Acta Theriol (2011) 56:123–127
DOI 10.1007/s13364-010-0018-5
ORIGINAL PAPER
Body temperature and body mass of hibernating little brown
bats Myotis lucifugus in hibernacula affected by white-nose
syndrome
Jonathan J. Storm & Justin G. Boyles
Received: 24 May 2010 / Accepted: 13 October 2010 / Published online: 22 December 2010
# Mammal Research Institute, Polish Academy of Sciences, Białowieża, Poland 2010
Abstract Populations of hibernating bats in the northeastern
USA are being decimated by white-nose syndrome (WNS).
Although the ultimate cause of death is unknown, one
possibility is the premature depletion of fat reserves. The
immune system is suppressed during hibernation. Although
an elevated body temperature (Tb) may facilitate an immune
response, it also accelerates the depletion of fat stores. We
sought to determine if little brown bats Myotis lucifugus Le
Conte 1831 hibernating in WNS-affected hibernacula have
an elevated Tb and reduced fat stores, relative to WNSunaffected Indiana bats Myotis sodalis Miller and Allen 1928
from Indiana. We found that WNS-affected M. lucifugus
maintain a slightly, but significantly, higher skin temperature
(Tskin), relative to surrounding rock temperature, than do M.
sodalis from Indiana. We also report that WNS-affected M.
lucifugus weigh significantly less than M. lucifugus from a
hibernaculum outside of the WNS region. However, the
difference in Tskin is minimal and we argue that the elevated
Tb is unlikely to explain the emaciation documented in
WNS-affected bats.
Communicated by: Karol Zub
J. J. Storm (*)
Division of Natural Sciences and Engineering,
University of South Carolina Upstate,
Spartanburg, SC 29301, USA
e-mail: [email protected]
J. G. Boyles
Center for North American Bat Research and Conservation,
Department of Ecology and Organismal Biology,
Indiana State University,
Terre Haute, IN 47809, USA
Present Address:
J. G. Boyles
Department of Zoology and Entomology, University of Pretoria,
Pretoria 0002, South Africa
Keywords Geomyces destructans . Hibernation . Immune
response . Psychrophilic fungus . Thermal preference
Introduction
Populations of hibernating bats in the northeastern USA are
being decimated by white-nose syndrome (WNS). The
characteristic feature of WNS is a white fungus that covers
the muzzle, ears, and/or wing membranes of affected bats.
The fungus was first discovered at four caves during the
winter of 2006–2007 in New York State and since then has
spread rapidly to caves throughout the eastern USA (Frick et al.
2010; current maps at www.fws.gov/WhitenoseSyndrome/).
The fungus associated with WNS, Geomyces destructans
(Gargas et al. 2009), is a recently described psychrophilic
(cold-loving) species that invades the living tissue of
hibernating bats (Meteyer et al. 2009). Even for WNSaffected bats that survive winter, infection with G. destructans
can inflict lasting damage to the wing membranes and impair
summer foraging (Reichard and Kunz 2009). The proximate
cause of death in many WNS-affected bats may be starvation,
but the ultimate cause is yet to be determined. Four possible
links between the fungus and starvation have been hypothesized: (1) WNS affects summer feeding or physiology,
causing bats to enter hibernation underweight, (2) affected
bats reduce time spent in torpor in order to mount an immune
response, (3) epithelial irritation associated with the fungus
causes bats to alter hibernation patterns, thereby increasing
energy expenditure (Boyles and Willis 2010), and (4) bats
with WNS cannot maintain water balance and must arouse
frequently to drink water (Cryan et al. 2010).
Using a modeling approach, Boyles and Willis (2010)
concluded that mortality from WNS is likely due to
changes in hibernation physiology, not pre-hibernal body
124
condition. During hibernation, the immune system of most
hibernators is partially or completely arrested (Carey et al.
2003). Following an immune challenge, the pattern of
arousal from hibernation suggests a need to periodically
raise body temperature (Tb) to mount an immune response
against pathogens (Luis and Hudson 2006; Prendergast et
al. 2002). Assuming the immune system of hibernating bats
is suppressed as it is in other hibernating mammals, WNS
may trigger a tradeoff in hibernation strategies. On one
hand, affected bats must remain torpid to conserve energy,
while on the other hand, they must maintain a sufficiently
high Tb, or arouse often enough, to mount an immune
response. Fighting an infection likely presents a more
immediate priority than conserving energy; therefore,
WNS-affected bats may reduce the expression of hibernation to mount an immune response.
Several hypotheses have been proposed for how
changes in hibernation lead to mortality. WNS-affected
bats may: (1) arouse more frequently than unaffected bats;
(2) remain euthermic for longer periods during each
arousal; or (3) increase mean Tb during hibernation. The
first two hypotheses have been modeled and discussed
elsewhere (Boyles and Willis 2010). The purpose of this
study was to evaluate the likelihood of the third
hypothesis. We sought evidence about whether WNSaffected bats thermoregulate at a higher Tb relative to the
surrounding rock temperature than unaffected bats. An
elevated Tb would lead to a higher torpid metabolic rate
and thus could lead to an increased rate of fat metabolism
during hibernation.
There is evidence that a majority of, but not all, WNSaffected bats are underweight at the time of death (Blehert
et al. 2009); however, there are no published data on
whether living WNS-affected bats are underweight during
hibernation. If WNS-affected bats are underweight during
hibernation, this would suggest that body fat is being used
too quickly throughout winter, not just immediately before
death. Thus, we also compared energy (fat) reserves of
affected and unaffected bats by comparing the mass of
WNS-affected bats in New York State to unaffected bats
from Ohio (Boyles et al. 2007). If WNS-affected bats are
underweight, but survive hibernation, they may still be
susceptible to death or decreased reproductive success
following spring emergence from hibernation.
Material and methods
Skin temperature of Myotis lucifugus
We took thermal images of 97 individuals or clusters of
naturally hibernating little brown bats M. lucifugus Le
Conte 1831 in two mines (Williams Lake Mine and
Acta Theriol (2011) 56:123–127
Williams Hotel Mine) in Ulster County, New York, USA,
on 12–13 February 2008. White-nose Syndrome was
documented at these mines in 2008 (Blehert et al. 2009),
shortly before the current study. All images were taken
using a ThermaCAM PM575 (FLIR Systems, North Billerica,
MA, USA) thermal imaging camera. Images were recorded as
digital files with ±0.1°C resolution. Upon entering hibernacula,
we took images of clusters from below so the bats’ lightly
furred faces comprised the majority of most images. To ensure
that only hibernating individuals were included, we avoided
clusters that exhibited thermal heterogeneity in skin temperature (Tskin) or contained active individuals. We used matching
photographic images to determine the number of individuals
in each cluster. We followed the methods of Boyles et al.
(2008) so that our data are comparable to those collected on
Indiana bats Myotis sodalis Miller and Allen 1928 in six
hibernacula within southern Indiana, USA. Boyles et al.
(2008) collected thermal image data on 202 clusters of
hibernating WNS-unaffected M. sodalis during January–
February 2007. M. sodalis and M. lucifugus are closely
related (Stadelmann et al. 2007) and share many physiological and ecological traits (Whitaker and Hamilton 1998).
These species often select similar microclimates within
hibernacula (Brack 2007) and form mixed-species clusters
during hibernation (J. Boyles, personal observation). As such,
we suggest that a comparison between M. lucifugus and M.
sodalis, while not perfect, still provides valuable data
regarding WNS. We recorded whether any bats in a cluster
were visibly affected by WNS, but subsequent research has
shown visible clues to be a poor indicator of WNS status
(Meteyer et al. 2009) and that most bats in affected caves
likely have WNS.
We analyzed thermal images using IRwin Research 2.01
software (FLIR Systems, North Billerica, MA, USA). We
calculated the mean and standard deviation of cluster
temperature within a polygon traced around each cluster.
Because the lightly furred faces of bats constituted the
majority of the picture, we were essentially measuring the
Tskin of bats in the cluster. Skin temperature closely reflects
Tb in small mammals (Dausmann 2005), including torpid
bats (Willis and Brigham 2003), so we considered the mean
temperature within the polygon to be a proxy of the mean
Tb of individuals in the cluster (Boyles et al. 2008). We
calculated mean rock temperature in an irregularly shaped
polygon adjacent to, but not touching, each cluster. The size
of each polygon could not be standardized because thermal
images of clusters were taken at varying distances. We
report raw cluster temperature data but we used the
difference between mean cluster and mean rock temperature
as the variable of interest in our analyses (Boyles et al.
2008). The difference between mean cluster and mean rock
temperature is an indication of the level of thermoregulation
used by bats in the cluster, while raw cluster temperature
Acta Theriol (2011) 56:123–127
125
data are likely to be strongly influenced by the temperature
of the hibernaculum.
We used an analysis of covariance in Statistica 6.0 to
determine if M. lucifugus in WNS-affected hibernacula
maintained a higher Tb relative to rock temperature than
WNS-unaffected M. sodalis in Indiana. We considered
population (Indiana or New York) as the independent
variable, difference between cluster temperature and rock
temperature as the dependent variable, and cluster size and
absolute rock temperature as covariates. Cluster size was
log-transformed to account for the large variation in cluster
size.
Mass of M. lucifugus
On 13 February 2008, we weighed 100 WNS-affected M.
lucifugus (52 males, 48 females) in the Williams Lake Mine
using a digital balance (Scout II; Ohaus, Pine Brook, NJ,
USA) accurate to 0.1 g. Mass alone (or mass corrected for
forearm length) is not always an effective indicator of fat
mass. However, body mass and fat mass are correlated in
M. lucifugus (Kunz et al. 1998). In addition, metabolic
water is formed during the catabolism of fat, potentially
skewing measures of body mass. Following data collection,
we released bats on a surface near where they were
obtained. Data were compared to measurements taken on
M. lucifugus from a WNS-unaffected limestone mine in
Preble County, Ohio, USA. During autumn and winter of
2007–2008, the average temperature at the New York and
Ohio sites were comparable, although Ohio was typically
slightly warmer than New York (Fig. 1). At the Ohio site,
we had previously collected mass data for 443 (169 males,
274 females) M. lucifugus on 12 February 2007 (Boyles et
al. 2007) and collected data on an additional 64 (34 males,
30 females) M. lucifugus on 9 February 2008. We tested for
differences in body mass of WNS-affected bats from New
York and WNS-unaffected bats from Ohio using t tests.
Although we do not have data on the body mass of bats
from either population at the onset of hibernation, we
assume that the relative similarity in climate between sites
makes this comparison feasible.
Results
Cluster size was variable for both WNS-affected M.
lucifugus in New York and WNS-unaffected M. sodalis in
Indiana (Table 1). The Tskin of WNS-affected bats was
above the surrounding rock temperature, while the Tskin of
WNS-unaffected bats was below rock temperature. Interestingly, the difference between Tskin and surrounding rock
temperature was significantly greater for WNS-affected bats
(F=195.1, df=1, 294, p<0.0001). This suggests that WNSaffected bats defend a higher Tb than WNS-unaffected bats.
Averaged across years, WNS-unaffected males from Ohio
had a body mass of 7.2±0.5 g, while WNS-affected males
from New York weighed 6.2±0.5 g (Fig. 2; data presented as
mean ± SD). Females from Ohio had a mean body mass of
8.0±0.7 g and females from New York weighed 6.5±0.5 g.
Male WNS-affected M. lucifugus from New York had a
significantly lower body mass than WNS-unaffected M.
lucifugus from Ohio during both 2007 (t=13.5, df=90,
p<0.001) and 2008 (t=9.1, df=58, p<0.001). In addition,
female WNS-affected M. lucifugus had less mass than WNSunaffected M. lucifugus in both 2007 (t=17.7, df=82,
p<0.001) and 2008 (t=9.8, df=51, p<0.001).
Discussion
30
Ohio
New York
Average Temperature (oC)
20
10
0
-10
-20
Sept 1
Oct 1
Nov 1
Dec 1
Jan 1
Feb 1
Fig. 1 Ambient temperature in western Ohio and southern New York
during autumn and winter of 2007–2008. Notice the similarity in
temperature between the two sites used in the body mass experiment.
Data taken from the United States Historical Climatology Network
(http://cdiac.ornl.gov/epubs/ndp/ushcn/daily_doc.html)
Our data represent the first published assessments of Tb and
body mass during hibernation for WNS-affected bats. We
found that M. lucifugus with WNS had a significantly
higher Tskin relative to surrounding rock temperature than
WNS-unaffected M. sodalis. However, given their relatively
low Tb, the WNS-affected M. lucifugus should also have a
low torpid metabolic rate. Although we did not collect
microclimate data in New York, we think the most likely
cause of this Tskin difference is microclimate differences
between hibernacula in New York and Indiana. The rock
temperatures (and therefore hibernaculum temperatures) are
considerably higher in Indiana than New York, suggesting
that M. sodalis in Indiana might lower Tb as close to ambient
temperature as possible in order to conserve energy, while
the colder temperatures in New York hibernacula mean that
M. lucifugus are not forced to abandon all thermoregulation.
Conversely, the ambient temperature in Indiana caves is
126
Acta Theriol (2011) 56:123–127
Table 1 Thermal data for WNS-affected little brown bats Myotis lucifufus from New York and WNS-unaffected Indiana Myotis Myotis sodalis
from Indiana
Group of bats
Cluster size
Cluster size range
WNS-affected M. lucifugus
WNS-unaffected M. sodalis
10.8±25.3
74.5±367.8
1–169
1–5,044
Tskin
Trock
Tskin −Trock
5.0°C±1.7°C
9.9°C±1.9°C
4.8°C±1.5°C
10.1°C±1.7°C
0.1°C±0.3°C
−0.2°C±0.3°C
Data for M. lucifugus were collected on 12–13 February 2008 during the current study, while data for M. sodalis were collected in a separate study
during January–February 2007 (Boyles et al. 2008). Data presented as mean ± SD
likely variable as evidenced by the large number of clusters
with temperatures below rock temperature (Boyles et al.
2008). A Tskin below rock temperature is typically caused by
air movement in the hibernaculum or variable ambient
temperatures and differential rates of heating and cooling
between clusters and the surrounding rock. We argue that the
difference between Tskin and rock temperature in WNSaffected bats is far too minor (the largest difference we
recorded was 0.7°C) to explain the mortality seen in WNSaffected populations.
Unfortunately, available data on metabolic rates and
torpor patterns have low resolution (i.e., measurements
usually taken every 3–5°C; Dunbar and Brigham 2010).
Therefore, any attempt at modeling the effect of such a
small increase in Tskin on survival would be dubious. Thus,
we are left making relatively qualitative comparisons
between the likelihood of WNS causing an increased Tb
during hibernation and changing patterns of arousals and
torpor bouts (Boyles and Willis 2010). Given the relatively
small increase in Tskin relative to rock temperature in WNSaffected bats, we suggest it is unlikely that bats challenge
the WNS infection by maintaining an elevated Tb. However,
the increase in Tb necessary to heighten the immune
response of bats is unknown.
9
Female
Male
Body Mass (g)
8
7
6
OH 2007
OH 2008
NY 2008
OH 2007
OH 2008
NY 2008
Fig. 2 Body mass of hibernating female and male little brown bats
(Myotis lucifugus) from Ohio (OH) and New York (NY). Data were
collected during February 2007 and 2008 for Ohio and February 2008
for New York
Our body mass measurements for M. lucifugus represent
some of the first empirical evidence that hibernating bats
affected by WNS are underweight during hibernation. We
found that the mean mass of hibernating M. lucifugus in
Ohio was 1.0 and 1.4 g higher than New York bats for
males and females, respectively. While data are not
available for the mass at the beginning of hibernation in
either Ohio or New York, we can draw some conclusions
about different rates of mass loss through the use of
bioenenergetic models (see Boyles and Brack 2009 and
Boyles and McKechnie 2010 for a complete description of
the model). The M. lucifugus in Ohio hibernate at ambient
temperatures between 2°C and 12°C (Brack 2007). For
these bats to attain the mean mass obtained in this study
and in Boyles et al. (2007), the beginning mass is predicted
to be between 8.0 and 9.2 g for males and 8.8 and 10.0 g
for females. Conversely, assuming the mass Kunz et al.
(1998) reported for M. lucifugus on 16 October at Aeolis
Cave in Vermont (8.6±0.33 and 8.9±0.38 g for males and
females, respectively), the WNS-affected M. lucifugus from
New York should have averaged 7.7±0.3 and 8.0±0.4 g for
males and females, respectively, during mid-February.
Thus, bats from both populations likely began the season
with similar mean body masses and the WNS-affected bats
from New York were 1.5 g lighter than that predicted by the
bioenergetic model. These results are in agreement with the
reduced fat stores found in necropsied WNS bats (Blehert et
al. 2009). The drastic differences in body mass between
populations, in addition to reduced fat stores found in
newly dead bats (Blehert et al. 2009), solidify the suspicion
that WNS leads to diminished energy reserves in affected
bats. Our mass data suggest that WNS-affected bats expend
energy too quickly throughout the winter as opposed to
expending their fat reserves in a short time period prior to
death. However, our data should be viewed with caution.
While environmental temperatures were similar between
the two sites (Fig. 1), we do not know whether the
hibernation period began near the same time for bats in
New York and Ohio, nor do we know the initial fat mass of
the bats at the onset of hibernation. In addition, we do not
have data on the fatty acid composition of foods consumed by
WNS-affected bats prior to hibernation. This may be important, as previous work has shown that the level of polyunsat-
Acta Theriol (2011) 56:123–127
urated fatty acids in the diet can influence the expression of
torpor in hibernating mammals (Frank et al. 2008).
Future laboratory studies will likely examine the relationship between WNS and Tb (or Tskin) during hibernation using
more controlled experiments than our preliminary field
research. Our data suggest that starvation in WNS-affected
bats is likely not the result of elevated Tb during hibernation.
We suggest instead that changes in hibernation patterns (e.g.,
increased arousal frequency and/or duration of euthermy;
Boyles and Willis 2010) are more likely to explain starvation
of WNS-affected bats. We suggest that future studies of
WNS-affected bats focus on arousal frequency, level of
activity during arousals (e.g. feeding), and the duration of
euthermy over an entire hibernation period.
Acknowledgments We thank A. Hicks of the New York Department
of Environmental Conservation (NYDEC) for organizing trips into
WNS-affected mines, and A. Hicks, J. Reichard, and M. Moore for
assistance in collecting data. Funding was provided by the Indiana
State University Center for North American Bat Research and
Conservation.
Ethical standards All procedures were approved by the Indiana
State University Animal Care and Use Committee (Protocol: JOW/JB
9-18-2006).
References
Blehert DS, Hicks AC, Behr M, Meteyer CU, Berlowski-Zier BM,
Buckles EL, Coleman JTH, Darling SR, Gargas A, Niver R,
Okoniewski JC, Rudd RJ, Stone WB (2009) Bat white-nose
syndrome: an emerging fungal pathogen? Science 323:227–227
Boyles JG, Brack V Jr (2009) Modeling survival rates of hibernating
mammals with individual-based models of energy expenditure. J
Mammal 90:9–16
Boyles JG, McKechnie AE (2010) Energy conservation in hibernating
endotherms: why “suboptimal” temperatures are optimal. Ecol
Model 221:1644–1647
Boyles JG, Willis CKR (2010) Could localized warm areas inside cold
caves reduce mortality of hibernating bats affected by white nose
syndrome? Front Ecol Environ 8:92–98
Boyles JG, Dunbar MB, Storm JJ, Brack V Jr (2007) Energy
availability influences microclimate selection of hibernating bats.
J Exp Biol 210:4345–4350
Boyles JG, Storm JJ, Brack V Jr (2008) Thermal benefits of clustering
during hibernation: a field test of competing hypotheses on
Myotis sodalis. Funct Ecol 22:632–636
127
Brack V Jr (2007) Temperatures and locations used by hibernating
bats, including Myotis sodalis (Indiana bat), in a limestone mine:
implications for conservation and management. Environ Manage
40:739–746
Carey HV, Andrews MT, Martin SL (2003) Mammalian hibernation:
cellular and molecular responses to depressed metabolism and
low temperature. Physiol Rev 83:1153–1181
Cryan PM, Meteyer CU, Boyles JG, Blehert DS (2010) Wing
pathology associated with White-nose Syndrome in bats
suggests life-threatening disruption of physiology. BMC
Biology 8:135. doi:10.1186/1741-7007-8-135
Dausmann KH (2005) Measuring body temperature in the field—
evaluation of external vs. implanted transmitters in a small
mammal. J Therm Biol 30:195–202
Dunbar M, Brigham RM (2010) Thermoregulatory variation among
populations of bats along a latitudinal gradient. J Comp Physiol
B 180:885–893
Frank CL, Carpovich S, Barnes BM (2008) Dietary fatty acid
composition and the hibernation patterns in free-ranging arctic
ground squirrels. Physiol Biochem Zool 81:486–495
Frick WF, Pollock JF, Hicks AC, Langwig KE, Reynolds DS, Turner
GG, Butchkoski CM, Kunz TH (2010) An emerging disease
causes regional population collapse of a common North
American bat species. Science 329:679–682
Gargas A, Trest MT, Christensen M, Volk TJ, Blehert DS (2009)
Geomyces destructans sp nov associated with bat white-nose
syndrome. Mycotaxon 108:147–154
Kunz TH, Wrazen JA, Burnett CD (1998) Changes in body mass and
fat reserves in pre-hibernating little brown bats (Myotis lucifugus).
Ecoscience 5:8–17
Luis AD, Hudson PJ (2006) Hibernation patterns in mammals: a role
for bacterial growth? Funct Ecol 20:471–477
Meteyer CU, Buckles EL, Blehert DS, Hicks AC, Green DE, ShearnBochsler V, Thomas NJ, Gargas A, Behr MJ (2009) Histopathologic
criteria to confirm white-nose syndrome in bats. J Vet Diagn Invest
21:411–414
Prendergast BJ, Freeman DA, Zucker I, Nelson RJ (2002) Periodic
arousal from hibernation is necessary for initiation of immune
responses in ground squirrels. Am J Physiol—Reg I 282:R1054–
R1062
Reichard J, Kunz T (2009) White-nose syndrome inflicts lasting
injuries to the wings of little brown Myotis (Myotis lucifugus).
Acta Chiropt 11:457–464
Stadelmann B, Lin LK, Kunz TH, Ruedi M (2007) Molecular
phylogeny of New World Myotis (Chiroptera, Vespertilionidae)
inferred from mitochondrial and nuclear DNA genes. Mol
Phylogenet Evol 43:32–48
Whitaker JO Jr, Hamilton WJ (1998) Mammals of the eastern United
States, 3rd edn. Cornell University Press, Ithaca
Willis CKR, Brigham RM (2003) Defining torpor in free-ranging bats:
experimental evaluation of external temperature-sensitive radiotransmitters and the concept of active temperature. J Comp
Physiol B 173:379–389