The risk faced by the early bat: individual plasticity and mortality costs of the timing of spring departure after hibernation

Hibernation is a widespread adaptation in animals to seasonally changing
environmental conditions. In the face of global anthropogenic change,
information about plastic adjustments to environmental conditions and
associated mortality costs are urgently needed to assess population
persistence of hibernating species. Here, we used a five-year data set of
1,047 RFID-tagged individuals from two bat species, Myotis nattereri and
Myotis daubentonii that were automatically recorded each time they entered
or left a hibernaculum. Because the two species differ in foraging
strategy and activity pattern during winter, we expected species–specific
responses in the timing of hibernation relative to environmental
conditions, as well as different mortality costs of early departure from
the hibernaculum in spring. Applying mixed-effects modelling, we were able
to disentangle population-level and individual-level plasticity in the
timing of departure. To estimate mortality costs of early departure, we
used both a capture mark recapture analysis and a novel approach that
takes into account individual exposure times to mortality outside the
hibernaculum. We found that the timing of departure varied between species
as well as among and within individuals, and was plastically adjusted to
large-scale weather conditions as measured by the NAO (North Atlantic
Oscillation) index. Individuals of M. nattereri, which can exploit milder
temperatures for foraging during winter, tuned departure more closely to
the NAO index than individuals of M. daubentoniid which do not hunt during
winter. Both analytical approaches used to estimate mortality costs showed
that early departing individuals were less likely to survive until the
subsequent hibernation period than individuals that departed later.
Overall, our study demonstrates that individuals of long-lived hibernating
bat species have the potential to plastically adjust to changing climatic
conditions, although the potential for adjustment differs between species.