|
of the
FLORIDA STATE MUSEUM
Biological Sciences
Volume 27 1982 Number 2
GROWTH AND MATURITY OF THE GOPHER TORTOISE
IN SOUTHWESTERN GEORGIA
J. LARRY LANDERS
W. ALAN MCRAE
AND
JAMES A. GARNER
UNIVERSITY OF FLORIDA
GAINESVILLE
Numbers of the BULLETIN OF THE FLORIDA STATE MUSEUM, BIOLOGICAL SCIENCES,
are published at irregular intervals. Volumes contain about 300 pages and are not necessarily
completed in any one calendar year.
OLIVER L. AUSTIN, JR., Editor
RHODA J. BRYANT, Managing Editor
Consultants for this issue:
JOHN F. DOUGLASS
JOHN B. IVERSON
Communications concerning purchase or exchange of the publications and all manuscripts
should be addressed to: Managing Editor, Bulletin; Florida State Museum; University of
Florida; Gainesville, Florida 32611.
Copyright 1982 by The Florida State Museum of the University of Florida
This public document was promulgated at an annual cost of $1,990.00
or $1.99 per copy. It makes available to libraries, scholars and all inter-
ested persons the results of researches in the natural sciences, emphasiz-
ing the circum-Caribbean region.
Publication date: January 5, 1982
Price $2.00
GROWTH AND MATURITY OF THE
GOPHER TORTOISE IN SOUTHWESTERN GEORGIA
J. LARRY LANDERS, W. ALAN MCRAE,
AND JAMES A. GARNER'
SYNOPSIS: Growth rates of gopher tortoises (Gopherus polyphemus) were determined by mea-
suring individuals during two consecutive years and by correlating plastral annuli counts and
measurements with shell dimensions. Ninety-three percent of growth occurred from April-
October. The annual increment in carapace length (CL) for immatures was 11-12 mm; this
varied by CL class. Growth increments were greatest for specimens 100-190 mm CL (about
4-11 yr of age), and gradually decreased thereafter. However, body volume increments
peaked as tortoises grew from 220-230 mm CL, shortly before attainment of somatic ma-
turity. Sexual maturity typically occurred at age 16-18 years (230-240 mm CL) in males and
at 19-21 years (250-265 mm CL) in females. Thereafter, growth in both sexes was reduced,
but females grew at greater rates and became significantly larger. Development of tortoises
living in sand ridge habitats in southwestern Georgia was much slower than in Florida: ma-
turity was delayed about 7 years. Growth was influenced by habitat quality, ambient tem-
perature, and length of the annual activity season. The extended period of time required for
attainment of maturity is potentially a limiting factor in population growth, and becomes
more important in the northern portions of the species' range.
TABLE OF CONTENTS
INTRODUCTION ..................... ............. ............ 82
ACKNOW LEDGEMENTS .. . .................... .................... 82
M ETHODS ............................................................... 83
Study Area ................... ........ .......... ........... 83
Data Collection and Analysis ............................................. 83
SEASONAL GROWTH CYCLE ............... ............................... 85
ANNUAL GROWTH RATE .................................. ........... 87
Carapacial Increment ................................................... 87
Allom etric Relations ........................ ....................... 92
Volumetric Increment ............... .................................... 97
SIZE IN RELATION TO LIFE HISTORY ...................................... 98
Immature Stages .......................... ......................... 98
Size at Sexual Maturity .................... .......................... 101
Age at Sexual Maturity ................... ..... .......... .......... 102
Post Maturity ................. ........ .. ............................ 103
SURVIVAL AND ADAPTIVE IMPLICATIONS ................................. 104
APPENDIX ............................................................. 108
Synopsis of Methods Used to Estimate Growth ............................... 108
LITERATURE CITED ................. ................................. 109
'The authors are Wildlife Research Biologists for International Paper Company at Southlands Experiment Forest, Bain-
bridge, Georgia 31717.
LANDERS, J. LARRY, W. ALAN MCRAE, AND JAMES A. GARNER. 1982. Growth and maturity of
the gopher tortoise in southwestern Georgia. Bull. Florida State Mus., Biol. Sci. 27(2):81-110.
BULLETIN FLORIDA STATE MUSEUM
INTRODUCTION
The gopher tortoise is one of four extant species of the genus Gopherus
and the only tortoise now indigenous to the southeastern United States. In
recent years this species has drawn much attention, due both to the eco-
logical significance of its burrows (Landers and Speake 1980) and its ques-
tionable population status (Auffenburg and Franz 1975). Gopher tortoise
populations have declined steadily over much of the range, due largely to
habitat alteration and human predation (Franz and Auffenberg 1978).
Few data exist on factors that influence the capacity of depleted popula-
tions to return to their former levels. A major aspect of population dy-
namics is the growth rate of individuals, especially in relation to sexual
maturity. In reference to terrestrial turtles, Auffenberg and Iverson
(1979) stated that the span of time between the reproductive period and
prereproductive period is most important: it determines the amount of
energy required and the length of time an individual is exposed to imping-
ing factors (e.g., predation and disease) before entering the breeding
population.
Unfortunately, no growth data are available on known-age gopher
tortoises living in the wild. Growth has been estimated in only two areas.
In central Florida, Goin and Goff (1941) determined annual growth in-
crements of 33 specimens captured at approximately one-year intervals.
In north-central Florida, Auffenberg and Iverson (1979) correlated plas-
tron lengths of 168 gopher tortoises with ages estimated from shield annuli
counts. Iverson (1980) utilized these two data sets to predict the age at sex-
ual maturity of necropsied females collected from north-central Florida.
No comparable data have been contributed from other portions of the
range, and environmental factors that influence growth have not been
determined.
This study includes samples from southwestern Georgia and north-
central Florida and employs several methods commonly used for growth
analysis of turtles. Major objectives were: (1) to determine annual growth
increments in relation to age and size, (2) to estimate age at sexual ma-
turity, and (3) to determine the influence of selected environmental fac-
tors on growth. Information on geographic variations in growth parame-
ters and comparisons with certain other turtle species are also included.
ACKNOWLEDGMENTS
This work is part of a study on gopher tortoise ecology sponsored by International Paper
Company. It was partially financed with grant-in-aid funds under Section 6 of the Endan-
gered Species Act of 1973 (PL 93-205) administered through the Georgia Department of
Natural Resources (Project No. 8-933). W. Auffenberg and R. Franz provided specimens for
study from the Florida State Museum and contributed additional data from Marion County,
Florida. Without their assistance, parts of this study would never have been completed.
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 83
J. Iverson provided data on maturity of females in the FSM sample, and L. Hunt kindly
brought in many specimens from various locations in southwestern Georgia. J. Douglass and
J. Iverson reviewed the manuscript and offered many helpful suggestions. We especially ap-
preciate the interest of members of the Gopher Tortoise Council; their efforts to conserve the
remaining populations of gopher tortoises provided the impetus for this study.
METHODS
STUDY AREA
Most data used in this study were collected on Silver Lake Station, a 1400-ha tract of
International Paper Company's Southlands Experiment Forest, and on adjacent lands in
Decatur County, Georgia. The forested area was dominated by natural stands of longleaf
pine (Pinus palustris) (965 ha). Sand ridges with scrub oaks (Quercus incana, Q. margaretta,
and Q. laevis) and scattered longleaf pines comprised approximately 210 ha.
A dense groundstory of wiregrass (Aristida stricta), bracken fern (Pteridium aquilinum),
and running oak (Q. pumila) grew beneath the longleaf pine stands, but these plants were
sparse on sand ridges. Open habitat conditions have been maintained by late-winter burning,
and fire swept through sand ridge habitats once every 2-4 years. Large colonies of gopher tor-
toises were found only on xeric sites characterized by Lakeland and Troup soils. Sites with
pure stands of longleaf pine were more mesic (Orangeburg and Norfolk soil series) and sup-
ported relatively few tortoises. On cultivated sites adjacent to the forested tract, isolated in-
dividuals or small groups of tortoises were located along fence-rows and field borders.
Elevations in the study area range from 24-31 m above mean sea level. The average maxi-
mum and minimum temperatures for the summer months are 32C and 21 C, respectively;
annual rainfall averages 1270 mm.
DATA COLLECTION AND ANALYSIS
From March 1978-March 1980, gopher tortoises were captured by hand or in pit-fall traps
placed in front of dens. Hatchlings from nests in the area were included, and data were col-
lected on tortoises found dead along roadsides. Several specimens in the Florida State Museum
collection were examined, and data on tortoises from Marion County, Florida, were also
utilized.
When possible, the age, sex, reproductive status, shell dimensions, and body weight of tor-
toises were recorded. Prior to release, tortoises were marked by notching the marginal scutes
according to a numbering scheme.
Age was estimated by counting annuli where they were most distinct, usually on the ab-
dominal shields (Fig. 1). This technique has been used to estimate age in G. poly-
phemus (Auffenberg and Iverson 1979), in Geochelone gigantea (Grubb 1971), and in several
species of water turtles (Sexton 1959, Moll 1976). False rings, formed occasionally by de-
creased growth during an activity season, were discounted; these were usually discontinuous
and were bounded by relatively shallow marginal grooves. The ability to discern false rings
was developed from observations of many specimens inspected seasonally and annually.
The sex and reproductive status of tortoises found dead were determined during necropsy;
otherwise, the sex of larger specimens was determined by monitoring sexual behavior in the
field and by measuring dimorphic characteristics (McRae et al. 1980). Subdentary glands,
which function in courtship (Auffenberg 1966), were inspected each time a tortoise was ex-
amined. These glands were recorded as active if they were enlarged and contained copious
fluid. Observations of courtship and mating of marked tortoises provided additional data on
reproductive status.
Measurements taken were straight-line dimensions of plastron length (PL), carapace
BULLETIN FLORIDA STATE MUSEUM
length (CL), and (at a point near the middle of the body) maximum width (W) and thickness
(TH). The carapace was chosen as the primary parameter because it is most commonly used
in linear growth analysis. Also, in G. polyphemus, the carapace is less variable than the plas-
tron: the latter is sexually dimorphic in length and the anterior portion (gulars) becomes
reduced by abrasion with age.
Tortoises were weighed as soon as possible after capture on a triple-beam balance. Many
individuals excreted profusely before they could be weighed, and this loss coupled with that
which occurs during winter dormancy made weight change data too variable for accurate
analysis. A box-model volume (CL x W x TH) was calculated as an indicator of total size;
this parameter has been used effectively as a size index in water turtles (Mosimann 1958).
Three methods were employed to determine growth rates: (1) capture-recapture analysis;
(2) age-dimension correlations; (3) annuli measurement-dimension predictions. These three
approaches were undertaken to allow comparisons with the findings of various other studies.
When possible, growth parameters were calculated by more than one method to provide
comparative checks of results.
D B Annuli
4th
3 rd
Shield 2 nd
at birth 1St
SCALE 20
0 5
FIGURE 1. Measurements of abdominal annuli used to estimate carapace length in previous
years. Total annuli width (A) was correlated with CL at present age (here, N = 4 yr) to form
regression equation (Fig. 2). CL for first previous year (N-l) was calculated using distance A
minus D, for second previous year (N-2) using A-C, etc.
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 85
Capture-recapture method: A total of 106 tortoises were caught in corresponding months
of two consecutive years. The annual growth increment was taken as the difference in
measurements from one year to the next (cf. Goin and Goff 1941).
Age-dimension correlations: These included measurements regressed against estimated
age at the time of each capture (N = 579). Only tortoises with distinct plastral annuli were in-
cluded in this analysis (cf. Auffenberg and Iverson 1979).
Annuli measurement-dimension predictions: These were made to discern the growth
history of selected tortoises. A regression equation was developed to predict carapace length
from a measurement of cumulative annuli width on abdominal scutes (Figs. 1 and 2) for all
previous years of an individual's life. Growth of 72 tortoises from sand ridge habitats was
compared to that of 12 others living nearby in cultivated fields. Comparisons were also made
with samples from Florida (15 from Alachua County, 30 from Marion County). Studies of
various species of water turtles (Sexton 1959, Gibbons 1968, Moll 1976) and a giant tortoise
(Geochelone gigantea) (Grubb 1971) have shown the utility of body dimension-annuli
measurement relationships in growth analysis. Unless otherwise stated, differences in means
were assessed using t-tests.
SEASONAL GROWTH CYCLE
During this study it was noted that tortoises of all sizes were essentially
dormant during winter (December-March). In early spring they showed
increased activity, especially on warm days; they were most active during
summer (late May-August). Tortoises were seen more infrequently as fall
progressed.
200
E
E
150 -
* 100- *
,- 9
CL 500
C.)O
10 40
Total Annuli Width Imml
FIGURE 2. Relationship between carapace length and an abdominal annuli measurement;
total annuli width (A) as in Figure 1.
BULLETIN FLORIDA STATE MUSEUM
In an attempt to determine the relationship between seasonal activity
and growth, young tortoises were chosen because they appeared to grow
at a rapid, fairly uniform rate from year to year. Mean monthly CL
plotted against age indicated that growth occurred from spring through
fall and ceased during winter (Fig. 3). Mean bimonthly CL increments
were averaged through the first five years of life and expressed as a per-
centage of annual growth (Fig. 4). There was slight growth during early
spring, a rapid increase to a constant rate (34 % bimonthly) during late
spring and summer, and a gradual decrease through fall to zero growth
during winter dormancy. Ninety-three percent of total growth occurred
from April-October. Growth was negligible when mean maximum tem-
perature was 18-20 C (December-February). It increased slightly at tem-
peratures of 21-27 C (March-April, October-November), peaked when
the mean maximum temperature was fairly stable at 32C during the
warmer months (May-August). Eighty percent of growth occurred during
May-September, the peak growth period noted in north-central Florida
by Auffenberg and Iverson (1979).
120-
3
110-
415
1. 3 1
100- 1 ..- 3
E
S90 2
S1
02
S 80- 1 1
70-
2
.4 Activity Season
S 2W --- inter Dormancy
60- 6 6
.12 -
50 ----- I I II
1 2 3 4 5
Age [ years)
FIGURE 3. Seasonal variation in growth as indicated by CL of young tortoises during various
months of the year. Numerals indicate sample sizes by month of capture.
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 87
-32
35
I-
30
0 30 cc
25 I
_,, B26 m E
Z 20 cc- 24 "
z
15 22
0 W 1
U 4)
S 10 cc |20 2
Z m
U 5 18
.U
JF MA MJ JA SO ND
FIGURE 4. Percent of total annual growth by 2-month intervals in relation to mean maximum
ambient temperature. Bars represent the mean bimonthly CL increment averaged through
the first 5 years of life and expressed as percentage of annual growth.
ANNUAL GROWTH RATE
CARAPACIAL INCREMENT
Measurements of tortoises captured during corresponding months of
two consecutive years were initially used to determine growth in-
crements. Capture-recapture data for tortoises of CL <240 mm were
compared with those obtained by Goin and Goff (1941) in central Florida
(one of the specimens in the latter sample (i.e., number 13) was deleted
because it showed no growth). For purposes of comparison, Goin and
Goffs (1941) data were adjusted to a growth interval of 12 months based
on the calculated percentage growth per month for each individual (Fig.
4). In both Goin and Goff's (1941) sample and our own, the mean CL in-
crement was about 12 mm per year (Table 1). The Florida sample includ-
ed only three individuals in smaller size classes (initial CL <130 mm),
and these represent the stages of most rapid development. Larger tortoises
grew at a mean rate of 11.1 mm per year, while those in our sample grew
BULLETIN FLORIDA STATE MUSEUM
TABLE 1. Annual carapacial increments of gopher tortoises from southwestern Georgia and
central Florida. Data were obtained by capture-recapture.
First year CL Mean CL increment (mm)
class (mm) Southwestern Georgia Central Floridaa
Range x N Range x N
50-59 7-20 13 15 10 1
60-69 13 1- -
80-89 7-15 12 4 -
90-99 8-22 14 4 13 1
100-109 18 2 -
110-119 15-20 17 2 17 1
120-129 10-16 13 4 -
130-139 7-9 8 3 3-19 13 4
140-149 4-19 11 2 13-60 37 2
150-159 11 1 4 1
160-169 8-19 13 2 3-18 11 4
170-179 4-16 10 4
180-189 14 1 4-14 8 4
190-199 7 1 1-9 6 4
200-209 8 1 6-9 8 3
210-219 5-9 7 2 -
220-229 11 1 4 1
230-239 4-24 10 4 3-18 10 2
All classes 7-18 11.8 16 4-37 11.6 13
All individuals 4-24 12.5 50 1-60 11.2 32
aData adapted from Goin and Goff (1941).
9.8 mm annually (about 12% less). The greater potential for growth in
the Florida sample was evidenced by specimen number 9, which grew 42
mm in 9 months, or about 60 mm per year (our estimate). This rate is ap-
proximately twice that of our fastest-growing specimen. Douglass and
Layne (1978) presented information that also indicates more rapid
growth in more southerly populations: in an area near Lake Placid,
Florida, they found tortoises to exceed 100 mm in PL by age 3-4 years;
this size was attained (on the average) after 4-5 years in our part of
Georgia.
A second approach to determining relative carapacial growth was
derived by estimating CL in previous years from annuli width measure-
ments (see METHODS). The difference between reconstructed CL from
one year to the next was plotted against CL class.
Figure 5 depicts an increment curve in relation to initial size based on
two methods. Small individuals with CL of about 50 mm grew rapidly,
but at 60-90 mm CL experienced below average (<12 mm per year) de-
velopment. This trough was followed by a growth surge at about 100-120
mm CL, following which tortoises grew regularly at above average rates
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES
20
18
16H
14"
12H
lo10
. Capture Recapture
* Annuli Measurement
* Mean
01 I I I I I I
60 80 100 120 140 160 180 200 220 240
Carapace Length Imml
FIGURE 5. Annual carapacial increments in relation to carapace length. Data points are mean
increments of individuals in 10-mm CL classes, plotted over class midpoints.
until they reached 180-190 mm CL. Growth generally decreased beyond
that point, but a slight surge was noted in tortoises 220-230 mm CL.
A comparison of growth increments in relation to age, as estimated by
three methods (Table 2), shows most rapid development to occur in
younger age classes, followed by sharply decreasing increments in later
years. The similarity in results of these methods indicates that any of the
three methods can be used to estimate growth.
A summary of CL growth per 4-year class as estimated from annuli
measurements is given in Table 3; only individuals in wiregrass habitats
were used in this analysis. Of 599 annuli measurements for growing
seasons 1-12, over 67% represented CL increases of at least 10 mm. Dur-
ing these years of immaturity, annual growth was characteristically 5-20
TABLE 2. Annual growth in CL in 4-year groups of G. polyphemus as determined by each of
three methods. Specimens are from natural habitats in southwestern Georgia.
CL-age Inter-annuli
Capture-recapture correlation Measurement
Age N x N x N R
1-4 23 12.6 82 11.8 271 12.0
5-8 15 12.3 36 12.0 210 12.1
9-12 2 11.8 25 11.6 118 11.5
13-16 5 9.3 24 9.8 70 9.5
17-20 5 5.7 37 6.2 35 7.8
BULLETIN FLORIDA STATE MUSEUM
mm (X = 12.0 mm) (Table 3). Auffenberg and Weaver (1969) suggested
that growth in CL in immature G. berlandieri averaged 16-18 mm per
year. Giant tortoises grow 15-20 mm per year in CL early in life, then
steadily decline in growth rate to about 5 mm annually by age 15-20
years, depending upon population density (Grubb 1971). Occasionally,
annual increments of 20 mm or more were detected in our study area;
only rarely did tortoises <20 years of age grow <5 mm per season.
Growth in the 17-20 year age class was typically only 5-10 mm per year
(80%), and was occasionally 10-15 mm per year (13%).
Most tortoises (55 of 69) grew in an intermittent pattern, with peak
CL increases typically occurring at 3 to 6 year intervals. Grubb (1971)
found that giant tortoises grew intermittently, often exhibiting sudden in-
creases in growth rates after several years of very slow growth. In our
study, the most rapid growth usually occurred during 1 to 3 year periods
within the 6th-10th growing seasons of an animal's life. Carapace length
increases of 15 mm occurred on the average once every 5 years in the
life of an individual; increments of 10-15 mm were twice as common, oc-
curring an average of once every 2.5 years in the lives of individuals.
Growth surges of > 15 mm per season were manifested by individuals for
periods of 1-4 years only; 44 such surges were confined to a single year, 12
occurred in two successive years, and only three each occurred during 3
and 4 consecutive years.
Fourteen of 69 individuals (20.3%) grew at fairly uniform rates
throughout life (i.e., variation in CL growth was not significant [P> 0.10]
among years). These individuals generally grew at rapid rates ( x = 13.3
mm/yr, N = 6) or at comparatively reduced rates (x = 8.8 mm/yr,
N = 8). Varying nutritional regimes are possible determinants of these
unusual growth patterns.
Of 52 tortoises living in wiregrass habitats, 13 experienced growth of
> 20 mm CL in a single season; five of these grew at least 20 mm during
TABLE 3. Increases in CL, per 4-year period, of 72 tortoises living in natural habitats. Incre-
ments were estimated from annuli measurements using regression equation (see
METHODS). Numbers in the body of the table are percentages of total annuli (N)
measured that represent an estimated CL increase falling within each increment
class.
Carapace growth/yr (mm classes)
Year N <5 5-9.9 10-14.9 15-19.9 20-24.9 25-29.9 _30
1-4 271 6.2 28.1 41.0 19.1 5.6 0.0 0.0
5-8 210 3.8 28.6 38.1 20.9 4.8 3.3 0.5
9-12 118 1.7 30.5 35.6 20.3 6.8 4.3 0.8
13-16 70 9.1 42.4 30.3 15.2 3.0 0.0 0.0
17-20 35 6.7 80.0 13.3 0.0 0.0 0.0 0.0
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 91
two or more years. All of these atypical growth surges were experienced
during growing seasons 2, 6-11, or 13, and usually during 7-8. Only one of
25 tortoises aged 13 and older grew more than 20 mm per year in CL after
the 12th growing season. Only two single-year increases were over 30
mm; one occurred in the 8th year, the other in the 9th year of life. It ap-
peared that surges of above-average growth (>12 mm/yr) did not occur
in any specific year of life, but tended to occur during the 6th-10th grow-
ing seasons.
To determine if there was significant variation in growth in relation to
rainfall or other environmental factors, annual carapacial increments
were estimated for the years 1967-1978 (Fig. 6). Two 5-year age cohorts,
2-6 and 7-11, were used in the analysis; these groups were chosen because
of the insignificant (P>0.20) variation in annual growth within each
group. There were no significant differences in growth rates among the 12
years in either of the two samples (P > 0.20). Nor were annual increments
in growth correlated with rainfall in the spring, summer, or preceding
fall, although seasonal rainfall was quite variable among these years. An-
nual growth of G. polyphemus in this area is determined to a greater ex-
tent by the age and growth potential of the individual and by food quality
(see below) than by seasonal rainfall. This is to be expected as rainfall in
this area, especially in spring and early summer, is typically sufficient for
adequate plant development.
673 74 76 77 16
16b 77
S,72 15
E 15 67 7
E 69 14
E 14
0 13
E 131
122
11
" 10
C. 10 ; 7th-11th Growing Seasons
S2nd 6 th Growing Seasons I 9
0 9
8
8 67 68 69 70 71 72 73 74 75 76 77 78
Growing Seasons
FIGURE 6. Estimated annual growth in carapace length for two age-groups (2-6 and 7-11 yr)
by calendar year. Sample sizes were 16-50 ( = 32) per year.
BULLETIN FLORIDA STATE MUSEUM
Annual growth rates thus did not vary significantly with rainfall; the
effects of nutrition and geographic location on growth were then ex-
amined. Depicted in Fig. 7 are CL growth rates estimated for years 1-10
for G. polyphemus from north-central Florida and from sand ridge and
agricultural habitats in southwestern Georgia. Individuals in natural
habitats in Georgia were significantly (P< 0.01) more likely to grow 10 mm
per year than were individuals in Florida or in fertilized Georgia sites.
The influence of improved nutrition was evident: tortoises in agricul-
tural situations consistently grew about 4 mm more per year (R incre-
ment = 16.2 mm) than those in natural communities ( = 12.0 mm).
This was to be expected: for example, Jackson et al. (1978) reported CLs
of 236 mm at 4 years in G. agassizii captives on highly nutritious diets,
while those living in natural habitats require 15-20 years to reach this size
(Woodbury and Hardy 1948). Nearly 20% of the gopher tortoises in a
colony in a ruderal habitat in Florida exceeded 318 mm CL, while less
than 7% of the individuals in each of 11 other colonies were this large
(Alford 1980). Significant differences in growth rates of Chrysemys picta
on diets differing in nutritional quality are well documented (Gibbons
1967; Ernst 1971; Christiansen and Moll 1973). Giant tortoises in two
low-density populations grew much more rapidly (Swingland 1977) and
reached sizes over 2.4 times larger (Coe et al. 1979) than those in a food-
limited, high-density population.
That the warmer climate and longer growing season in north-central
Florida led to a greater annual CL increment ( X = 17.7 mm) seems clear
(Fig. 7); this increment is identical to that determined from data pre-
sented by Auffenberg and Iverson (1979). The growing season at the
Florida locality was 295 days, approximately 12% longer than that in
southwestern Georgia (259 days). Over 75% of Georgia G. polyphemus,
regardless of habitat, were in increment classes of less than 20 mm; in-
dividuals in Florida were significantly (P <0.01) more likely to grow more
than 20 mm per year. In an additional sample of 30 tortoises from Marion
County, Florida (provided by W. Affenberg), projected CL increase ex-
ceeded 13 mm annually during the first 17 years of life and was 15-16 mm
at ages 1-8 years. These values also are significantly greater than corres-
ponding data in our samples (P<0.01). Earlier maturity, faster growth
rates, and larger body size of southern versus northern Kinosternon subru-
brum (Ernst et al. 1973) and Chrysemys picta (Christiansen and Moll
1973) have been attributed to longer annual growing seasons in the south.
Jackson et al. (1978) felt that year-round activity was a significant factor,
in addition to nutrition, in the rapid growth of G. agassizii captives.
ALLOMETRIC RELATIONS
Proportional changes in various body dimensions are important com-
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 93
m] Georgia, Wiregrass
8 t Georgia, Agricultural
S] Florida, Woodland
4 20,- 20
0- 10
<5 5-9.9 10-14.9 15-19.9 20-24.9 25-29.9 >30
Carapace Increase /Year (mm)
FIGURE 7. Comparison of mean annual carapacial increment during growing seasons 1-10 for
gopher tortoises in Georgia (wiregrass and agricultural habitats) and Florida (woodland
habitat).
ponents of shape and development (Mosimann 1958). In this section,
allometry is utilized to depict changes from the juvenile to subadult to
fully mature form in G. polyphemus. The relative rates of increase in CL,
width (W), and thickness (TH) are apparent in Fig. 8. As has been noted
for CL increase, mean body widths and thicknesses for age cohorts 0-17
were not significantly different from values predicted from the linear
regression lines (P>0.20). Thus it is assumed that deviations about the
line (Fig. 8, notably at age 9) are due to small sample sizes in some age
cohorts.
Mean annual increments for various dimensions are given in Table 4.
The major axis of growth through age 17 was antero-posterior, width and
thickness increasing at only 73 and 41% of the CL rate of increase, respec-
tively. The annual increase in CL is a larger percentage of carapace
length (compared to relative growth in W and TH) for 4-5 years after
hatching. Mosimann (1958) noted dissimilarity between hatchlings and
later forms and suggested that this reflected selection for "form-changing"
types, i.e., the advantageous form is produced mainly just after hatching
through differential growth. This also seems to characterize the relative
growth relations in G. polyphemus.
From about age 5 to just before maturity, growth is linear with age at
the same relative rate in all 3 dimensions. Grubb (1971) reported isometry
between CL and width in giant tortoises. Beginning at about age 17, as
individuals reached adult form, annual increments in all body dimensions
decreased sharply (Fig. 8). Growth rates in linear dimensions for the first
11 years as adults ranged from 23-54% of rates for the younger group
(Table 4).
BULLETIN FLORIDA STATE MUSEUM
mm
150 -
Th F
100 M
soo1- 0mom cn on to N
0 5 10 15 20 25 30 35
Age (years)
FIGURE 8. Relative growth in carapace length (CL), body width (W), and body thickness
(TH) in relation to age estimated from annuli counts for male (M) and female (F) tortoises.
Symbols are mean values for tortoises in 1-year age classes; sample sizes are indicated above
the age axis. Through age 15, regression lines are used; thereafter, lines are fitted to the points
by hand.
TABLE 4. Mean annual increments in body dimensions of young (0-17 yr) and older (18-28 yr)
gopher tortoises. Increments for the first group are the slopes of regression equations
(see text); for the older group, increments are values for the 26-30 age-cohort
(x = 28) minus values at age 18 divided by 11 yr. Sample sizes in parentheses.
Group 1 Group 2
0-17 yr 18-28 yr
(298) Males (63) Females (61)
Dimension x 5 % Group 1 x % Group 1
Carapace length (mm) 11.3 2.7 23.9 5.2 46.0
Width (mm) 8.3 2.5 30.1 4.5 54.2
Thickness (mm) 4.7 1.3 27.7 1.8 38.3
Volume (cc) 210.6 181.8 86.3 345.5 164.1
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 95
Allometric relations of older tortoises were clearly sexually dimorphic
(Table 4). Width increases of males were almost equal to CL increments,
and growth in thickness, although greatly reduced from the rate for the
smaller group, was more rapid relative to CL increase. In females,
however, CL and W continued to increase at approximately half the rates
in the younger cohort, contrasted to rates in males of less than 1/3 those in
the younger cohort (Table 4). The thickness of females increased at a
much lower rate (38%). As a result of these allometric changes, mature
females were longer and wider relative to their thickness than they had
been during the latter stage of rapid growth (ages 12-17 yr). Increase in
TH of both sexes and increase in CL and W of males typically was asymp-
totic soon after maturity was reached; CL and W of females usually in-
creased substantially for several more years. Relative growth in the three
dimensions is depicted in Figure 9.
Box-model volume (CL x W x TH) was a curvilinear function of
both CL and age (Fig. 10). As weight data were too variable for accurate
analysis, volume was employed as an index to size. Regression analysis us-
ing 605 individuals demonstrated that weight and volume (V) of gopher
tortoises were linearly correlated (r2 = 0.95). It was found that patterns of
development of secondary sex characters (e.g., plastral concavity) were
Hatchling
I CL
12 TH
5 1 28 Years C
17 Years 4.5
Pm5
2 28 Years 9
SCALE Imm]
0 10 30
FIGURE 9. Three-dimensional growth models of Gopherus polyphemus in the hatchling,
subadult, and mature stages of development. Arrows represent growth vectors of the three
body dimensions, expressed in terms of mean annual increments (mm) between developmen-
tal stages.
BULLETIN FLORIDA STATE MUSEUM
7 M
6 3000 "
0 5-
0- 0
S 4- -2000 m
E 3
> 2 o Box Model Volume 1000
Body Weight
1 -
0 5 10 15 20 25 30 35
Age (Years)
FIGURE 10. Volumetric growth and body weight in relation to age. Data were obtained using
the volume-age correlation method, and the line for volume was fitted by eye.
highly correlated with body volume in linear relationships (McRae et al.
1980).
Since volume is a quadratic of three dimensions (CL, W, TH), all of
which increase linearly with age (through 17 yr), its exponential increase
(Fig. 10) is expected. The 90% greater rate of volumetric increase in 18 to
28 year old females (Table 4) relative to males was due mainly to con-
tinued growth in CL and W of females. Older females almost doubled in
body volume during this 11-year span (Fig. 10), while males began to ap-
proach terminal size.
The exponential increase in total body size demonstrates a pronounced
change that is not clearly reflected in linear dimensions taken separately.
Volumetric change is a more meaningful indicator of growth; it best
reflects the rapid development that is apparent in weight change with age
(Table 5). A very close relationship exists between the box-model estimate
of volume and body mass from age 0-17 years; the carapace in this age
cohort is characteristically dome-shaped. In older, larger tortoises, the
carapace becomes more dorsoventrally compressed and the body con-
forms more closely to the box model. This change in shape has also been
noted in certain aquatic turtle species (Mosimann 1958). The body mass
per cubic unit of box-model volume consequently increases greatly in
older individuals (Fig. 10).
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 97
VOLUMETRIC INCREMENTS
Estimates of annual volumetric increments were determined by cal-
culating the mean volume of tortoises in each age class (Fig. 10) and then
determining differences in volume from one year to the next. Values were
plotted over CL to provide a general comparison with CL increments
(Fig. 5). This curve (Fig. 11) shows a slight decrease in added size at CL of
60-70 mm, after which volume increased at an increasing rate. The in-
flection point occurred at 220-230 mm CL, beyond which tortoises were
still growing, but at decreasing rates. This growth change occurred at the
size of most rapid development of sexually dimorphic external characters
(McRae et al. 1981).
TABLE 5. Body weights (g) of gopher tortoises, by age, in natural habitats in southwestern
Georgia.
Age N x SD
0 130 34.4 3.1
1 40 50.1 10.4
2 4 64.1 12.4
3 10 100.1 19.1
4 12 181.4 28.8
5 16 238.1 41.8
6 13 319.3 45.2
7 10 400.2 47.4
8 8 519.7 100.2
9 5 779.1 81.6
10 7 817.6 118.3
11 3 1103.7 338.4
12 9 1272.5 303.1
13 5 1427.9 220.0
14 8 1581.2 358.6
15 6 2064.0 188.6
16 5 2165.4 366.9
17 5 2278.3 325.0
18-20 M 25 3012.3 575.9
F 7 3184.2 513.7
21-25 M 44 3287.4 688.2
F 18 3959.6 1102.0
26-30 M 58 3603.2 558.7
F 56 4205.3 958.7
31+ M 32 3651.9 573.6
F 42 4911.6 775.5
BULLETIN FLORIDA STATE MUSEUM
500
400 o
E 0
300
00
.o 200 -
100
I I I I
50 100 150 200 250
Carapace Length (mm)
FIGURE 11. Annual volumetric increment in relation to CL. Increments are differences in
mean volume between age classes, plotted over mean CL for each age class. Sample sizes as in
Figure 8.
SIZE IN RELATION TO LIFE HISTORY
IMMATURE STAGES
As shown in previous sections, gopher tortoises develop most rapidly
in early years, though not at a uniform rate. The cumulative effect of
these variable rates is an overall growth curve, which is useful in depict-
ing life history stages.
The progressive development of the carapace was estimated by three
methods. Capture-recapture data from Table 1 were applied, starting at
hatchling size (50 mm CL) and adding CL increments; theoretically, as a
tortoise entered a new CL class, the mean annual increment for that class
was added as the next year's growth. The second method was to plot CL
over estimated age at the time of capture (data from Table 6). A curve
was developed by connecting CL values obtained in September-October,
when termination of yearly growth occurred. Carapacial increases
estimated from annuli widths provided a third estimate (see Table 6).
These approaches indicate similar, curvilinear relationships (Fig. 12),
beginning with a steep linear increase extending through age 11 years
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 99
(190 mm CL). This pronounced growth coincides with the above-average
growth reflected in annual increments (Fig. 5). Thereafter growth
gradually decreased; it was greatly reduced beyond age 17 years, espe-
cially in males.
TABLE 6. Carapace lengths of young gopher tortoises, by age, in natural habitats in south-
western Georgia. Values calculated by CL-age correlation are data for the middle
of the growing season; annuli measurements provide values for the end of the grow-
ing season (winter).
Gr
sea
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
aA(
Carapace length (mm)
owing CL-age correlation Annuli measurement
son N x SD N x SD
130 50.la 2.1 -
40 59.4 5.5
52 64.6 0.6
4 69.8 7.6
44 76.0 1.0
10 79.4 4.2
44 88.2 1.4
12 97.4 6.0
42 100.5 1.9
16 106.4 6.9
31 111.6 2.7
13 122.6 5.3
29 122.9 3.1
10 134.1 5.5
23 136.2 3.5
8 144.4 7.5
20 148.9 4.6
5 159.6 7.6
17 162.6 6.4
7 163.0 5.1
16 177.5 7.6
4 181.8 15.0
16 187.5 7.9
9 191.7 11.9
12 197.0 10.8
5 194.8 15.6
11 207.1 10.9
8 206.4 14.9
11 216.2 10.1
6 223.2 16.1
7 225.0 10.2
5 231.1 13.3
7 228.8 10.4
5 232.1 11.8
7 237.9 10.4
actual size during first day of life.
BULLETIN FLORIDA STATE MUSEUM
SCapture recapture
*CL- age correlation
o Annuli measurement
Age yearsl
FIGURE 12. Comparison of CL-age relationships estimated by three methods of analysis. The
curves are fitted to the CL-age correlation data only.
Tortoises passed through two general life history stages before reach-
ing maturity. The juvenile stage lasted until the carapace was about
100-120 mm in length. The shells of young tortoises were very soft, and
carapacial scutes usually had distinct yellow centers. Laminal spurs on
the humeral and femoral shields were pronounced. During the following
few years the juvenile coloration faded as tortoises entered the subadult
stage. Laminal spurs became indistinct from abrasion, and the shell less
compressible. During the latter portion of the subadult stage (190-220
mm CL), the shell was nearly rigid and sexual dimorphism became more
apparent; body volume increased rapidly during this stage (Fig. 8). There
was no evidence of social interaction (i.e., visits to dens of others) in the
juvenile stage, and very little interaction was observed among subadults.
Auffenberg and Iverson (1979) studied aging in relation to plastron
growth in the Florida State Museum (FSM) collection. The growth curve
in that sample appeared to be sigmoid, compared to the somewhat
parabolic curve determined for tortoises at the southwestern Georgia
locality (Fig. 13). Student's t-tests indicated that yearly means of PL
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 101
growth increments for the Florida tortoises 7-14 years of age were
significantly larger (P<0.01). Means at ages 6 and 15 were different only
at the 10% level, and at other ages the means were not significantly dif-
ferent (P > 0.10). The Florida specimens were collected near Gainesville,
approximately 140 km south of our study site. The differences in growth
increments seem attributable to dissimilar yearly activity periods, as
noted earlier.
SIZE AT SEXUAL MATURITY
Several criteria were used to determine size at maturity. Estimating
the size at which males are functionally mature is a complex matter.
Necrospy data showed that physiological maturity, as indicated by sperm
production, occurs in individuals with a CL of 210 mm, and possibly
smaller. Somatic maturity, including rapid development of secondary sex
characters, usually occurs from that point up to 240 mm CL or more
(McRae et al. 1981). Of greater importance is psychogenic maturity,
which occurs when males acquire a propensity to breed. Determining
that stage from field observation is difficult because large males attempt
to prevent smaller ones from visiting females (Douglass 1980).
Subdentary glands were active from spring-fall in 55 males over 230
mm CL; glands of 18 others (160-235 mm CL) were inactive, but those of
two males (222 and 227 mm CL) were slightly active. During a study of
the mating system in southern Florida, Douglass (1980) observed seven
males that visited females at their dens; plastron lengths of these males
250r o
250-NC Florida *
0 SW Georgia .
200 .
E ... "* or
150
o :: '
C 100- 00
Q. 50 .
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Estimated Age IYears I
FIGURE 13. Plastral growth in relation to age in southwestern Georgia (this study) and north-
central Florida (from Auffenberg and Iverson 1979). Symbols represent means for 0.1-year
age classes.
BULLETIN FLORIDA STATE MUSEUM
were 222-256 mm (227-251 mm CL, our estimate). Of 32 males seen
copulating or visiting females in our study, all were at least 240 mm CL.
Taking all factors into account, we estimate that males usually be-
come functionally mature in this region as they reach 230-240 mm CL.
The lower size limit of mature males indicated by Auffenberg and Iverson
(1979) was also 230 mm CL.
Females in our study area reach somatic maturity at about 250 mm
CL (McRae et al. 1981). The minimum CL of confirmed sexually mature
females (N = 44) was 252 mm. Ten small females (215-261 mm CL),
necropsied during April and early May, contained no oviducal eggs,
enlarged follicles, or corpora lutea; reproductive activity would probably
have been discernible at this season (e.g., Iverson 1980). Subdentary
glands were active during spring for females 255 mm in length and larger,
and inactive in smaller individuals.
From the FSM collection we also examined 23 females for which re-
productive data were provided by J. Iverson. External sex characters in
this sample were evident at smaller sizes than in our area (McRae et al.
1981). The smallest mature females in the FSM collection were 242, 245,
and 249 mm CL. Other mature females reported from Florida have been
as small as 238 mm CL (Auffenberg and Iverson 1979) and 241 mm CL
(Hallinan 1923). Behavioral evidence presented by Douglass (1980) in-
dicated that seven females 242-303 mm PL were frequently visited at their
dens and often showed interest in male courtship; two of the larger in-
dividuals (245 and 303 mm PL) apparently laid eggs. Smaller females
(184-240 mm PL) were rarely visited by males, and showed no interest in
courtship. Based on a review of the literature and examination of the FSM
and other samples, Iverson (1980) concluded that females mature at
220-230 mm PL (226-236 mm CL) in northern Florida.
We examined four immature females from our study area that were
243-261 mm CL: the smallest mature female was 252 mm CL, and all
other confirmed mature individuals were 260 mm in CL or larger. From a
collection made near Douglas, Georgia, Jackson et al. (1974) examined a
female 241 mm CL that contained enlarged follicles, but this is not con-
clusive evidence of maturity. In turtles ovarian follicular development
can occur several years prior to ovulation; this process is demonstrated in
studies of Chelydra serpentina (Christiansen and Burken 1979) and
Geochelone gigantea (Swingland 1977). Therefore we estimate that
females in this part of Georgia reach maturity at some point between
250-265 mm CL.
AGE AT SEXUAL MATURITY
Growth rates of the two sexes were very similar up to about 240 mm
CL, but began to diverge beyond that size. Volumetric increments peaked
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 103
at 220-230 mm CL (Fig. 8). After divergence the reduced growth rates of
males and the greater rates in females led to sexual dimorphism in size
beyond age 17 years (Table 7). Males attained the size of estimated sexual
maturity (230-240 mm CL) at 16-18 years while females reached mature
size (255-265 mm CL) at 19-21 years. The latter period is considerably
greater than the 10-15 years estimated by Iverson (1980) for attainment of
maturity by females in northern Florida. As stated earlier, the Florida
specimens examined became mature at smaller sizes, and the minimum
age was determined from faster-growing tortoises (Auffenberg and Iver-
son 1979).
Estimated ages at maturity in other Gopherus species in the wild vary
considerably. Gopherus agassizii is said to mature at 15-20 years (Wood-
bury and Hardy 1948; Berry 1976). Burge and Bradley (1976) suggested
that both sexes mature at 215-220 mm CL in that species, and data ob-
tained by Medica et al. (in Auffenberg and Iverson 1979) indicate that
this size is attained some time after 17 years of age. Miles (1953),
however, stated that G. agassizii can reach maturity at ages as low as 10
years.
TABLE 7. Mean CL (mm) of large gopher tortoises, by sex and age group, in southwestern
Georgia.
Males Females
Age N SD N R SD
18-20 25 248.2 10.0 7 251.8 8.7
21-25 44 254.9 12.7 18 271.6 7.7
26-30 58 261.9 10.5 56 288.6 12.6
31+ 32 263.7 16.1 42 294.3 15.9
In the Texas tortoise (G. berlandieri), attainment of breeding size has
been estimated to occur at 15-20 years of age (Anonymous 1967, in
Douglass 1980). However, Auffenberg and Weaver (1969) estimated that
maturity in this species is reached at 105-128 mm CL, at ages of only 3-5
years.
Apparently the only information on sexual maturity in G. flavomar-
ginatus is that of Legler and Webb (1961). They suggested that both sexes
mature at 220-300 mm CL, and that the age of one female in this size
range was about 11 years.
POST MATURITY
After maturity is reached, growth occurs occasionally, but at reduced
rates. Unusually large size in adult Chrysemys picta (Gibbons 1967) and
BULLETIN FLORIDA STATE MUSEUM
Geochelone gigantea (Grubb 1971) has been attributed to exceptionally
rapid growth as immatures rather than to continuing growth as adults.
Auffenberg and Weaver (1969) noted a sharp decrease in growth of the
Texas tortoise after attainment of sexual maturity. Abrasion of the shell of
gopher tortoises over the years can hinder accurate age estimation. Males
in this part of Georgia typically stop growing at some point between
250-270 mm CL. Females grow to be significantly larger; mean CL for
adult females in our area was 283 mm (N = 102); the largest captured was
335 mm CL. Sexual dimorphism in size may be related to differential
energy intake and expenditure. Movement data from radio-instrumented
tortoises in this area (unpublished) and data presented by Douglass (1980)
show that males travel much more and spend more time in mate-seeking
and courtship than do females.
By applying the mean growth rate of adults that grew measurable
amounts from one year to the next (Table 8), we estimated that the age of
the largest female in our sample was at least 57 years, and of the largest
male (290 mm CL) was 46 years. So about 53% of recaptured adults did
not grow during a one-year period, sporadic growth from year to year is
indicated. Probably some tortoises at our locality live to be perhaps
80-100 years of age. Hubbard (1894) suggested that tortoises of this species
may live more than 100 years.
TABLE 8. Annual growth of 56 adult gopher tortoises in southwestern Georgia, as determined
by capture-recapture.
Malesa Femalesa
Growing Tortoises
N 11 15
x increment (mm) 2.1 2.3
Increment range (mm) 0.2-5.2 0.4-5.8
All Tortoisesb
N 24 32
x increment (mm) 0.9 1.1
Increment range (mm) 0.0-5.2 0.0-5.8
aCL > 240 mm for males, > 265 mm for females.
blncludes tortoises that did not show growth (13 males, 17 females).
SURVIVAL AND ADAPTIVE IMPLICATIONS
Data gathered thus far suggest a latitudinal dine in body size in
G. polyphemus, with a general northward increase. Notable exceptions
are record-sized individuals from Florida, such as one of 368 mm CL
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 105
(Conant 1958) and another of 343 mm CL (Carr 1952). Local variations
in growth due to differences in habitat quality and nutrition, documented
in this study and by Gibbons (1967), Jackson et al. (1978), and Swingland
(1977) for other species, may explain these exceptions.
The typical upper size limit for this species, according to Cochran
(1952), is about 305 mm CL in Florida, and the largest reported by
Ditmars (1946) and Hutt (1967) in Florida are less than 300 mm CL.
Douglass and Layne (1978) noted that adults of PL greater than 300 mm
were rare in a large sample (N = 366) from south-central Florida, and
Iverson (1980) reported a mean CL of 262 mm for adult females in a
north-central Florida sample (N = 16). Alford (1980) reported that only
4% of the individuals in 12 northern Florida colonies exceeded 300 mm in
CL. Contrasting with these reports are: (1) in our study area in south
Georgia, a mean CL for adult females of 283 mm (over 24% with CL ex-
ceeding 300 mm); and (2) observations that the largest individuals of a
Florida-Georgia sample were in the fall-line hills of Georgia, at the north-
ern limit of the range (D. Franz, pers. comm.). Further evidence that
gopher tortoises in the south are generally smaller is implied by existing
data on clutch size, which increases with the size of females (Iverson 1980;
Landers et al. 1981). The mean and upper limit of clutch size in our study
area (7 + and 12 eggs, respectively) exceed corresponding data from
Florida (see Landers et al. 1981) with only two exceptions, both from
Dade County (a clutch of 15 eggs and a group of 19 enlarged follicles in a
female, Iverson 1980). Considerable evidence suggests a similar north-
south dine in clutch size in some aquatic turtles (Cagle 1954; Tinkle 1961;
Christiansen and Moll 1973).
The higher growth rates of Florida G. polyphemus relative to those in
our study area seem attributable to longer growing seasons in the south.
The reasons why females in Florida attain mature form and lay eggs at
smaller sizes (McRae et al. 1980; Landers et al. 1981) and are apparently
smaller on the average than those in southwestern Georgia are probably
more complex. Several possible explanations are presented here for con-
sideration.
One hypothesis is based on differential physiological age. In south-
western Georgia tortoises are dormant for 3-4 months during winter.
Those in the southern portion of the range are active for much longer
periods during a given year, and in extreme southern Florida are active
year-round (W. Auffenberg, pers. comm.). Thus a southern specimen liv-
ing the same number of calendar years as a more northerly one is physio-
logically older. If physiological age is a primary determinant of maturity,
internal and external morphological traits accompanying maturity would
be expressed in fewer calendar years and at smaller sizes in southern
animals. Maturity is followed by a cessation or great reduction in growth,
BULLETIN FLORIDA STATE MUSEUM
and, consistent with the hypothesis, terminal sizes appear to be smaller in
the south.
Another possible explanation for these size differences is based on the
probable tendency of gopher tortoises to modify their habitats through
grazing pressure. A longer growing season (i.e., Florida vs. Georgia)
results in earlier maturity and may induce more frequent additions to the
population. If a high density ensues, overgrazing may reduce the more
nutritious herbs in the feeding stratum and serve to limit the growth of
larger individuals, which require greater quantities of food. Work on the
giant tortoise (Swingland 1977; Coe et al. 1979) is interesting in this con-
nection: Adults in a dense, food-limited population attained sizes only
40% as great as those attained in two low-density populations.
Additionally, environmental influences may select for terminal body
size. If high-density populations have occurred more frequently and ex-
erted stronger grazing pressure in southern portions of the range, selection
for smaller, more energy-efficient tortoises could have resulted.
Selection for thermal efficiency may also be at work. Southern
specimens not only tend to be smaller, they are also lighter in color on the
carapace and plastron. In southwestern Georgia the carapace is dark gray
and the plastron mottled black and yellow, while at the northern limits of
the range (e.g., the Georgia fall-line hills) the carapace is even darker and
the plastron is often totally black (our observations and those of R. Franz,
pers. comm.). It seems that smaller, lighter-colored tortoises may be bet-
ter adapted to reflect heat (i.e., in the south), while larger, darker indi-
viduals probably conserve heat more efficiently (i.e., northern portions of
the range).
Selection for larger females may be linked to fecundity. For example,
in a study of geographic variation in size and reproduction of Ster-
notherus odoratus, Tinkle (1961) hypothesized that selection in the south
must be mainly for smaller body size, while in the north, selection for
both larger size and higher reproductive potential seems to exist. These
selective forces may also affect gopher tortoises. The less frequent addi-
tion to colonies in southern Georgia, as indicated by longer periods of
growth to sexual maturity, may be somewhat compensated by selection
for larger females producing larger clutches. Growth is probably further
reduced in more northerly populations, which may be even less produc-
tive. This species is distributed patchily in the fall-line hills of Georgia,
and sand ridge habitats exist in many areas north of the present and
known historic range of these animals. As annual temperature cycles in-
fluence growth and age at sexual maturity, it is possible that the length of
the growing season is a major factor limiting the range of this species.
As Auffenberg and Iverson (1979) pointed out, growth to the age of
sexual maturity is a very important parameter in population dynamics.
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 107
This period is critical, as individuals are exposed to predators and diseases
before becoming reproductively active.
A study made in this area (Landers et al. 1981) revealed that female
G. polyphemus nest at most once per year, and most nests (87%) are
destroyed by predators. Many hatchlings are killed in the nests or shortly
after emerging by predatory mammals and fire ants (Solenopsis
saevissima) and, for at least two years after hatching by several species of
snakes and mammals. In some areas, larger tortoises are preyed upon by
canids (Causey and Cude 1978) and black bears (Ursus americanus)
D. Speake, pers. comm.). In addition, gopher tortoises are known hosts
for several types of parasites (e.g., Petter and Douglass 1976), but the ef-
fects of these and other possible agents of disease on populations are
unknown. During this study, three immature tortoises (aged 2, 4, and
5 yr) and four adults were found dead in or near their burrows, but the
causes of death could not be determined.
Low reproductive rates and the high incidence of postnatal mortality
in this species no doubt greatly impede the comeback of depleted popula-
tions. The extended period of time required for hatchlings to reach
maturity (e.g., 10-15 yr in Florida, 16-21 yr in our study area) is an addi-
tional limiting factor, especially in more northerly parts of the range.
BULLETIN FLORIDA STATE MUSEUM
APPENDIX
SYNOPSIS OF METHODS USED TO ESTIMATE GROWTH
Results of this study are based on application of three methods commonly used in
testudinid growth estimation. In using these methods, it was noted that each had its merits
and shortcomings. This section is a comparison of the approaches, and is presented as an aid
on future studies.
The capture-recapture method is most time-consuming. Due to varying rates of increase
among and within size classes, a large sample is required (i.e., at least 100 specimens). It is
difficult to capture gopher tortoises at the same time in two successive years because they
relocate frequently and become wary of even well-camouflaged traps. Recapture success was
only 27% in this study and 25% in that of Goin and Goff (1941). If measuring seasonal
growth is a study objective, this method should not be used because frequent capture may
cause gopher tortoises to move to other burrows and may alter normal patterns of movement
and feeding. If volumetric increments are to be studied, even larger samples are required; ex-
ponential increases make inferences from small samples of little value. The advantage of the
capture-recapture method is that it yields exact measurements of linear increments over time.
Its effectiveness can be improved by concentrating trapping efforts on juveniles and
subadults, as adults grow very little.
The accuracy of age-dimension correlations is dependent upon the regularity of ring
deposition in a given location and the ability of the observer to discern actual annuli. Both can
be understood by close examination of specimens captured at various time intervals. To depict
the entire aging process, a sufficient sample is needed for each 1-year class through maturity.
In gopher tortoises, 5-10 per class is adequate to determine the overall pattern of growth, but
not to estimate accurately increments between yearly age classes. The advantages of this
method over the capture-recapture procedure are: (1) data can be gathered each time a live or
dead specimen is available; (2) seasonal growth can be estimated by comparing sizes of the in-
dividuals captured during various months of the year. This method is also the most practical
means of discerning trends in volumetric increase, as adequate data are easily obtained.
The utility of annuli measurement-prediction equations is also dependent upon accuracy
in discerning rings. Total annuli width on the abdominal scutes was considered to be the best
measurement for our purposes because (1) it takes into account the breadth of corresponding
(paired) abdominal annuli; (2) it provides more precise points of measurement in comparison
to other dimensions of that scute; (3) rings on the other plastral scutes are difficult to measure
because of their irregular shapes and greater susceptibility to abrasion. Annuli are also
deposited on the carapace, but measurements made on the convex shields are somewhat in-
consistent from one tortoise to the next and false rings are not so easily detected as on the
plastron. Resulting equations are valuable in estimating linear dimensions in past years, but
not volume; the logarithmic relationship involved and the variable nature of the data prevent
predicting volume from annuli measurements within acceptable limits. The advantages of
this method are: (1) few specimens are required to obtain an adequate data set; (2) both
preserved and live specimens can be utilized. Growth histories can also be used to compare
the effects of differing environmental conditions over long periods of time.
Studies of growth and maturity of this and similar species are most thorough when all
three methods can be employed. Capture-recapture data provide an essential check of results
from other approaches. Seasonal growth and volumetric increase are most efficiently deter-
mined by age-dimension correlation, and long-term environmental effects and individual
growth histories by the annuli-prediction equations.
Vol. 27, No. 2
1982 LANDERS ET AL.: S.W. GEORGIA GOPHER TORTOISES 109
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BULLETIN FLORIDA STATE MUSEUM
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Vol. 27, No. 2
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