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2012 SOUTHEASTERN NATURALIST 11(1):89–98
Decline in the Winter Population of
Gray Bats (Myotis grisescens) in Florida
Jeffery A. Gore1,*, Louis Lazure1, and Mark E. Ludlow2
Abstract - The number of Myotis grisescens (Gray Bat) observed at hibernacula has
increased across most of the species’ range in recent years as more roost caves have
been protected from humans. In Florida, most Gray Bats have been found to hibernate
in one primary cave, where the number of hibernating individuals decreased from several
thousand in the 1950s to zero in 1975 due to disturbance by humans and improper
gating of the cave. After removal of the gate in 1982, the number of hibernating Gray
Bats increased, averaging 166 bats during 1990–1998. The number then declined again,
and <10 Gray Bats per year were counted during 2002–2011. The cause of this recent
decline is uncertain. Temperatures in the cave remain suitably cold, visitation by humans
is restricted, and no mortality from floods or disease has been observed. Most of
the Gray Bats may have moved to larger and more suitable hibernacula in northern Alabama
Myotis grisescens A.H. Howell (Gray Bat) roosts year-round in caves in the
southeastern United States, primarily in Missouri, Arkansas, Kentucky, Tennessee,
and Alabama (Decher and Choate 1995). Gray Bats are less abundant in
adjacent states, but small populations have persisted outside the primary range,
including in northern Florida (Gore 1992). Because Gray Bats roost in large
colonies in caves that have particular physical features, their populations are
vulnerable, and disturbance of roost caves by humans can cause a decline in
populations (Ellison et al. 2003, Tuttle 1979). Populations are especially vulnerable
in winter because relatively few caves provide the cold, stable conditions
required by hibernating Gray Bats (Tuttle 1979).
Florida represents the southern extent of the Gray Bat’s range, and only 2
hibernacula are known there, both of them in northwestern Florida on the border
with Alabama (Gore 1992, Rice 1955b, Wenner 1984). Although the population
in Florida always has been relatively small, the species’ status here is of broader
interest for three reasons. First, because the population in Florida is so distant
from northern populations, its loss would decrease the Gray Bat’s geographic
range. Second, because Florida is the southern limit for the species, individuals
hibernating in Florida are likely to be the first impacted by rising temperatures
associated with climate change (Humphries et al. 2002, McNab 1974). Finally,
because caves in Florida are distant from other caves occupied by Gray Bats and
Florida winters are relatively short, bats hibernating here are likely to be the least
1Florida Fish and Wildlife Conservation Commission, 3911 Highway 2311, Panama City,
fl32404. 2Torreya State Park, 2576 NW Torreya Park Road, Bristol, fl32321. *Corresponding
90 Southeastern Naturalist Vol. 11, No. 1
vulnerable to white-nose syndrome, a deadly disease in bats associated with a
cold-adapted fungus (Blehert et al. 2009, Boyles and Willis 2010).
During the past century, populations of Gray Bats decreased greatly throughout
the species’ geographic range, and by the 1970s, nearly all Gray Bats were
hibernating in only 9 caves (Barbour and Davis 1969, Tuttle 1979). In recent years,
populations have increased across much of the geographic range, presumably a
result of increased protection of caves from disturbance by humans (Elliot 2008,
Martin 2007, Sasse et al. 2007). Also, Gray Bats recently have returned to hibernate
in a few caves from which they had been absent for several years (Martin 2007).
The apparent increase in the population has led to a more optimistic outlook for the
species (USFWS 2009). However, Florida appears to be an exception to this positive
trend, and here we summarize observations of hibernating Gray Bats in Florida
and discuss the apparent decline in the wintering population.
Although the fossil record indicates that Gray Bats formerly occurred in
north-central Florida (Martin and Webb 1974), all caves recently used by this
species are in an area of karst topography in Jackson County in northwestern
Florida (Gore 1992). Gray Bats have been observed in <10 caves in Florida,
and only Old Indian Cave and nearby Dugong Cave are used as roosts in winter
(Tuttle 1976; Wenner 1984; J.A. Gore, pers. observ.). The other caves support
maternity, bachelor, or transient roosts, or their entrances have been sealed, making
them unavailable to bats (Wenner 1984). Old Indian Cave is within Florida
Caverns State Park, and Dugong Cave is <100 m south of the Park; both caves
are in the floodplain of the Chipola River. Old Indian Cave has multiple rooms,
ceilings >3 m high, and >150 m of passage. Dugong Cave contains a single room
<25 m long with a ceiling <1.5 m high.
We searched the literature to obtain estimates of the number of Gray Bats
wintering in caves in Florida. Because the primary hibernaculum, Old Indian
Cave, lies within Florida Caverns State Park, we also examined previous counts
recorded in files of the Florida Park Service. During 1988–2011, we made 20
midwinter counts of Gray Bats in Old Indian Cave, and during 1997–2011,
we made 7 counts in Dugong Cave. We conducted all counts at these caves
between 15 January and 28 February. We made incidental visits in winter to
all other known bat roosts in northwestern Florida, but we did not search them
regularly because they were not cold enough (<12 °C) for hibernating Gray
Bats and because Gray Bats have been reported more than once only in Old Indian
and Dugong caves. Wenner (1984) labeled Miller Cave in Florida Caverns
State Park as a transient hibernaculum based on a record from 1970, when disturbance
by humans was a problem at nearby Old Indian Cave. Miller Cave is
fenced to restrict public access and no Gray Bat has been observed in the cave
during >10 surveys since 1970.
In some years, we conducted 2 counts and used the larger count as our estimate
of colony size. Because Old Indian and Dugong caves have small rooms and
2012 J.A. Gore, L. Lazure, and M.E. Ludlow 91
low ceilings and the wintering colonies are small, we usually were able to make
a direct count of individual bats. In 1988, the colony was relatively large, and
we estimated the number of bats to minimize disturbance. To estimate size of the
colony, we measured height and width of the cluster of roosting bats and multiplied
the area by a density of 1828 bats/m2 (Tuttle 1975). Myotis austroriparius
Rhoads (Southeastern Myotis) and Perimyotis subflavus Menu (Tri-colored Bat)
also roosted in each cave, but Gray Bats typically roosted separately from the
other species and low on the cave wall, facilitating identification of species. To
reduce disturbance, we illuminated roosting bats with visible red light or with
infrared light, limited sound, and restricted the time we spent in each cave.
We were uncertain about how early estimates of colony size were made, and
we suspected that counts based on estimates of area covered by roosting bats
were not precise; therefore, we used only direct count data from 1990–2011 for
statistical analyses. Because the data were not distributed normally, we used
Spearman-rank correlation to analyze changes in abundance among years. Statistical
analyses were performed using SAS (PROC CORR; SAS Institute Inc.,
We measured wall and air temperature in Old Indian Cave near the roosting
Gray Bats. Air temperature was recorded hourly during 8 December 2010–28
February 2011with a data logger (HOBO model U22-001, Onset Computer
Corporation, Southern, MA) installed 1–2 cm from the wall near the spot
where Gray Bats usually aggregate in winter. We used an infrared thermometer
(Extech Instrument, Waltham, MA) to measure temperature of the wall on 2
Data collected in Old Indian Cave before 1975 were not precise, and the
methods used are not described in detail. Nevertheless, the reported information
indicates a steep decline in number of hibernating Gray Bats from thousands
of bats prior to 1960 to zero by about 1975 (Table 1). Subsequent observations
indicated that the population of Gray Bats increased after 1975 to a maximum of
about 600 in 1988 (Table 1). After 1988, the population again decreased, but the
wintering population still averaged 166 individuals in 8 counts during 1990–1998.
The population then declined sharply, averaging 28 during 1999–2001. In 2002,
only 1 Gray Bat was observed, and <10 individuals ( x = 3.1) were recorded in
each of 7 subsequent counts (Table 1). Data beginning in 1990, when we began
direct counts, through 2011 were correlated negatively with year (Spearman ρ =
-0.089, P < 0.001). In all years, most Gray Bats wintered in Old Indian Cave.
Dugong Cave contained Gray Bats in only 3 of the 7 years in which it was visited
and only once contained >2 individuals (Table 1).
On 2 February 2011, we measured temperature of the wall as 6.7 °C where
Gray Bats were roosting in Old Indian Cave. However, these bats were using a
different, smaller room than in previous years, when >100 bats hibernated in the
cave. At the usual hibernation spot, in the larger main room, the wall temperature
92 Southeastern Naturalist Vol. 11, No. 1
was 10.0 °C on 2 February. During 8 December–28 February, temperature of
the air at that location in the main room averaged 8.3 °C (SD = 1.919, range =
The trend in numbers of Gray Bats that hibernate in Old Indian Cave is difficult
to track closely because of the lack of regular and precise estimates, especially
before 1990. Lee and Tuttle (1970) speculated that >125,000 Myotis historically
wintered in Old Indian Cave, but they provided no quantitative evidence for that
estimate. Wenner (1984) presented the same estimate for the 1940s with no additional
supporting information. Rice (1955a) reported that about 5000 Gray Bats
Table 1. Number of Gray Bats (Myotis grisescens) hibernating in Old Indian Cave and Dugong
Cave, Jackson County, FL.
Date Old Indian Dugong Method Source
1940s Thousands Unknown Wenner 1984
1954 5000 Unknown Rice 1955a
1969 5–1000 Unknown M.D. Tuttle, pers. comm.; Wenner 1984;
Humphrey and Tuttle 1978; Lee and
1975 0 Direct count Humphrey and Tuttle 1978
2 FEB 1976 2 Direct count M.D. Tuttle, pers. comm.
2 FEB 1981 <150A Unknown Wenner 1984
7 FEB 1986 125–150 Unknown O.G. Brock, Florida Park Service,
Tallahassee, FL, pers. comm.
22 FEB 1988 600B Area estimate This study
28 FEB 1990 269 Direct count This study
27 FEB 1991 120 Direct count This study
1 FEB 1993 187 Direct count This study
19 JAN 1994 163 Direct count This study
15 JAN 1995 180 Direct count This study
22 JAN 1996 149 Direct count This study
28 JAN 1997 93 60 Direct count This study
26 JAN 1998 108 Direct count This study
27 JAN 1999 11C Direct count This study
18 FEB 2000 31 Direct count This study
22 JAN 2001 42 Direct count This study
30 JAN 2002 1 0 Direct count This study
22 JAN 2003 1 Direct count This study
25 JAN 2006 3 1 Direct count This study
25 JAN 2007 0 Direct count This study
31 JAN 2008 7 2 Direct count This study
30 JAN 2009 0 0 Direct count This study
17 FEB 2010 5 0 Direct count This study
2 FEB 2011 3 0 Direct count This study
AReported as Myotis, and the majority likely were Southeastern Myotis.
BPossibly overestimated due to uneven shape of the cluster of bats.
CSome Gray Bats within a group of Southeastern Myotis were not counted.
2012 J.A. Gore, L. Lazure, and M.E. Ludlow 93
were in Old Indian Cave in the winter of 1954–1955 and later quoted accounts
by staff at the Park of 15,000 “brown bats” in this cave in winter (Rice 1955b).
Using even the smallest of these early estimates, the number of Gray Bats in Old
Indian Cave clearly declined, because by 1969, a few hundred bats at most were
hibernating there ( Humphrey and Tuttle 1978, Lee and Tuttle 1970).
Probable causes of this initial decline were vandalism in Old Indian Cave and
unintentional disturbance by Park visitors who had free access to the cave (Lee
and Tuttle 1970, Ludlow and Gore 2000). To minimize disturbance of the bats,
staff at the Park closed the cave to visitors in 1969 and installed steel bar gates
and concrete pillars to block the entrances (Ludlow and Gore 2000). Unfortunately,
these structures impeded bat flight and air flow and were likely the reason
that Old Indian Cave supported few Gray Bats in the 1970s even after visitation
by humans had been curtailed sharply (Humphrey and Tuttle 1978, Tuttle 1977).
After barriers at the primary or eastern entrance were removed and replaced by
a chain-link fence in May 1982, the number of roosting bats increased (Ludlow
and Gore 2000). In 1995, bars across the western entrance were removed and the
entrance was fenced. More Myotis used the western entrance after the gate was
removed (Ludlow and Gore 2000).
We recorded the largest number of Gray Bats in 1988, and during that year, the
number present was estimated from the area covered by roosting bats. Meretsky
et al. (2010) reported that estimates of the number of hibernating bats obtained
by visually estimating the area of the clustered bats usually were inaccurate and
they recommended counts from photographs of bats when clusters contained
>100–200 individuals. Because of the imprecision of the area and estimates of
density, we are uncertain of the accuracy of the estimate of 600 bats present in
1988. Regardless of the exact magnitude of the peak in abundance, the surprising
finding is that rather than continuing to increase with reduced disturbance by
humans at Old Indian Cave, the number of bats fluctuated between 100 and 200
over most of the next decade and then declined sharply, as it did in the 1960s and
1970s (Table 1).
Unlike the earlier decline, the more recent decrease in number of wintering
Gray Bats is probably not a result of disturbance by humans. Since 1981, entry
into Old Indian Cave has been limited largely to 1 visit by biologists per year in
winter. Park staff has detected few illegal entries into the cave and no activity
near the bat roost. In addition, we have seen little evidence of human entry into
Dugong Cave. Therefore, the hibernating Gray Bats rarely have been disturbed.
In addition, records of other bat species suggest little disturbance by humans.
The number of Southeastern Myotis and Tri-colored bats that winter in Old
Indian Cave did not decrease as the number of Gray Bats declined (J.A. Gore,
Other potential causes of the recent decline in Gray Bats include an increase
in temperature in caves, flooding of caves, and movement of bats to other caves.
Because Gray Bats require cold caves for hibernation, an increase in temperature
might lead them to abandon a hibernaculum (Tuttle 1976). However, the mean
temperature of 8.3 °C that we recorded in Old Indian Cave in winter 2010–2011
94 Southeastern Naturalist Vol. 11, No. 1
was within the range recorded in other caves used by hibernating Gray Bats
(Tuttle 1979). For example, at 3 of the largest hibernacula used by Gray Bats in
Alabama and Tennessee, Tuttle (1976) recorded mean daily temperatures of
6.7–10.0 °C, with a range of 2–12 °C. Not only was the recent temperature in
winter at Old Indian Cave suitably cold, there was no evidence that temperature
in winter has increased in recent decades. For example, Rice (1955a:46) stated
that the temperature in Old Indian Cave was 10–15 °C. On 2 February 1975,
the air temperature was 9.5–14.6 °C throughout the cave and 9.5–11.5 °C near the
roosting Gray Bats (M.D. Tuttle, Austin, TX, pers. comm.). Furthermore, mean
air temperature during winter (December–February) at Tallahassee, fl(100 km
southeast of Florida Caverns State Park), did not increase significantly during
1940–2010 (NCDC 2011; r = -0.155).
Although we did not have a continuous record of temperature in Old Indian
Cave, available data suggest that the portion of the cave used by Gray Bats remains
as cold as, or colder than, it has been for several decades and is within the
range of temperatures required by hibernating Gray Bats. Although changes in
temperature do not explain the recent decline in hibernating Gray Bats, future climate
change may result in increased temperatures. If mean annual temperatures
increase, Gray Bats likely would move north to cooler caves (Humphries et al.
2002, McNab 1974, Newson et al. 2009, Tuttle 1976), further hampering efforts
to restore populations of the Gray Bat in Florida.
There also is no reason to suspect that the recent decline of Gray Bats in
Florida is related to white-nose syndrome, which has decimated many northern
colonies of hibernating bats (Blehert et al. 2009). The first cases of white-nose
syndrome were not observed until several years after populations of Gray Bats
began declining in caves in Florida, and no affected bat has been reported south
of Tennessee (Blehert et al. 2011). In addition, we have seen no evidence of
fungal growth on hibernating bats during winter in Florida, nor have we found
dead bats or bats exhibiting odd behavior, as is reported for colonies affected by
white-nose syndrome (Blehert et al. 2009).
Flooding can be a significant cause of mortality and roost abandonment
among Gray Bats (Tuttle 1979). Old Indian Cave occasionally is inundated by
the Chipola River, and from 1988, when hibernating populations of Gray Bats
were at their most recent maximum, until 2002, when they first declined to less than 10
individuals, the Chipola River exceeded flood stage 3 times (USGS 2011). None
of those floods were in winter, when hibernating Gray Bats would have been
vulnerable. Furthermore, not all of Old Indian Cave is subject to flooding and the
cave has multiple entrances that would allow bats to escape slowly rising water.
Another possible explanation for the decline in number of hibernating Gray
Bats in Florida is that the population moved to more northern caves. In the past,
some Gray Bats that spent summer in caves in Florida migrated several hundred
kilometers north to hibernate, ostensibly to make use of caves with more suitable
conditions for hibernation (Rice 1955b, Tuttle 1976). In spring, these bats
migrated south to caves in Florida that had warm and stable temperatures ideally
suited for development of young bats (Tuttle 1976). The benefits of occupying
2012 J.A. Gore, L. Lazure, and M.E. Ludlow 95
ideal maternity or hibernation sites apparently outweighed the energetic costs
and low rates of survival associated with long-distance movement between caves
(Tuttle 1976). However, several thousand bats historically wintered in Old Indian
Cave and did not migrate (Table 1). As noted above, temperatures in Old
Indian Cave have been suitable for Gray Bats since at least the 1950s and the
cave could support many more bats than have been observed there. These conditions
suggest that some bats were making the risky and energetically expensive
migration to northern caves unnecessarily; perhaps, due to fidelity to northern
caves where they first hibernated (Tuttle 1976). Inseminated females may benefit
from the longer periods of hibernation available in more northern caves (McNab
1974), and most of the Gray Bats that migrated from Florida to northern caves
were females (Tuttle 1976). The bats might have come to Florida because the
maternity caves there offered optimal conditions for raising young (Tuttle 1975),
but another reason might have been widespread disturbance of maternity roosts
in northern caves by humans (Tuttle 1979).
If disturbance of northern caves by humans has declined, and maternity colonies
have grown since the 1970s (Martin 2007, Sasse et al. 2007), the number of
Gray Bats migrating between Florida and Tennessee and northern Alabama may
also have decreased because bats stayed at the northern caves year-round. We
are uncertain how many, if any, Gray Bats still use maternity caves in Florida
because the large numbers of Southeastern Myotis in the maternity caves make
it difficult to detect Gray Bats. An estimated 48,000 Gray Bats were present in
summer in Florida in the 1960s (Lee and Tuttle 1970), and as many as 10,000
may have occupied maternity caves into the 1980s (Gore 1992, Wenner 1984).
Since then, observations of Gray Bats have been rare in summer. Although the
primary maternity cave and two lesser maternity caves have been well protected
from disturbance by humans, no Gray Bat has been recorded in caves in Florida
during summer since 1990 (J.A. Gore, unpubl. data). If Gray Bats have stopped
coming to caves in Florida during summer, the few individuals now seen in winter
may be the only survivors of the population in Florida. If that is true, loss of
the population is imminent with the death of the few non-migratory individuals
still observed in Old Indian Cave.
The only other hibernaculum occupied by Gray Bats that is nearly as far
south as Old Indian Cave is Sanders Cave, in Conecuh County, AL, about 160
km northwest of Old Indian Cave (Tuttle 1976). This cave was surveyed on 20
February 2010, and 150 hibernating Gray Bats were present (K. Hudson, Alabama
Division of Wildlife and Freshwater Fisheries, Montgomery, AL, 2010
unpubl. data). Summer use of this cave by Gray Bats has not been studied, so
it is not clear whether bats are migrating from there each year. If there is no
summer colony, the small colony of hibernating Gray Bats at Sanders Cave
may be a relict population, similar to the winter colony in Old Indian Cave.
The colony at Sanders Cave might also contain bats that formerly hibernated
in Old Indian Cave.
The number of Gray Bats has increased dramatically in recent years in numerous
hibernacula and maternity roosts throughout the species’ geographic
96 Southeastern Naturalist Vol. 11, No. 1
range (Elliot 2008, Ellison et al. 2003, Martin 2007, Sasse et al. 2007). Martin
(2007) estimated that the rangewide population of Gray Bats increased about
104% during 1982–2007. However, during the same period, the wintering
population of Gray Bats in Florida decreased precipitously, to near zero. Unfortunately,
none of the possible causes of the recent decline in the population
of Gray Bats in Florida seems compelling to us. Disturbance by humans has
been the factor most frequently associated with declines in populations (Elliot
2008, Sasse et al. 2007, Tuttle 1979), but the population in Florida has declined
despite strong protection from disturbance for nearly 30 years. The Gray
Bat continues to be listed as endangered under the federal Endangered Species
Act, and protection of caves remains the most important conservation priority
identified for this species (Elliot 2008, Sasse et al. 2007, USFWS 2009). If few
Gray Bats now migrate south from larger roosts in northern caves, continued
protection of caves in Florida will not be sufficient to maintain the population
of Gray Bats in the state. Monitoring of hibernacula in Florida should continue
to reveal whether Gray Bats continue their decline. More intensive monitoring
of colonies in summer would help determine whether Gray Bats are absent
from the state or if some are still taking advantage of favorable conditions in
We thank the Florida Park Service, Deborah Kelly, and other landowners for access
to the caves. Several individuals assisted with surveys over the years, and we thank
them all, particularly Dana Bryan, Greg Brock, and Catherine Florko for their help
with some of the earlier surveys. Erin Leone kindly reviewed the statistical analyses.
Troy Best, Bland Crowder, Blake Sasse, and Melissa Tucker provided helpful comments
on the manuscript. funding was provided through the Florida Nongame Wildlife
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