at a site in Michigan revealed that some individuals have survived 7 consecutive winters at a site where *Pd* occurs [43]. Studies on hibernating free-ranging *M. lucifugus* conducted during the first several years after the appearance of *Pd* revealed that although most individuals developed severe *Pd* infections with a high density of skin lesions that resulted in WNS, some individuals developed only moderate *Pd* infections that produced far fewer skin lesions, avoiding WNS and surviving the winter. Furthermore, the mean (± SE) torpor bout duration of these individuals with fewer lesions was 13.96 ± 4.30 d, which was not significantly different from that observed for *M. lucifugus* hibernating with no cutaneous *Pd* infections [25]. The differences in torpor bout lengths between individual *M. lucifugus* hibernating at the same site where *Pd* is found is illustrated by the skin temperature (Tskin) recordings of 2 adult females hibernating at the Williams Preserve Mine in New York State during the November–December period of 2008 (**Figure 1**). Skin temperature is equivalent to body temperature in small bats [44]. The first bat (**Figure 1A**) began hibernation with torpor bouts that were normal in length (15–20 d), but torpor bout length decreased to just 7–9 d during December 2008, indicating that this individual had succumbed to WNS. The second *M. lucifugus* (**Figure 1B**) examined, however, maintained torpor bouts that were 15–20 d long throughout the study period, demonstrating that it was not afflicted with WNS.

These studies indicate that for *M. lucifugus*, some individuals within certain populations are more resistant to *Pd* infections than others, and these are the bats that are surviving consecutive winters despite the presence of *Pd*. The consistent survival of some *M. lucifugus* in the presence of *Pd* has led to a partial recovery of some populations in New York State. A small maternity colony of *M. lucifugus* in NY examined by Dobony and Johnson [45] during the summers of 2006 through 2017 demonstrated that the size of it decreased by 88% after the first appearance of *Pd*, then stabilized during 2010–2014, and has been increasing since 2014. The New York Department of Environmental Conservation has been conducting annual counts of hibernating bats at the Williams Preserve Mine and Hailes Cave since 1999. These are 2 of the 6 bat hibernation sites where *Pd* first appeared during the winter 2007–2008. The number of *M. lucifugus* hibernating at the Williams Preserve Mine during the winter of 2008–2009 was just 12% of that observed prior to the first appearance of *Pd*, and the number at Hailes Cave was just 9% of the pre-*Pd* level for this site. The number of *M. lucifugus* observed at these sites during subsequent hibernation periods has since consistently increased, however. The number of *M. lucifugus* increased to 41% of pre-*Pd* levels by 2017 at the Williams Preserve Mine, and increased to 31% of pre-*Pd* levels at Hailes Cave by 2017 [23]. Another field study conducted at the Williams Preserve Mine indicates that this *M. lucifugus* population has evolved a higher resistance to *Pd* growth on their wings [46].

A field study conducted at a single hibernation site in NY during the winter of 2014–2015, about 6 years after *Pd* had arrived, indicated that the mean (± SE) torpor bout duration of *M. lucifugus* surviving the winter was 12.0 ± 10.8 d [47], which

#### **Figure 1.**

*(A) Skin temperatures of two different female Myotis lucifugus hibernating at the Williams preserve mine during November and December 2008. (B) Recordings of Tskin began 45 min after a temperature-sensitive radio transmitter was placed on each bat during Julian day 305. Radio signals were continuously recorded at 10–15 min intervals, torpor is defined as when Tskin < 24°C. \*indicates a period when the bat was out of the range of the automated radio receiver during an arousal episode. Data are from Frank et al. [23].*

is close to the normal torpor bout duration of 15–20 d previously reported for this species, thus indicating that most were hibernating normally. Another field study on *M. lucifugus* hibernating in the Williams Preserve Mine revealed that the bats hibernating at this site 1 year after the arrival of *Pd* (2008–2009) had: a) a mean torpor bout duration of 7.6 d, b) no depot fat reserves remaining by March, and c) an apparent over-winter mortality rate of 88%. The *M. lucifugus* hibernating at this same site 6–9 years after the arrival of *Pd*, in contrast, had: a) a mean torpor bout duration of 14.7 d, b) depot fat remaining in March, and c) an apparent over-winter mortality rate of 50% [23]. Interpreting these studies together reveals that some populations of *M. lucifugus* have recently evolved a greater resistance to cutaneous infections with *Pd*, thus reducing the frequency of WNS.
