Survival
The highest survival was observed in Persisting 2 (Cold + Wet) (29
survivors of 30, 96.67% surviving), mirroring the pattern of infection
severity and host body condition (Fig. 4; Supplemental Table 5). The
lowest observed survival occurred in Persisting 1 (Cold + Dry) (12
survivors of 30, 40% surviving) while Extirpated (Warm + Wet) displayed
an intermediate level of survival (24 survivors of 30, 80% surviving).
We found no effect of early hibernation body mass on survival
(Supplemental Table 6). The higher degree of mortality at Extirpated
(Warm + Wet) may have been related to the high pathogen growth rate
within the site, while the high mortality at Persisting 1 (Cold + Dry)
may be attributable to the particularly dry conditions, which appeared
to exacerbate infection severity. Additionally, low humidity conditions
increase evaporative water loss during infection with P.
destructans 78, so the high degree of mortality within
the cold and dry site may be attributable to the high infection severity
driving higher rates of water loss78–84. However, in
all sites, the level of survival observed was higher than that observed
during the initial epidemic within the same sites, suggesting traits
adaptive to surviving WNS are promoting persistence in these populations
(Fig. 4).
An additional line of evidence for adaptive traits promoting persistence
in this system comes from a similar study in 2009 in which bats naïve to
WNS were translocated to the same extirpated site used in this study. In
the historical experiment, all experimental bats died within 114 days
(compared to 110 days, the length of this experiment; Supplemental Fig.
4).85 Effectively, this represents two replicates of
extirpation within this site, in which all naïve bats succumbed to
disease during the initial epidemic and subsequent re-introduction in
2009. However, in this translocation experiment, 24 of the 30 (80%)
little brown bats we translocated from persisting colonies to the same
extirpated site survived hibernation, suggesting these populations now
have traits adaptive to surviving infection. Additionally, compared to
the bats in the extirpated site during the historical translocation
experiment, bats in our study had low disease severity scores as
indicated by UV fluorescence (Supplemental Fig. 5).