Title: Environmental effects on a coastal population of gopher tortoises
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Permanent Link: http://ufdc.ufl.edu/UF00066432/00001
 Material Information
Title: Environmental effects on a coastal population of gopher tortoises
Physical Description: Book
Creator: Kushlan, James A.
Mazzotti, Frank J.
Publisher: Journal of Herpetology
Publication Date: 1984
General Note: Drawn from Journal of Herpetology, Vol. 18, No. 3, pp. 231-239, 1984
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Bibliographic ID: UF00066432
Volume ID: VID00001
Source Institution: University of Florida
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Journal of 'ol 18, No. 3, pp. 231-239, 1984
Copyrigh ..*. for the Study of Amphibians and Reptiles

Environmental Effects on a Coastal Population of
Gopher Tortoises


National Park Service, South Florida Research Center, P.O. Box 279, Homestead, Florida 33030, USA

ABSTRACTr -Gopher tortoises (Gopherus polyphemus) typically occur in upland xeric habitats,
where long-term stability is thought to be required for population persistence. The southernmost
population on Cape Sable, Florida, occupies 113 ha of unstable coastal dunes. This isolated popu-
lation is relatively dense, estimated at 11.3 tortoises/ha, and consists of over 1200 tortoises. Burrows
are randomly distributed over the coastal grassland habitat. The size distribution of burrows sug-
gests the occurrence of larger animals than in other populations studied, probably owing to the
long-term absence of human predation. Population density seems to have increased substantially
in the 16 years since the last hurricane passed over the area. Environmental conditions, particularly
storms and drought, may explain the recent population trend, size structure, and distribution of
this coastal population.

The gopher tortoise (Gopherus poly-
phemus) is widely distributed along the
southeastern coastal plain of North
America. Its population core is in south-
ern Alabama and Georgia and in north-
ern and central Florida, where it inhab-
its inland xeric habitats, i .'i. pine
scrub (Ernst and Barbour, 1972). Nearly
all published information on the pop-
ulation ecology and status of gopher
tortoises is from these upland habitats
(Auffenberg and Iverson, 1979; Alford,
1980; Iverson, 1980; Auffenberg and
Franz, 1982; Landers et al., 1982). Auf-
fenberg and Franz (1982) noted that
tortoises are generally absent or scarce
in low-lying areas near the coast, par-
ticularly in the poorly drained southern
half of the Florida peninsula. Gopher
tortoises do occur in coastal dune hab-
itats (Auffenberg, 1978), although little
is known about their ecology there. The
southernmost population occupies the
coastal beach dunes of Cape Sable, Flor-
ida (Fig. 1).
The gopher tortoises of Cape Sable are
disjunct from currently and previously
occurring populations further north.

I Present address: Department of':. ..
Pennsylvania State University, University Park,
16802, USA.

The population is also notable in its
being protected from human predation
for decades, owing to its location in Ev-
erglades National Park. The dune en-
vironment of Cape Sable is unstable be-
cause of periodic tropical storms, a factor
that may affect tortoise population char-
acteristics. In this respect Alford (1980)
has suggested that long-term habitat
stability may be required for the persis-
tence of tortoise populations. The lit-
erature contains only the barest men-
tion of the existence of the Cape Sable
population (Carr and Goin, 1955; Auf-
fenberg, 1978) and no analysis of its sta-
tus or population structure. The pur-
pose of the current study was to analyze
the size, structure, and distribution of
this isolated island population of go-
pher tortoises and to consider possible
environmental factors affecting popu-
lation characteristics.

Cape Sable (Fig. 1) is the southwest-
ern tip of the Florida peninsula. Its gen-
eral characteristics have been described
by Craighead (1971). The cape is iso-
lated from the mainland by Whitewater
Bay and by a canal between Whitewater
and Florida Bays at the developed area
of Flamingo. Sandy beaches of marl,


shell, and some quartz occur along the
shorelines of East, Middle, and North-
west Capes. Middle and East Capes are
separated from the rest of Cape Sable
by Lake Ingraham and canals connect-
ing the lake to tidewater. The spatial
sequence of vegetation away from the
beach (Fig. 2) is similar on all three
capes. Inland from the beach is a xeric
coastal prairie grassland, the nearly ex-
clusive habitat used by gopher tortoises
on East and Middle Capes. The prairie,
covering a series of low dunes and
swales, is composed of Lantana involu-
crata, Schizachyrium semiberbe, Andropo-
gon virginicus, Boerhavia repens, and
clumps of Cocos nucifera, Sabal palmetto,
and Agave sp. Inland, on higher eleva-
tions, are stands of hardwood hammock
forest containing well-developed trees
of Bursera simaruba, Piscidia piscipula, Fi-
cus aurea, and tall Yucca aloifolia. We
found few burrows on the edge of the
hammock forest on Middle Cape. Be-
yond the hammock are mangrove
swamp forests, dominated by Rhizoph-
ora mangle, stretching to Lake Ingra-
ham. The grassland occupies more area
on Middle Cape than on Northwest
Cape or East Cape. As a result of a larger
catchment area, the groundwater of
Middle Cape is fresh, potable water oc-
curring 50 m from the beach even dur-
ing extreme droughts (Russell 1971). On
Northwest Cape, fresh groundwater is
lost during droughts.

All upland areas on the three capes
were searched for gopher tortoise bur-
rows. Because tortoises occurred only in
prairies, our quantitative study was
confined to that habitat. The area cov-
ered by prairie vegetation was deter-
mined from aerial photographs using
dot grids. The census technique was
modified from that of Auffenberg and
Franz (1982). Sixty 7-m wide transects
were walked from the beach inland to
the tree line and back to the beach. Be-
cause distance to the tree line varied,
transect lengths differed. The census



MEXICO --. ..- ~ V -'"

10 ki ""
S. p of Ce Sabl, -Florida. In

map of Florida showing the location of Cape Sa-
ble and the range of the gopher tortoise (stip-

team counted all burrows within the
transect boundaries. Five persons
walked those transects longer than 100
m, whereas shorter transects were ade-
quately covered by two people. Follow-

ing Auffenberg and Franz (1982), we
classified burrows as active (signs of
recent use), inactive (leaves, twigs in
CAPE I, *. -- -- ,

hole, no signs of recent use), or aban-
doned. The first two are considered in

population totals. The distance of each
burrow from shore was recorded. The
transects covered 13.8% of the total
available habitat of prairie vegetation.
The census was conducted on 22-23
August 1979 on Middle Cape and 18
November 1981 on East Cape. Studies
elsewhere have shown that on the av-
erage 38.6% of all unabandoned bur-
rows are unoccupied (Auffenberg and
Franz 1982). Lacking similar informa-
tion for the study population, we ap-
plied their correction factor of 0.614
turtles/burrow to the count of active
and inactive burrows in order to esti-
MIDDLE ,,j L "
CAPE \ ['|_-^- ._ MFAMINGO


Cape Sb .e

FIG. 1. Map of Cape Sable, Florida. Insert is a
map of Florida showing the location of Cape Sa-
ble and the range of the gopher tortoise (stip-

team counted all burrows within the
transect boundaries. Five persons
walked those transects longer than 100
m, whereas shorter transects were ade-
quately covered by two people. Follow-
ing Auffenberg and Franz (1982), we
classified burrows as active (signs of
recent use), inactive (leaves, twigs in
hole, no signs of recent use), or aban-
doned. The first two are considered in
population totals. The distance of each
burrow from shore was recorded. The
transects covered 13.8% of the total
available habitat of prairie vegetation.
The census was conducted on 22-23
August 1979 on Middle Cape and 18
November 1981 on East Cape. Studies
elsewhere have shown that on the av-
erage 38.6% of all unabandoned bur-
rows are unoccupied (Auffenberg and
Franz 1982). Lacking similar informa-
tion for the study population, we ap-
plied their correction factor of 0.614
turtles/burrow to the count of active
and inactive burrows in order to esti-


FIG. 2. Aerial view of Middle Cape, Cape Sable, Florida, showing tortoise habitat, coastal prairie
grassland with scattered trees. Mangrove swamp forest surrounds Lake Ingraham, in the background,
and also occurs in a forested patch near the point of the cape. Hardwood hammock forest is located
along the inland edge of the prairie.

mate the number of tortoises in the
population. Further information can be
found in a report available from the
South Florida Research Center (Kush-
lan and Mazzotti 1982).
A permanent plot of 1.45 ha was es-
tablished in August 1979 on Middle
Cape. All burrows in the plot were
counted, marked, and measured (width
and height) below the mouth at the
point the burrow narrows. The plot was
recensused on 8 November 1980 and 22
February 1982. In 1982 we also mea-
sured burrow width as far into the bur-
row as possible to correspond to the
method used by Alford (1980). For con-
version of burrow width to carapace
length, we used the equation logi0Y =
0.879 logoX + 0.149 derived by Alford
(1980). Dispersion of burrows was ana-
lyzed using the nearest neighbor meth-
od (Poole 1974).

Data from the plot were also used to
evaluate the accuracy of the transect
census technique. Transects run within
the plot produced an estimated density
of 51.82 burrows/ha, whereas the plot
actually contained 51.03 burrows/ha.
This comparison inspires confidence
that the transect data were adequate to
estimate tortoise burrow density over
the entire study area.

Extensive survey work showed that
gopher tortoises were confined to Mid-
dle and East Capes (Fig. 1). The prairie
habitat on these capes was 113.3 ha in
extent. On the census of 60 transects
covering 13.8% of occupied area, we lo-
cated 253 active and inactive burrows.
From this, we calculated the cape con-
tained 2077 burrows at a density of 18.3
burrows/ha, and we estimate the pop-



FIG. 3. Distribution of gopher tortoise burrows along each transect. Sixty transects were run per-
pendicular to the shoreline at intervals from Middle Cape to East Cape. The horizontal axis is the
shoreline, straightened for clarity. Transect No. 1 was at the point of Middle Cape shown in Fig. 2.
Transect 1 to 15 were on Middle Cape, 51 to 60 were on East Cape, the res, in between. Dots show
location of burrows away from the shoreline along each transect. The horizontal lines show the
location of the inland edge of the prairie and beginning of the hammock forest.

ulation to be 1275 tortoises at a density
of 11.3 tortoises/ha.
Burrows were scattered throughout
the dune area, from the foredune to
slightly beyond the tree line edge of
hammock forest (Fig. 3). We found only
three burrows within 10 m of the beach,
suggesting tortoises avoided digging
burrows in areas subject to flooding
from overwash. Tortoises were active in
this area though, as shown by the pres-
ence of scat and trails. The dispersion
of burrows within the study plot in Au-
gust 1979 was random (R = 1.09, Z = 1.5,
N = 73), but there did appear over the
entire habitat to be a concentration of
burrows on ridges with fewer in the in-
tervening sales. There was no appar-
ent preferred use by tortoises of vege-
tation i. within the prairie; some
burrows were located under trees.
The three censuses conducted on the
study plot provided a complete count
of burrows there. In 1979 we found 74
active and inactive burrows; in 1980 we
found 84; in 1982 we found 86. These
three complete counts imply that the

population on the plot may have been
increasing during the study period.
Some idea of the relative size struc-
ture of the population can be obtained
from our measurements of burrows (Ta-
ble 1). The presence of small burrows
less than 15 cm wide suggests that re-
cruitment is occurring within the pop-
ulation. The smallest burrow was 6.0 cm
wide. These were made by young tor-
toises, although probably not by hatch-
lings (Alford, 1980; Arata, 1958). Most

TABLE 1. Distribution of the widths of the
mouths of gopher tortoise burrows in a 1.45-ha
study plot on Middle Cape.

Size range (cm) August 1979 February 1982
5-9.9 4 9
10-14.9 7 13
15-19.9 3 3
20-24.9 5 4
25-29.9 11 14
30-34.9 11 26
35-39.9 22 13
40-44.9 6 1
45-49.9 5 3


a ---- i
S 1 1
z 57 9.3 129 16.5 201


237 273

309 34.5 38.
309 34,5 38.1

i ; - I

57 93 12.9 165 20.1 23.7 27.3 309 34.5' 38 1
FIG. 4. Frequency distribution of calculated
carapace lengths (in centimeters) of the gopher
tortoise population on Cape Sable contrasted with
one in north Florida (Alford, 1980). Size classes
are 1.8 cm wide matching those of Alford I:', ..

burrows were between 25 and 40 cm
Alford (1980) determined the rela-
tionship between burrow width and
carapace ...: I. of gopher tortoises. In
1982, we measured burrow width in a
way comparable to Alford's and so used
his relationship to generate a size-fre-
quency distribution for the population
segment in the study site (Fig. 4). The
resulting distribution shows a predom-
inance of animals in the 25 to 35 cm
carapace length classes, with good rep-
resentation of smaller animals.
In 1982 and 1983, burrows were ex-
amined for the presence of commensal
animals. In 1982 we excavated two bur-
rows and found slugs and a whip scor-

pion. To investigate further, A. Beck and
J. Butler took vacuum samples (Butler
and Gibbs, 1982) of 26 burrows finding
1'i !. and greenhouse frogs (Eleu-
. .,
In order to determine whether arche-
ological evidence existed for the early
occurrence of gopher tortoises on Cape
Sable, G. P. Fischer (pers. comm.) exam-
ined records of material collected in two
Indian middens on Cape Sable. He was
able to find no -- i:.. of turtle remains.

We estimate that the southernmost
gopher tortoise population, located on
Cape Sable, Florida, includes in excess
of 1200 individuals in 113 ha. This pop-
ulation had a within-colony density of
11.3 tortoises per hectare of habitat, a
density relatively high compared to
other populations (J. Diemer, J. Butler,
pers. comm.). T. Logan (pers. comm.)
found that tortoise populations on 17
federally managed areas in Florida had
densities ranging from 0.3 to 11.9 tor-
toises per hectare of habitat. Our data
confirm that, of natural habitats, coastal
areas tend to have the highest tortoise
densities (Auffenberg and Iverson, 1982;
J. Diemer, pers. comm.).
The gopher tortoise population on
Cape Sable is entirely isolated from oth-
ers, but it could have derived :::. :::i
by rafting from Gulf Coast populations
or by rafting down the Everglades from
inland populations along the coast (Fig.
1). The nearest coastal population was
on Marco Island, 100 km NW. Gopher
tortoises occur, at least sporadically in
the pine forest 82 km inland. Recent ob-
servations were in 1955, 1957, 1960,
1979, and 1981 (Everglades National
Park files). The infrequency of obser-
vations indicates that these animals had
been released in the park rather than
being remnants of a :- ::. occurring
It has been suggested that gopher tor-
toises may have been introduced onto
Cape Sable by man (McKinney 1970).
Tebeau (1968) in discussing the history


of human occupation of the cape, noted
that it was i .. ,' 1. used only sporad-
ically by Indians. The first non-Indian
occupation of the site was in 1838 dur-
ing the Seminole War when a tempo-
rary fort was constructed on East Cape.
exist of occasional transient
settlement, farming, and cattle ranch-
ing in the 19th century and of a coconut
plantation that was : .. ':i destroyed by
the hurricane of 1935. It is possible that
any of these human occupants of the
cape may have introduced tortoises
there. The cape has been unoccupied
since the national park was established
in 1947.
McKinney (1970) argued for intro-
duction because his excavation of five
burrows failed to produce any com-
m ensal animals, ... 1...:.. any 1:, ,i.
commensals characteristic of gopher
tortoise burrows (Young and Goff, 1939).
He suggested that their absence indi-
cated that the tortoises were ....: .. i
transported to the cape by man. Al-
though we were able to find nonobli-
gate commensals, the Cape Sable bur-
rows were very barren of such animals
compared to those in other parts of the
state (D. Beck, pers. comm.). Consider-
ing that storms, which .. ...: .- :.. flood
the cape (Craighead, 1971), could easily
kill obligate commensal invertebrates,
their absence on Cape Sable provides
little information on the tortoises' his-
tory there.
The lack of tortoises on Northwest
Cape has been cited as evidence of their
introduction after the construction of
the canal separating Northwest and
Middle Capes (Robertson, 1957). This
canal was dug in the early 1920's (Te-
beau, 1 .- however, Simpson (1920)
reported that tortoises were present on
Cape Sable prior to 1920. It is likely that
the tortoises absence on Northwest Cape
may be due to habitat differences, per-
haps related to the lack of persistence
of a fresh ground water lens during
drought. In any case turtles occurred on
Cape Sable prior to 1920, but how much

earlier is not known. Barring the for-
tuitous discovery of datable archeolog-
ical remains, the question of the origin
of this population remains unsettled.
We note, however, that occupancy of
coastal beaches by gopher tortoises is
not .:. ..*1.* (Auffenberg and Franz,
1982) and that Cape Sable represents the
southernmost suitable habitat.
The .- :...: .:.... was rediscovered on
Middle Cape on 9-10 October 1949 by
W. Dilley, R. O. Woodbury, and J. B.
Earle. Subsequently, on 2 March 1950,
J. Moore (1953, pers. comm.) found a
number of burrows and signs of activity
on East Cape. Tortoises were later ob-
served in 1951, 1952, and 1955. In this
period, the population must have been
considerably smaller, or at least less no-
ticeable than at .. .. because Albert
Schwartz (pers. comm.) spent several
days camping on Middle Cape in the
1950's but remained unaware of their
existence when he assembled his mono-
graph on south Florida reptiles (Duell-
man and Schwartz 1958). Tortoises were
present on both East and Middle Cape
in 1960 and 1961, proving their survival
after Hurricane Donna in 1960. On 2
April 1966, after Hurricane : ., E. T.
Christensen (memo Everglades Nation-
al Park files) censused 81 hectares and
found 0.2 burrows/ha (0.12 tortoises/
ha). From this history it appears that the
population was present but small in the
1950's and 1960's. It has increased in
subsequent years, the present popula-
tion density on Middle Cape being 90
times that recorded in 1966. The popu-
lation on the study plot also seems to
be increasing, as evidenced by the
number of burrows :-- ..- '-; by 16%
between A .. ... 1979 and January 1982.
Although the higher number of bur-
rows could have been caused by immi-
gration from outside the plot (McRae et
al., 1980), the increase in small burrows
(Table 1) suggests that reproduction was
the primary factor. A rapid population
increase since 1966 may have been pos-
sible because of the large clutch size in


south Florida compared to more north-
ern areas (Iverson, 1980; Landers et al.,
The population has a relatively high
proportion of larger, presumably older
animals, the predominance of animals
being 25 to 35 cm long (calculated car-
apace 1-.!il.' This population includes
larger animals than Alford (1980) found
in northern Florida, where most ani-
mals were 19 to 27 cm long (Fig. 4). Al-
ford interpreted his data as demonstrat-
ing a relatively high number of large
tortoises in his population. He stated
that the likely explanation was a slow-
ing in the development rate of older
tortoises, leading to the bunching of
older cohorts. On Cape Sable, tortoises
are free from human exploitation, and
even older age classes seem to be rep-
resented (Fig. 4). Information on the ac-
tual, rather than calculated, size distri-
bution of Cape Sable tortoises would
allow further consideration of this
We believe that the population on
Cape Sable is affected primarily by en-
vironmental conditions. Seasonal
drought may adversely impact tortois-
es, especially where fresh water does not
persist during the dry season, as on
Northwest Cape. Rainfall floods bur-
rows in swales in the summer; Moore
(1953) reported how raccoons use such
water-filled holes for drinking. Flood-
ing of burrows would affect details of
burrow placement, even if it did not kill
tortoises directly. Storms may have an
especially profound effect on popula-
tion size and distribution. The low pop-
ulations of the mid-1950's followed
hurricanes in 1947 and 1951. Tortoises
seem to have survived the hurricanes of
1960 and 1965, in relatively small num-
bers. After the 1965 storm, Christensen
found the tortoises 60 to 150 m back
from the beachfront, with the greatest
number of holes 150 m inland, close to
the hammock ridge. This distribution
was very different than at present (Fig.
3). The burrow distribution found in

1966 suggests that tortoises closer to the
beach on lower ground did not survive
the hurricane. It is possible that the re-
cent population expansion may have
coincided with the latest interhurricane
Storms may also account for the cur-
rent size structure of the population.
Although growth rates of Cape Sable
tortoises have not been studied, infor-
mation exists for other populations.
Growth rates of gopher tortoises are
complex, ,. ; : with age, nutrition,
and latitude (Landers et al., 1982) and
are relatively higher in more southerly
populations (Landers et al., 1982), pre-
sumably also on Cape Sable. If Alford's
(1980) estimates of growth rates in Flor-
ida are not greater than those of Cape
Sable tortoises, members of the modal
size class (30.9 cm mid-point, Fig. 4)
would be over 15-years old and would
have survived the last major hurricane
in 1965. The dip centered in the 22 cm
size class (Fig. 4) may represent animals
lost in that hurricane.
Cape Sable is part of Everglades Na-
tional Park, and the population is there-
by protected from the ?..:..:. that oc-
curs elsewhere in i :: .. Thus, the
most severe threat to the tortoise seems
to be from natural causes, especially
hurricanes. The Cape Sable coastal sys-
tem of dunes and sales of successive
deposition was in large part shaped by
such storms (Craighead, 1971). Al-
though Alford (1980) concluded that
long-term habitat :. i:::: seems to be
needed for tortoise survival, coastal
beaches are inherently unstable. On
Cape Sable, the tortoise population ap-
pears to have survived storms, but in
reduced density. If inter-colony move-
ment were low under the best of con-
ditions (Alford, 1980), recolonization of
isolated population centers such as Cape
Sable would be an unlikely event. Thus,
local extinction of the population on
coastal beaches of Cape Sable may be an
eventual possibility, and at any time its
numbers and structure are determined


by the recent history of environmental

Ack. .. .' ..' -We thank the
:: :. people who assisted in this study.
T. Logan arranged the preliminary cen-
sus and established the transect proce-
dures. We thank those who helped with
the field work, suffering the abuses of
1, : spears and cactus spines, includ-
ing B. Brown, D. Brown, S. Christman,
W. Dunson, R. Franz, P. Frohring, T.
Jacobsen, P. Moler, A. Muller, D. Peck,
S. Raney, C. R. Smith, and D. Smith. A.
Schwartz, W. B. Robertson, Jr., F.
Whitehead, and W. Dunson discussed
their observations and contributed in-
formation on the historical status of the
population. We especially thank J.
Moore who graciously made available
his unpublished notes to us. Various
observations that we cite were made by
W. Dilley, R. O. Woodbury, J. B. Earle,
J. C. Moore, M. Duesing, D. O. Karrak-
er, R. McDowell, F. C. Craighead, Sr.,
K. Vernick, and D. Culliard. The census
of E. T. Christensen in 1966 was espe-
cially important. We thank A. Beck and
J. Butler for collecting vacuum samples
of burrow inhabitants. P. Meylan, G.
Robinson, G. P. Fischer, T. Logan, and
W. Dunson assisted us in finding infor-
mation. W. Seelee, D. B. Means, and the
Gopher Tortoise Council were most
supportive. R. A. Alford, J. Diemer, J. F.
Douglass, R. Franz, J. N. Layne, P. Mol-
er, and C. R. Robins, and R. Ruibal re-
viewed the manuscript and provided
their advice. Dottie Anderson, Betty
Curl, Jessie Brundige, and Dee Childs
typed the drafts.
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pherus polyphemus in northern Florida. J. Herp.
ARATA, A. A. 1958. Notes on the eggs and young
of Gopherus polyphemus : . Q .... Quart. J. Fla.
Acad. Sci. 21:274-280.
AUFFENBERG, W. 1978. Gopher tortoise. Pp. 33-
35. In R. W. McDiarmid (ed.), Rare and endan-
gered biota of Florida. Vol. 3, Amphibians and
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- AND R. FRANZ. 1982. The status and dis-
tribution of the gopher tortoise (Gopherus poly-
phemus). Pp. 95-126. In R. B. Bury (ed.), North
American tortoises: conservation and ecology.
U.S. Fish and Wildlife Service, Wildlife Re-
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AND J. B. IVERSON. 1979. :. .. ..,. of
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IVERSON, J. B. 1980. The reproductive biology of
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ida. 15 pp.
1980. Reproduction of gopher tortoises (Go-
pherus polyphemus) in southwestern Georgia.
Herpetologica 36:353-361.
- W. A. MCRAE, AND J. A. GARNER. 1982.
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southwestern Georgia. Bull. Florida State Mus.
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