Comparisons with free-flying bats
At the end of the experiment in late hibernation, we sampled free-flying
bats found roosting near the cages within each persisting site. At
Persisting 1 (Cold + Dry), we found that free-flying bats had
significantly lower tissue invasion compared to the bats within cages
(Fig. 5; Supplemental Table 7). Conversely, free-flying bats at
Persisting 2 (Cold + Wet) displayed a similar degree of infection
severity to caged bats within the same site. Free-flying bats had higher
late hibernation body mass than caged bats in both persisting sites
(Supplemental Fig. 6; Supplemental Table 8), and this difference was
more pronounced in Persisting 1 (Cold + Dry).
DISCUSSION
Our data suggest that environmental conditions interact with host traits
to jointly drive persistence of host populations. We found evidence for
elevated on-host growth of P. destructans with increasing
roosting temperature in bats that originated in Persisting 1 (Cold +
Dry), but no relationship in bats that originated in Persisting 2 (Cold
+ Wet). Infection severity, host body condition, and survival also
appeared to be influenced by site humidity, with higher disease severity
and lower survival associated with over-winter exposure to the driest
conditions in Persisting 1 (Cold + Dry). However, within the dry
conditions, bats sampled from outside of cages in late hibernation
displayed significantly lower infection severity compared to their caged
counterparts, whereas no such difference was detected in Persisting 2
(Cold + Wet). This suggests that bats within Persisting 1 (Cold + Dry)
may be utilizing a variety of microclimates and not remaining in these
dry environments for the entire winter period, as the caged bats
experienced. Importantly, the survival we observed during this
experiment was significantly higher than survival during the initial
epidemic within the same sites in all cases, as well as during a similar
experiment in 2009. Furthermore, we observed declines in pathogen loads
on 10 individuals, nine of which were in persisting sites. These data
collectively suggest that persisting little brown bat colonies in the
northeast United States have evolved traits beneficial to surviving
WNS88,89, but that these host traits interact with
environmental conditions such that protection against severe disease and
mortality depends to a strong degree on temperature and humidity.
We detected a positive relationship between average roosting temperature
during hibernation and on-host pathogen growth rate. This is
corroborated by data from the initial WNS epidemic, where the most
severely impacted colonies were those hibernating in relatively warm
hibernacula3. Infection severity and over-winter
weight loss showed a similar trend when humidity was high, with lower
values occurring at Persisting 2 (Cold + Wet) compared to Extirpated
(Warm + Wet). However, despite the coldest ambient conditions, infection
severity and host weight loss were high under the dry conditions within
Persisting 1 (Cold + Dry) and were comparable to the extirpated site.
Under unfavorable ambient conditions, fungal pathogens may forgo
reproduction and instead commit resources to within-host growth and the
formation of spores that can survive stressful conditions. For example,Metarhizium anisopliae is a fungal pathogen of tick eggs that
invades the egg tissue and undergoes growth90,91.
Under humid, favorable conditions, the fungus will emerge from the egg
to undergo asexual reproduction. However, under dry, unfavorable
conditions, the fungus will instead remain within the egg host, continue
to undergo growth, and produce environmentally resistant
spores91. We suggest that, similarly, exposure to dry
conditions of Persisting 1 (Cold + Dry) over winter were unfavorable to
the survival of P. destructans in superficial infections, and
that the pathogen augmented tissue invasion to satisfy moisture
requirements, resulting in a high degree of infection severity.
Additionally, evaporative water loss from hibernating bats is highest in
dry conditions92,93 and is exacerbated by infection
with P. destructans 78, resulting in dehydration
and increased arousal frequency to re-hydrate78–84.
Increased frequency of arousal from torpor drives the premature
depletion of fat reserves during hibernation, resulting in weight loss
and starvation58,94. Therefore, the increased tissue
invasion and evaporative water loss in Persisting 1 (Cold + Dry) may
have operated synergistically to result in severe disease and ultimately
the lowest observed survival.
Colonies of little brown bats in dry hibernacula may be persisting
because of the availability of different microclimates. Microclimatic
conditions are not uniform throughout an entire hibernation site, but
vary with factors such as depth, air flow, and the height of the
ceiling95. Recent evidence suggests that bats may
arouse and move to different roosting locations periodically during
hibernation96, possibly in response to shifting costs
associated with hibernation97, which could expose them
to a variety of microclimates98. For example, some
data suggest that bats may transition from roosting in relatively warm
sections of hibernacula in early hibernation to the relatively cold
sections by late winter96. In our study, bats were
unable to select varying microclimates over the course of hibernation.
However, we observed hundreds of little brown bats roosting in the area
surrounding the cages during late winter in Persisting 1 (Cold + Dry),
whereas less than a dozen individuals appeared to use that specific
location in early hibernation, suggesting that bats do not roost in the
same location for the entirety of hibernation in this site. Given that
disease severity is highly dependent on environmental conditions within
hibernacula, this movement behavior may have been pre-adaptive to
surviving WNS if bats utilize microclimates that mitigate disease
severity for at least part of hibernation. For example, movement within
hibernacula may reduce the growth of P. destructans in late
hibernation if bats move to the relatively cold conditions that slow
pathogen growth, potentially affording them enough time to emerge from
hibernation in spring and clear infection. Within Persisting 1 (Cold +
Dry), free-flying bats sampled at the end of hibernation had
significantly lower infection severity than caged bats, which is the
expected pattern if movement within hibernacula is indeed beneficial to
mitigating disease severity. Furthermore, free-flying bats in both
persisting sites had higher late hibernation body masses, and this was
more pronounced in Persisting 1 (Cold + Dry). Behavioral responses that
moderate the severity of disease have also been proposed for snake
populations impacted by snake fungal disease99, caused
by the fungal pathogen Ophidiomyces
ophiodiicola 100. Snakes infected with O.
ophiodiicola exhibit changes to their behavior that include increased
surface activity and more time spent in exposed environments compared to
their disease-free conspecifics99,100, potentially a
sign of a behavioral fever response to infection101.
However, we make the important distinction here that because movement
within hibernacula was observed in bats prior to the WNS epidemic, this
behavior may have been pre-adaptive to surviving the disease rather than
a direct response to the disease itself. Future research should
investigate how the availability and utilization of varying
environmental conditions can influence the dynamics of WNS, and how this
may scale up to a population-level response.
During the initial epidemic, the cold conditions within hibernacula
utilized by colonies may have prevented total colony collapse, allowing
standing genetic variation for favorable host traits to
propagate3,24,68,70,102. Previous research has also
found genetic evidence from persisting little brown bat colonies
indicative of a selective sweep following the invasion of P.
destructans 89,103. However, our data suggest that
populations that appear to have evolved adaptive host traits are only
afforded protection within a narrow environmental space. These processes
have the potential to result in local adaptation, in which the
evolutionary response of populations to WNS and the resulting dominant
phenotype is specific to the local environmental conditions of
hibernacula104. We detected an effect of origin site
on pathogen growth rate within Extirpated (Warm + Wet), potentially a
signature of local adaptation to differing conditions in source
hibernacula. However, for local adaptation to occur, the strength of
selection must be high enough to combat the homogenizing effects of gene
flow105–108, and current genetic evidence suggests a
panmictic genetic landscape for bat
populations109–113, but see114,115.
Environmental conditions within hibernacula are sensitive to conditions
present aboveground95, and global climate change has
the potential to alter the host-pathogen interaction in this
system116,117. Disease severity is strongly linked to
temperature and humidity conditions, as is the protection afforded by
unique host traits, as this study suggests. Therefore, even slight
changes to the environmental conditions within hibernacula may alter
host survival within a given site, potentially resulting in more severe
disease and higher mortality if conditions deviate from the
environmental space within which bats coexist with the pathogen. Future
research should explore the potential for climate change to impact the
disease dynamics of WNS, as well as the potential for bat populations to
respond to shifting conditions.
As P. destructans continues to spread throughout North America,
bat population declines and regional extirpations will continue to
occur. However, this study strongly suggests that prior to the invasion
of P. destructans , host traits conducive to surviving WNS
circulated in little brown bat populations, which now offer some
colonies imperfect protection from the disease. These host traits do not
operate independently to promote population persistence with WNS, but
rather interact strongly with environmental conditions, specifically
temperature and humidity, to ultimately drive host-pathogen coexistence.
Therefore, we should not expect to see all little brown bat populations
across North America stabilize or rebound from declines, but rather the
persistence of colonies with the correct combination of host traits and
environmental conditions.
Host population response to the invasion of a virulent pathogen will not
be predictable by a single aspect of the host, environment, or pathogen.
Rather, host-pathogen interactions and coexistence will be strongly
mediated by environmental conditions, the result of which may be as
variable as the environment itself118,119. Underlying
variation in host and pathogen populations will set the stage for
subsequent coevolutionary processes and the likelihood of
coexistence102,120, but this interaction and the
resulting host population response may be influenced by environmental
conditions that vary over space and time118,121, as
illustrated by this study. Therefore, to achieve predictability in how
emerging infectious diseases will impact host populations, it is
essential to disentangle host-environment-pathogen interactions across a
geographic and temporal mosaic of host-pathogen coevolution.
MATERIALS AND METHODS