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2014 NORTHEASTERN NATURALIST 21(4):587–605
Hibernating Bats and Abandoned Mines in the Upper
Peninsula of Michigan
Allen Kurta1,* and Steven M. Smith2
Abstract - Prior to arrival of white-nose syndrome, we found bats hibernating in 82 of 119
abandoned mines in northern Michigan. Unoccupied sites typically were short (19 ± 17 m
SD) and/or experienced chimney-effect airflow, which led to temperatures near or below
freezing (-0.8 ± 2.9 ºC). Overall, occupied sites were more structurally complex, longer
(307 ± 865 m), and warmer (5.7 ± 3.0 ºC) than unoccupied mines. Number of bats varied
from 1 to > 55,000, although the median was 115. Perimyotis subflavus (Eastern Pipistrelle)
and Eptesicus fuscus (Big Brown Bat) accounted for only 0.5% of the total of 244,341 bats
that were observed. Ninety percent of hibernating animals were Myotis lucifugus (Little
Brown Bat), and almost 10% were M. septentrionalis (Northern Bat). Relative to Little
Brown Bats, Northern Bats were more common in the mines of the Upper Peninsula than
in hibernacula in the East and Ohio River Valley. Maximum ambient temperature, presence
of standing water, and water vapor pressure deficit were potential predictors of the number
of Myotis that was present. Seventy-five percent of Northern Bats and 22% of Little Brown
Bats roosted alone, rather than cluster with other bats. Little Brown Bats in Michigan were
solitary much more often than in the East.
Introduction
In temperate areas, such as the Great Lakes region, bats provide economically
important ecosystem services to humans that involve consumption of agricultural
and forest pests and potential vectors of human disease (e.g., Long et al. 2013,
Münzer 2008). Despite the economic importance of bats, these long-lived, slowreproducing
mammals are experiencing unprecedented mortality.The burgeoning
wind-power industry in the United States now kills an estimated 600,000 bats per
year through collisions with spinning turbine blades and barotrauma (Hayes 2013).
In addition, an introduced disease, white-nose syndrome, may have caused the
death of >5.5 million hibernating bats since 2006 (US Fish and Wildlife Service
2012). Pseudogymnoascus destructans (Gargas, Trest, Christensen, Volk, and Blehart),
the fungus that causes white-nose syndrome, induces frequent arousals from
hibernation, which ultimately lead to the starvation of these small mammals by depleting
fat reserves long before food (flying insects) is available in spring (Reeder
et al. 2012). The disease spread rapidly, northward and southwestward from its
epicenter in eastern New York, but it was not documented in the western basin of
the Great Lakes (Wisconsin and Michigan) until spring 2014 (Whitenosesyndrome.
org 2014).
1Department of Biology, Eastern Michigan University, Ypsilanti, MI 48197. 2S.M. Smith
Co., 1105 Westwood Avenue, Iron Mountain, MI 49801. *Corresponding author - akurta@
emich.edu.
Manuscript Editor: Hugh Broders
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Species of bats that are affected by white-nose syndrome hibernate underground
in caves and cave-like sites, such as mines and concrete tunnels, where temperatures
remain cool but above freezing throughout winter. Suitable subterranean
retreats are rare in many parts of the western Great Lakes region (Kurta 1995),
such as the Lower Peninsula and eastern Upper Peninsula of Michigan, where only
5 hibernacula are known (Kurta 2008, Slider and Kurta 2011). The western Upper
Peninsula, though, is geologically different from the rest of the state, and hundreds
of underground mines were excavated there in a search for iron and copper beginning
in the 1840s (Lankton and Hyde 1982, Reed 1957). Although none of these
mines is operating today, and most have become flooded or are permanently sealed,
some sites remain open and are used by overwintering bats (Kurta and Smith 2004).
Little information exists on bats in the mines of the Upper Peninsula. Available
publications do not focus on characteristics of the mines as hibernacula (e.g., Bricklin
et al. 2007, Stones and Branick 1969, Stones and Oldenburg 1967, Sullivan et al.
2012), and only 1 limited attempt has been made to catalog the species of bats that
are present (Stones 1981). We have examined more than 100 open mines throughout
the copper and iron ranges of Michigan since 1996, searching for hibernating
bats, conducting repeated censuses, and recording physical characteristics such as
length, temperature, and humidity. In this paper, we summarize that data to establish
a baseline for future comparison of the bats and their hibernacula following arrival
of white-nose syndrome. In addition, we describe characteristics of occupied and
unoccupied sites, investigate factors affecting the number of bats that are present in
a mine, quantify the tendency of the most common species to form clusters during
hibernation, and make pertinent comparisons with overwintering bats in the East
and in the Ohio River Valley.
Methods
Study area
Our surveys took place in the western third of the Upper Peninsula, which
is bordered by Lake Superior to the north and Wisconsin to the south and west
(46.79–47.43°N, 87.59–90.14°W). The area is sparsely populated by humans, with
densities as low as 1.6 persons/km2 in some counties (US Census Bureau 2014).
The region is largely covered by northern hardwoods and conifers and underlain by
igneous and metamorphic rocks (Albert et al. 1986). Elevation varies from 184 to
604 m above sea level, but maximum elevation above Lake Superior is only 427 m.
Winters are snowy, long, and cold. Although amount of snowfall varies with
elevation and distance from Lake Superior, average snowfall is up to 6.1 m/year
(Keen 1993). Average date of the first overnight minimum temperature of 0 °C for
14 communities in the study area is 18 September, and the last such minimum typically
occurs on 1 June. The average, lowest, minimum temperature reached each
winter varies from -34 to -25 °C (Albert et al. 1986). Mean annual temperature,
which greatly impacts the temperature of underground sites where bats hibernate
(Domínguez-Villar et al. 2013), is 4.2–5.5 °C (Albert et al. 1986).
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Mining terminology
To understand where bats are usually found within the mines, definitions of
basic mining terms are needed (Lankton and Hyde 1982, Reed 1957, Sherwin and
Altenbach 2009). A “shaft” is an entrance to a mine that penetrates the earth at
a steep angle. In Michigan, shafts may be vertical, especially at iron mines, but
many shafts for copper mines descend at an angle of 45–75°, following the dip of
mineral-bearing strata. An “adit” is a horizontal passage entering a mine that may
end blindly or connect with shafts and drifts. A “drift” is any horizontal passage,
extending from a shaft or adit, whereas a “crosscut” extends laterally from a drift.
Miners excavated drifts from shafts at regular intervals, typically every 30 m, and
each successive drift is considered a “level.” A “winze” is a shaft-like opening
downward from a drift, usually to connect with a lower level. A “stope” is a cavity
formed by removal of ore; stopes typically extend from the side of a drift and are
inclined upward, following the mineral-bearing rocks.
Field surveys
Physical characteristics of mines. After finding a mine, we determined its latitude
and longitude, using hand-held global-positioning units, and noted maximum width
and height of the largest entrance. Upon entering a new site, we conducted a simple
survey, recording approximate length, passage dimensions, and other significant features,
such as presence of different levels, stopes, and standing water. We arbitrarily
defined standing water as any pool at least 15-cm deep, 2-m long, and 1.5-m wide,
which was the typical width of adits and drifts. Standing water most commonly occurred
as pools in adits or drifts and occasionally in flooded shafts or winzes.
We also assigned each mine a number that indicated its complexity. A score of
1 was given to mines that consisted of a single adit and/or shaft. A complexity of 2
indicated mines with drifts or crosscuts, and a value of 3 was assigned to sites with
1–2 stopes and/or a second level that was less than 25% of the length of all passages
in the mine. The highest score of 4 was given to mines with 3 or more stopes and/or
2 or more levels, with the additional levels contributing more than 25% to the total
length of the mine.
We measured environmental variables as close to the ceiling as possible, and
the number of such measurements varied with the complexity of each mine. In
simple mines (complexity of 1–2), measurements occurred at the midpoint and
end of adits and at the end of drifts or crosscuts. In complex sites (3–4), additional
measurements were made at the top of stopes and on each level at the end of the
drifts. Relative humidity was determined with a sling psychrometer, and ambient
temperature was measured with various electronic instruments or the dry bulb of
the psychrometer. We calculated equilibrium vapor pressure using temperature and
the quadratic formula of Tabata (1973) and then determined actual vapor pressure,
based on relative humidity.
Evaporative water loss from an animal is proportional to the difference between
vapor pressure of the body fluids (as determined by surface temperature of the
body) and saturation vapor pressure (Hill 1976). However, if surface temperature
of the animal is equal to ambient temperature, as is often true for a hibernating
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bat, then vapor pressure of the body fluids is equal to the equilibrium vapor pressure
(Kurta 2014). Consequently, we used the water vapor pressure deficit (i.e., the
difference between equilibrium and actual vapor pressure) as an index to potential
evaporative water loss that a hibernating bat might experience.
Number and species of bats. In each site, we counted the bats that were present
with the aid of hand counters. Bats in northern Michigan begin entering hibernation
by late September and start leaving in late April, although a few are present into June
(Kurta et al. 1997, Stones and Fritz 1969). Consequently, we performed counts between
late December and March, although some visits unavoidably occurred outside
this window. In particular, entrances to 7 occupied mines usually became completely
blocked with ice by late autumn and necessarily were surveyed in late November
or early December prior to icing-up. Although surveys began in 1996, most mines
(93%) in which counts occurred were discovered or revisited between 2010 and 2013.
The authors often simultaneously counted bats in different sections of a site to
minimize time inside a hibernaculum. To compare our abilities, on 11 occasions,
both authors counted the same bats in sections of mines that contained 140–1576
bats. The mean difference between counts was 3.6 ± 17 (SD) bats and was not significantly
different from zero (paired t10 = 0.72; P = 0.49), indicating no consistent
bias between observers.
We also identified some bats to species and estimated relative abundance. Perimyotis
subflavus (Cuvier) (Eastern Pipistrelle) and Eptesicus fuscus (Palisot de
Beauvois) (Big Brown Bat) can be identified unambiguously from a distance, and
we recorded the presence of these species whenever we encountered them. However,
definitive identification of Myotis lucifugus (LeConte) (Little Brown Bat) and
M. septentrionalis (Trouessart) (Northern Bat) required close inspection of a roosting
animal and often handling the bats so that the ear and tragus were more visible.
To minimize disturbance to bats and the attendant use of stored fat (Speakman et
al. 1991, Thomas 1995), we generally did not identify each Myotis to species and
simply combined all Myotis in our counts. Nevertheless, we did examine samples
of Myotis, in hand, when time and safety allowed, to determine overall relative
abundance of Little Brown and Northern Bats. Samples were obtained by arbitrarily
delineating sections of passage, about 10 m in length, and identifying all bats that
were present. Samples were obtained before and after the midpoint of simple mines
and in the stopes and on different levels of complex mines.
Clustering. Many species of bats cluster while hibernating, and Langwig et al.
(2012) recently suggested that clustering occurs less often after P. destructans is
established. Therefore, we also gathered baseline information on clustering patterns
of Myotis at 13 sites during winters 2011–2012 and 2012–2013. All tunnels
for this survey were typical adits and drifts, with passages about 1.8-m high and
1.5-m wide, which allowed the senior author to inspect each roosting bat, identify
it to species, and note the number of individuals with which it was in contact. If a
survey yielded less than 10 individuals of a species, then data for that species from
that mine were discarded. Clusters containing more than 1 species were uncommon
and not included in the analyses.
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Analysis of data
Species of Myotis were the most common (>99%), and to investigate factors
affecting number of Myotis present in a site, we performed negative binomial regression,
using a loge-link function and loge of the length of the mine as an offset
variable (Coxe et al. 2009, Gardner et al. 1995). Although many factors may affect
the number of bats, we restricted our analysis to 5 variables to avoid overfitting
and spurious results (Anderson 2008). Based on our knowledge of bats and the
literature, we hypothesized that potentially important independent variables were
maximum ambient temperature, which affects energetic expenditure (Boyles et al.
2007); water vapor pressure deficit, which should reflect evaporative water loss
(Kurta 2014); presence of standing water, which would provide a source of drinking
water (Taylor and Tuttle 2007); and size of the largest entrance, which could
impact a bat’s ability to access the site. Because maximum temperature in an underground
site typically is limited by surface temperature (Slagstad et al. 2008), we
always paired maximum temperature squared with temperature, to reflect the expected
nonlinear relationship (Hayes et al. 2011). Height and width of the entrance
were highly correlated, so we used principal components analysis to create a single
variable reflecting overall maximum size of the entrance.
We analyzed 15 models that represented the null model (intercept only) and
all possible combinations of the variables of entrance size, water vapor pressure
deficit, presence of standing water, and the pair of temperature variables, as main
effects (see Results). We evaluated competing models using an informationtheoretic
approach and by calculating Akaike’s information criteria (AICc), model
probabilities, and evidence ratios (Anderson 2008). Only occupied mines (by bats
of any species) that had been completely surveyed and for which all variables had
been measured were used in this analysis.
In addition to the negative binomial regression, we examined some simple relationships
between variables using Pearson’s correlations. Comparisons between
means were analyzed with appropriate t-tests. All statistics were performed using
IBM Statistics 21 (IBM Corporation 2012). Means are presented with ± 1 standard
deviation, and alpha was set to 0.05.
Results
We examined 119 subterranean sites in the western Upper Peninsula, including
91 copper mines, 26 iron mines, 1 dolomite mine, and 1 putative gold mine. Mines
were not randomly distributed across the landscape, but their location was highly
dependent on local geology (Fig. 1). In particular, most copper mines were restricted
to a narrow outcropping of igneous rock called the Portage Lake Volcanics
(Butler and Burbank 1929), and most iron mines were located in the southeastern
part of the study area, in the Menominee Iron Range (Reed 1957).
Physical characteristics of mines
Unoccupied sites. Thirty-seven sites (31%) contained no bats of any species. At
1 mine, a modern building had been erected over the shaft, effectively excluding
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bats. The 36 other sites without bats were typically short (19 ± 17 m; Table 1), cold
(-0.8 ± 3.1 ºC; Table 1), and simple, with 97% receiving a complexity score of 1.
Most of these (84%) were blind-ending shafts (14 sites) or adits (17). Although 5
additional mines possessed 2–3 entrances, all underground passage was located between
the openings, leading to noticeable chimney-effect airflow and temperatures
near freezing (Sherwin and Altenbach 2009).
Occupied sites. Eighty-two of the 119 mines (69%) contained at least 1 hibernating
bat. Total length of passage for the occupied mines varied from 10 m to 6.4
km, with an average of 307 ± 865 m (Table 1; Fig. 2). Number of bats of all species
present was correlated with length of the mine (loge-transformed values: r = 0.50;
P < 0.0001). Although most occupied sites (57%) also received a complexity score
of 1, many were given values of 2 (23%), 3 (9%), or 4 (11%). Complexity and logetransformed
length were correlated (r = 0.79; P < 0.0001; n = 81). Maximum ambient
temperature in occupied sites was 5.7 ± 3.0 ºC (Table 1), and most (55%) mines used
by bats were between 5 and 9 ºC (Fig. 3). Mean vapor pressure deficit was (0.075 ±
0.071 kPa). Pools of standing water occurred in 52 (63%) of the 82 mines.
Figure 1. Map showing the location of 82 mines used for hibernation by bats in the western
Upper Peninsula of Michigan. Background shading represents different types of bedrock;
PLV = Portage Lake Volcanics.
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Table 1. Characteristics of mines that were unoccupied or occupied by different groups of bats in the western Upper Peninsula of Michigan. Listed values
are mean ± SD (median), range, n.
Maximum ambient Water vapor pressure
Type of site Length (m) Number of bats temperature (°C) Water vapor pressure (kPa) deficit (kPa)
Unoccupied mines 19 ± 17 (14), -0.8 ± 2.9 (-0.3), 0.697 ± 0.112 (0.747), 0.111 ± 0.112 (0.075),
2–84, -8.7–4.0, 0.509–0.817, 0–0.355,
36 22 14 14
All occupied mines 307 ± 865 (52), 2980 ± 8494 (115), 5.7 ± 3.0 (6.4), 0.889 ± 0.134 (0.915), 0.075 ± 0.071 (0.070),
10–6402, 1–55,685, -1.3–10.8, 0.484–1.111, 0–0.260,
79 82 81 72 72
Mines with Eastern Pipistrelles 374 ± 589 (145) 3.3 ± 2.3 (3), 7.3 ± 2.4 (8.0), 0.963 ± 0.103 (0.988), 0.043 ± 0.063 (0.005),
24–2692, 1–10, 1.5–10.8, 0.709–1.111, 0–0.198,
23 23 23 22 22
Mines with Big Brown Bats 241 ± 525 (72), 24 ± 62 (4), 5.5 ± 2.8 (6.0), 0.878 ± 0.136 (0.910), 0.079 ± 0.075 (0.068),
14–2692, 1–338, -1.3–10.8, 0.484–1.111, 0–0.026,
46 46 46 43 43
Mines with Myotis 314 ± 886 (48), 3173 ± 8733 (132), 6.0 ± 2.9 (6.5), 0.898 ± 0.126 (0.924), 0.071 ± 0.069 (0.068),
10–6402, 1–55,685, -0.1–10.8, 0.533–1.111, 0–0.251,
77 77 78 72 72
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Over the years, many entrances had become partly blocked by dirt and rocks,
often leaving an irregular space barely large enough for a human to pass. Maximum
height or width of the largest entrance was less than 1 m in 25 occupied mines
(32%), although average height and width was 1.6 ± 1.5 and 2.1 ± 1.9 m, respectively.
Sixty-nine mines (84%) had only a single entrance. In sites with more than
1 opening, few, if any, bats roosted in the passage between entrances; virtually all
bats (>95%) were concentrated instead in stopes above the tunnels that connected
the entrances, in blind-ending drifts or cross-cuts, or on different levels.
Number and species of bats
Eastern Pipistrelles. We counted 244,341bats representing 4 species—the Little
Brown Bat, Northern Bat, Big Brown Bat, and Eastern Pipistrelle, the latter of
which was the least common species. Only 75 Eastern Pipistrelles were discovered,
and the species was found at 23 (28%) of the 82 occupied sites and constituted
0.03% of all bats hibernating in the mines. Median number of Eastern Pipistrelles,
when present, was 3, and the maximum was 10 (Table 1). Eastern Pipistrelles were
Figure 2. Top: Percent of all
occupied mines of different
length and percent of the
total number of bats associated
with mines of different
length. Bottom: Percent of all
occupied mines of different
complexity and percent of the
total number of bats associated
with mines of differing
levels of complexity.
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never the only species present in a mine; all 23 sites were shared with Myotis, and
17 (77%) also contained Big Brown Bats. Hibernacula with Eastern Pipistrelles
were warm (7.3 ± 2.4 °C) and had low water vapor pressure deficits (0.043 ± 0.063
kPa; Table 1).
Big Brown Bats. A total of 1106 Big Brown Bats roosted in 45 (55%) of the 82
occupied mines, and the species represented about 0.5% of all bats overwintering
in the mines. Median number present was 5. Thirteen mines contained only 1
Big Brown Bat, 19 had 2–10 individuals, 9 sites sheltered 11–40 Big Brown Bats,
and 4 sites had more than 100 individuals, with a maximum of 338 Big Brown
Bats in any mine. Forty-one hibernacula (89%) occupied by Big Brown Bats also
contained at least 1 Myotis. Maximum air temperature in mines with Big Brown
Bats was 5.5 ± 2.8 °C, and the water vapor pressure deficit was 0.079 ± 0.075 kPa
(Table 1).
Little Brown and Northern Bats. Species of Myotis occurred in 77 of 82 (94%)
occupied mines and represented over 99% of all bats counted. Median number of
Myotis in mines that contained at least 1 Myotis was 132, but the 39 hibernacula
Figure 3. Top: Percent of
all occupied mines having
different maximum
ambient temperature and
percent of the total number
of bats associated with
different maximum ambient
temperatures. Bottom:
Percent of all occupied
mines having different water
vapor pressure deficits
and percent of the total
number of bats associated
with different water vapor
pressure deficits.
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with the number of Myotis at or below the median accounted for only 1052 bats
or 0.4% of total Myotis. Over half the Myotis (56%) were concentrated in 4 large,
complex sites. Almost all Myotis (99%) hibernated in mines with a maximum
ambient temperature of 4–10 °C, and 59% of Myotis occupied hibernacula with a
temperature between 7 and 9 °C. Mean water vapor pressure deficit was 0.071 ±
0.069 kPa (Table 1).
Samples containing as many as 875 Myotis were handled and unequivocally
identified to species in 48 sites, with a strong correlation between number of bats in
a sample and number of Myotis in the mine (loge-transformed values: r = 0.88; P less than
0.0001). Little Brown Bats constituted 90% of identified animals, and the other 10%
were Northern Bats. Although relative abundance was not estimated in all sites, our
notes indicated that Northern Bats and Little Brown Bats co-occurred in 92% of
the 77 mines occupied by Myotis, and the few exceptions contained only 5 or fewer
Myotis of either species.
For the negative binomial regression, a complete set of data was available for
66 mines. The best model (lowest AICc) for predicting number of Myotis contained
temperature, temperature squared, and presence of standing water, whereas the
second best-supported model included the same variables along with water vapor
pressure deficit (Table 2). The initial third- and fourth-place models involved
addition of entrance size to the first and second models. However, entrance size
appeared to be a “pretending variable” (sensu Anderson 2008) in both instances,
because ΔAICc was approximately 2 when compared with the less complex model,
Table 2. Candidate models and associated parameters for predicting number of Myotis in mines,
based on negative binomial regression, with a loge-link function and loge of the length of a mine as an
offset variable. Models were ranked using Akaike’s information criterion (AICc), model weights, and
evidence ratios (Anderson 2008). Variables are: ta = ambient temperature; ta2 = ambient temperature
squared; water = presence of standing water; wvpd = water vapor pressure deficit; and entrance = size
of entrance, as indicated by scores extracted from principal components analysis of maximum height
and width of entrance. Two models containing the variable entrance were discarded because entrance
appeared to be a “pretending variable” (Anderson 2008); these excluded models were ta ta2 water
entrance and ta ta2 water wvpd entrance.
Model AICc ΔAICc Weight Evidence ratio
ta ta2 water 807.71 0.00 0.652 1.00
ta ta2 water wvpd 809.22 1.51 0.307 2.12
water 814.93 7.22 0.018 36.98
water entrance 815.96 8.25 0.011 61.74
water wvpd 817.04 9.33 0.006 106.11
water wvpd entrance 817.41 9.70 0.005 127.55
ta ta2 entrance 821.29 13.58 0.001 888.03
ta ta2 822.03 14.32 0.001 1284.98
ta ta2 wvpd entrance 822.90 15.19 <0.001 1987.23
ta ta2 wvpd 824.27 16.55 <0.001 3934.35
intercept 829.95 22.24 <0.001 67,541.67
wvpd 830.44 22.73 <0.001 86,120.26
entrance 832.14 24.43 <0.001 20,1692.59
wvpd entrance 832.46 24.75 <0.001 23,6925.26
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the deviance of the model changed minutely after addition of entrance size, and
the 95% confidence intervals of the coefficient for entrance size included zero.
Therefore, these 2 models were removed from the analysis. None of the remaining
models was supported (all ΔAICc > 7).
Clustering
At 13 sites, we determined cluster size for 85–2585 Little Brown Bats (total
= 9640 bats) that roosted in 22–1169 groups per mine (total = 4390 groups, with
a solitary animal considered a group of 1). Solitary bats represented 52 ± 12% of
the groups and 22 ± 11% of all Little Brown Bats examined in each mine, whereas
clusters containing more than 10 bats were 4 ± 5% of the groups and involved 17 ±
16% of the individuals (Fig. 4). Average maximum number of Little Brown Bats in
a cluster was 21 ± 14, with a single highest value of 58. Actual ambient temperature
in these 13 mines at the time and location of the survey was 7.5 ± 2.3 °C, and the
percentage of Little Brown Bats that was solitary was positively correlated with
temperature (r = 0.67; P = 0.01; Fig. 5), i.e., Little Brown Bats were more likely to
cluster at lower temperatures.
Figure 4. Top: Average
percent of groups of varying
size and average percent
of the total number
of bats found in clusters
of varying size for Little
Brown Bats at 13 mines;
overall, 9640 Little Brown
Bats in 4390 groups were
surveyed (a solitary animal
was considered a
group of 1). Bottom: Average
percent of groups of
varying size and average
percent of the total number
of bats found in clusters of
varying size for Northern
Bats at 9 mines; overall,
214 Northern Bats in 186
groups were surveyed.
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For Northern Bats, cluster size was determined at 9 sites, containing 10–79
Northern Bats (total = 214 bats) in 8–72 groups per site (total = 186 groups).
Solitary animals represented 87 ± 12% of all groups and 75 ± 21% of all Northern
Bats (Fig. 4). Only 2 clusters, 1 with 4 and the other with 6 bats, contained more
than 3 Northern Bats. Ambient temperature in these 9 mines was 7.3 ± 1.5 °C, but
unlike Little Brown Bats, the percentage of Northern Bats that was solitary in each
mine was not significantly correlated with temperature (r = 0.36; P = 0.34; Fig. 5).
The proportion of Northern Bats that roosted alone was greater than for Little
Brown Bats (paired t8 = 7.99; P < 0.0001) in the 9 sites in which both species were
evaluated with a sufficient sample.
Discussion
Much of the literature on hibernating bats emphasizes the effect that the structure
of an underground site has on creating an internal temperature suitable for
torpor during hibernation. The oft-stated dogma is that caves and mines are too
warm internally and must be cooled for successful hibernation through cold-air
traps or chimney-effect airflow (McAney 1999, Nagel and Nagel 1991, Prather and
Briggler 2002, Sherwin and Altenbach 2009, Tuttle and Kennedy 2002, USFWS
Figure 5. Top: Percent
of Little Brown
Bats that was solitary
in each of 13
mines versus ambient
temperature. Bottom:
Percent of Northern
Bats that was solitary
in each of 9 mines
versus ambient temperature.
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2013). This pervasive view, however, is a southern perspective, applicable only to
warm regions with shorter winters and higher mean annual surface temperatures
than found in the Upper Peninsula.
For example, chimney-effect airflow throughout simple mines up to 100 m in
length in the Upper Peninsula causes air temperature to be near or below freezing,
making such sites unsuitable for overwintering bats. Even in mines over 500 m in
length, passages subjected to chimney-effect airflow are cold, drafty, and generally
devoid of bats, and it is only in complex sites, with drifts, crosscuts, stopes, or
multiple levels that chimney-effect airflow and large numbers of hibernating bats
can co-occur in the North. Similarly, blind-ending shafts are cold-air traps that can
contain ice into June in northern Michigan (Kurta and Smith 2009) and thus are not
used by hibernating bats. Indeed, more than 85% of all bats in the mines overwinter
in sites with a maximum air temperature (Fig. 3) greater than the mean annual
surface temperature of the region (4.2–5.5 °C; Albert et al. 1986), suggesting that
presence of warm-air traps, not cold-air traps, may be a factor in selection of hibernacula
by bats in the Upper Peninsula.
Number and species of bats
Eastern pipistrelles. Eastern Pipistrelles were detected at only 28% of 82 occupied
mines in Michigan, which contrasted with southern portions of the Midwest,
where this species occurred in a greater percentage of hibernacula than any other
species (Brack et al. 2003, Dixon 2011, Hall 1962). Throughout their range, Eastern
Pipistrelles most frequently hibernated in sites with ambient temperature between
8 and 12 °C (Brack 2007, McNab 1974, Rabinowitz 1981, Raesly and Gates 1987),
but temperatures of 8 °C or higher, occurred in only 1/3 of the mines in Michigan
(Fig. 3). In more southern locations, the Eastern Pipistrelle often was the only species
found in short (less than 50 m) hibernacula, which did not cool sufficiently at those
latitudes for use by other species (Brack 2007, Dixon 2011). Most short mines in the
Upper Peninsula, however, appeared too cold and variable in temperature (Table 1)
for successful hibernation by this species, and the Eastern Pipistrelle was never the
only species found in any mine in Michigan. The western Great Lakes region has
few natural caves (Culver 1999), and the Eastern Pipistrelle likely was not present
before mines were dug and abandoned, thereby providing the sites needed for hibernation
(Kurta 1995, Kurta et al. 2007, Stones and Haber 1965). A recent arrival
and cool available temperatures may explain the low proportion of occupied sites
and the small number of Eastern Pipistrelles per mine, compared to other states in
the core of the range of the species (Brack et al. 2003, Turner et al. 2011).
Big Brown Bats. The Big Brown Bat roosted in small numbers in many hibernacula,
most of which were shared with Myotis. These bats accounted for less
than 1% of the bats that were counted, although they likely were more abundant
than underground surveys indicated because most Big Brown Bats probably do
not migrate to the mines. Many Big Brown Bats hibernate in the walls or attics of
heated buildings (Whitaker and Gummer 2002), and some likely overwinter in deep
rock crevices (Neubaum et al. 2006).
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Little Brown and Northern Bats. Bats in the genus Myotis accounted for more
than 99% of bats hibernating in the mines and were widely distributed, with a few
individuals in many sites and most animals concentrated in a few, long, complex
mines. Our analysis, using negative binomial regression, indicated that maximum
ambient temperature and presence of standing water likely had a positive effect on
the number of Myotis that was present. Temperature greatly affects energetic expenditure,
and its importance for hibernating bats in the field has been implicated
before (e.g., Gates et al. 1984). We used maximum recorded temperature in a mine,
because all lower temperatures presumably are available in other parts of the site.
Bats in varying physical condition may choose different temperatures at which to
hibernate (Boyles et al. 2007), and the energetic cost of different metabolic activities
during hibernation, such as maintenance during torpor and functioning of the
immune system during an arousal, may be minimized at different temperatures
(Boyles and McKechnie 2010).
Presence of standing water has a number of potential benefits. During periodic
arousals, bats often drink (Boyles et al. 2006), and they typically do so on the wing
(Taylor and Tuttle 2007). Hence, a pool of water potentially enhances the ability
of a bat to maintain water balance during the long winter. Standing water also contributes
to higher ambient water vapor pressures, which reduce evaporative water
loss, and depending on location within a mine, the water may act as a deterrent to
would-be predators and disturbance-causing humans.
Our study is the first to indicate water vapor pressure deficit as a factor affecting
number of Myotis in a hibernaculum (Table 1). This variable is mathematically
proportional to the amount of evaporation that occurs when surface temperature of
an animal approximates ambient temperature (Hill 1976). Although earlier studies
(e.g., Bogdanowicz and Urbańczyk 1983, Kim et al. 2009, Raesly and Gates 1987)
suggested that relative humidity is a factor in determining the number of hibernating
bats, such conclusions may be spurious, because relative humidity does not reliably
predict evaporative loss in environments that vary in air temperature (Kurta 2014).
Nevertheless, our results provide statistical support for the concept that ambient
moisture is a factor in determining the number of hibernating bats in a mine.
Hibernating bats in the Upper Peninsula were dominated by 2 species of Myotis
that co-occurred in most mines. During our study, Northern Bats that were identified
in hand accounted for 10% of Myotis, which was similar to the 9.2% estimated
by Stones (1981), who examined over 18,000 Myotis in 16 discrete sites. The
Northern Bat, which is proposed for listing as an endangered species because of
white-nose syndrome (USFWS 2013), appeared more common, relative to Little
Brown Bats, in Michigan than in the East or the Ohio River Valley (Brack et al.
2003, Trombulak et al. 2001). In New York, for example, Northern Bats represented
only 0.2% of the combined number of Northern and Little Brown Bats that were
counted before arrival of white-nose syndrome (Turner et al. 2011).
Clustering
Although both Little Brown and Northern Bats frequently clustered with other
bats of their species, the most common “group” for both species contained only
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1 bat (Fig. 4). For Northern Bats, solitary animals constituted 87% of the groups
and 75% of the individuals, and maximum cluster size was only 6. A solitary nature
during hibernation seems typical of this bat (e.g., Barbour and Davis 1969, Brack
2007), although the frequency of clustered versus single individuals apparently has
not been quantified elsewhere.
For Little Brown Bats, single animals represented 52% of groups and 22% of individuals,
and the proportion of Little Brown Bats that was solitary was positively
correlated with temperature. The large number of solitary Little Brown Bats and
small clusters during 2011–2013 in Michigan matches our subjective impression
at most locations over 17 winters. For example, the original description (Kurta
1996:21) of the Mead Mine, which contains more than 15,000 bats, indicated that
“bats were spread evenly throughout the [mine], with no large concentrations, and
clusters were consistently small, rarely exceeding 10–15 individuals.” The percentage
of Little Brown Bats roosting alone in Michigan is much higher than the 1%
reported for New York prior to white-nose syndrome (Langwig et al. 2012), but the
reason for such a large difference is not clear. Air temperatures for the hibernacula
in New York are not published, but it is doubtful whether temperature can explain
the difference in clustering behavior between states. Even at our coldest site (2 °C)
during the survey of clusters, 10% of Little Brown Bats were solitary, and the lowest
percentage at any of the 13 mines was 7% (Fig. 5).
Future impacts of white-nose syndrome
The late arrival of P. destructans in Michigan, compared to other eastern states
(Whitenoseyndrome.org 2014), likely is attributable to the obstacles presented by
the Great Lakes to the east–west movement of bats and isolation of the copper and
iron ranges from areas of natural caves (hibernacula) in the East and the Ohio River
Valley (Culver 1999). Unfortunately, many factors suggest that the disease will be
devastating in the next few years. For example, high levels of ambient moisture and
temperatures of 4–10 °C in most hibernacula (Fig. 3) are conducive to growth of the
fungus (Verant et al. 2012), and the severe and prolonged winters make it unlikely
that afflicted animals can leave the hibernaculum and find food.
Large colonies may be at particular risk from this disease (Wilder et al. 2011),
and although hibernating bats are found in at least 82 sites in the western Upper
Peninsula, most bats are concentrated in a few mines (Fig. 2). Degree of sociality
within sites also may impact the severity of the infection and the ability of a
species to persist after introduction of the fungus (Langwig et al. 2012). In New
York, Little Brown Bats reduced their propensity to cluster after introduction
of P. destructans, with single animals representing 1% of hibernating animals
before the disease and 45% afterwards (Langwig et al. 2012). The much greater
occurrence of solitary Little Brown Bats in Michigan prior to arrival of the fungus
(22%) may slow the spread of the disease, but ultimately, it seems inevitable
that white-nose syndrome will envelope the Great Lakes region and the continent
(USFWS 2013).
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Acknowledgments
We thank B. Scullon, Michigan Department of Natural Resources, for consistent logistical
support and for bringing an awareness of flying mammals to a traditional management
agency. B. Turin, mine inspector for Ontonagon County, provided leads on the location of
many sites. M. Kurta, C. Rockey, and R. Slider offered field assistance at multiple sites, and
D. Derrick and D. Sabel often supplied technical support at vertical entrances. We are grateful
to the numerous owners and managers of property and mineral rights, especially R. Whiteman,
who allowed access to mines. The Michigan Department of Natural Resources provided
primary funding, with additional support from the Ottawa National Forest, Bat Conservation
International, and our own pockets. The senior author thanks A. Rodríguez-Durán for access
to the field station at Mata de Plátano, Arecibo, Puerto Rico, where many ideas were formulated
and analyses completed, while on sabbatical from Eastern Michigan University.
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