Incorporating evaporative water loss into bioenergetic models of hibernation to test for relative influence of host and pathogen traits on white-nose syndrome

Hibernation consists of extended durations of torpor interrupted by
periodic arousals. The ‘dehydration hypothesis’ proposes that hibernating
mammals arouse to replenish water lost through evaporation during torpor.
Arousals are energetically expensive, and increased arousal frequency can
alter survival throughout hibernation. Yet we lack a means to assess the
effect of evaporative water loss (EWL), determined by animal physiology
and hibernation microclimate, on torpor bout duration and subsequent
survival. White-nose syndrome (WNS), a devastating disease impacting
hibernating bats, causes increased frequency of arousals during
hibernation and EWL has been hypothesized to contribute to this increased
arousal frequency. WNS is caused by a fungus, which grows well in humid
hibernaculum environments and damages wing tissue important for water
conservation. Here, we integrated the effect of EWL on torpor expression
in a hibernation energetics model, including the effects of fungal
infection, to determine the link between EWL and survival. We collected
field data for Myotis lucifugus, a species that experiences high mortality
from WNS, to gather parameters for the model. In saturating conditions, we
predicted healthy bats experience minimal mortality. Infected bats,
however, suffer high fungal growth in highly saturated environments,
leading to exhaustion of fat stores before spring. Our results suggest
that host adaptation to humid environments leads to increased arousal
frequency from infection, which drives mortality across hibernaculum
conditions. Our modified hibernation model provides a tool to assess the
interplay between host physiology, hibernaculum microclimate, and diseases
such as WNS on winter survival.