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NATURAL HIST SURVE’

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ILLINOIS
NATURAL
HISTORY

SURVEY

Natural History Notes on Nesting, Nests, and 7
Hatchling Emergence in the Red-eared Slider

Turtle, Trachemys scripta elegans, in

West-central Illinois
John K. Tucker

Biological Notes 140
January 1997

Illinois Natural History Survey, Edward J. Armbrust, Acting Chief
607 East Peabody Drive

Champaign, Illinois 61820

(217) 333-6880

A Division of the Illinois Department of
Natural Resources

Printed by authority of the state of Illinois
RR80485GR-3M-1-97
US ISSN 0073-490X

Editors: Thomas E. Rice and Charles Warwick
Cover photograph by Michael Jeffords

Suggested citation:

Tucker, J.K. 1997. Natural history notes on nesting, nests, and
hatchling emergence in the red-eared slider turtle, Trachemys
scripta elegans, in west-central Illinois. Illinois Natural History
Survey Biological Notes 140. 13 pp.

Author's address: John K. Tucker, Illinois Natural History Survey,
LTRM Program, Reach 26, 4134 Alby St., Alton, IL 62002

INTRODUCTION

Detailed studies of terrestrial movements and activities of
aquatic turtles have focused mainly on the energetics or the
ecological benefits of such movements (e.g., Sexton 1959;
Gibbons 1986; Congdon and Gatten 1989). Such terrestrial
movements or activities include nesting forays, migration, and
post-incubation emergence. Studies of terrestrial activities of
the slider (Trachemys scripta) include those of Cagle (1937,
1950), Moll and Legler (1971), Morreale et al. (1984), and
Gibbons et al. (1990).

Despite the wealth of data on the ecological cost/benefits
of such movements, little is known about the proximal
environmental correlates of nesting in females or emergence
in hatchlings for most turtles (Gibbons 1990 for T: scripta and
Ernst et al. 1994 for other aquatic species). Most reports are
descriptive. For instance, Gibbons and Coker (1977) found
no evidence of a correlation between daily terrestrial activity
and either temperature or precipitation for the cooter
(Pseudemys floridana), but presented no statistical support for
their conclusions. In contrast, Christiansen et al. (1985)
reported that rainfall stimulated movements of the yellow mud
turtle (Kinosternon flavescens). But species apparently vary
in their response to rainfall. For example, Ernst et al. (1994)
noted that the wood turtle (Clemmys insculpta) does not nest
in the rain. Likewise, Vogt (1980, for the false map turtle
[Graptemys pseudogeographica]) and Burger and
Montevecchi (1975, for the diamondback terrapin
[Malaclemys terrapin]) found that rainfall inhibits nesting.
Vogt (1980) also suggested that cool nocturnal air tempera-
tures may inhibit female nesting movements. In the same
vein, Landers et al. (1980) reported that nesting in the gopher
tortoise (Gopherus polyphemus) occurred only on warm,
sunny days and apparently was not associated with rainfall.
Swingland and Stubbs (1985) suggested that fluctuations in
weather caused temporary halts in nesting of Hermann’s
tortoise (Testudo hermanni). Christens and Bider (1987)
posited that the length of the nesting season and timing of
nesting are probably more influenced by day-to-day weather
changes than general climatic conditions throughout the range
of a particular species. Despite all these comments and
suggestions, there have been no publications with data
supporting their conclusions detailing female nesting patterns

of aquatic turtles in relation to local weather conditions.

An important exception is the mild controversy concerning
correlation of mean temperatures during various portions of
the activity cycle with the onset of nesting. Christens and
Bider (1986) found a correlation between onset of nesting and
mean temperatures of the previous year in the painted turtle
(Chrysemys picta) from Quebec. In contrast, Obbard and
Brooks (1987, for the common snapping turtle [Chelydra
serpentina]), Congdon et al. (1983, for Blanding’s turtle
[Emydoidea blandingii}), and Iverson and Smith (1993, for
Nebraska C. picta) demonstrated a correlation between mean
spring temperatures and onset of nesting.

Information on hatchlings and their responses to environ-
mental cues is even more limited, particularly for freshwater
turtles. Other than records of the time of year of emergence or
comments on the evolutionary significance and variation in the
propensity for various species to overwinter in the nest (i.e.,
Gibbons and Nelson 1978; Jackson 1994; Holman and
Andrews 1994), only anecdotal information is available. For
example, Moll and Legler (1971) reported that 7. scripta
hatchlings in Panama emerged “en masse” after heavy rain.
Similarly, Mitchell (1988) and DePari (1996) suggested that
rainfall was associated with emergence of C. picta hatchlings
in North America, as did Alho and Padua (1982) for the giant
South American river turtle (Podocnemis expansa) and Polisar
(1996) for the Central American river turtle (Dermatemys
mawii) from South and Central America, respectively.

During 1994 and 1995, I collected more than 400 nesting
female T. s. elegans (Figures | and 2). During this time, and
particularly during the second year of the study, detailed daily
records were kept on the numbers of females encountered
throughout the nesting season. In the process, I observed 25
completed nests. I also observed hatchling emergence in 1995.
The purpose of this Biological Note is to report the relation-
ships of females, hatchlings, and their nests and environmental

conditions that they encounter in west-central Illinois.

MATERIALS AND METHODS

Females were collected from five nesting areas in Jersey and
Calhoun counties, Illinois (Figure 3). Swan Lake, Stump
Lake, Gilbert Lake, and Pohlman Slough were visited in 1994.
Flooding of usual travel routes in 1995 allowed nesting areas
near Gilead Slough to be added. Because females at Swan and

Stump lakes were the primary focus of other studies, I

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Figure 1. Front view of female Trachemys scripta elegans nesting at
the Stump Lake nesting area on | June 1995. Nest was constructed

in the grassy strip bordering the millet field.

alternated which site I visited first each day at these two
locations. The other sites lie along the route between Swan
and Stump lakes. The number of turtles collected each day
was combined for all sites. In all, | examined 129 females in
1994 and 321 females in 1995.

Nesting turtles were sought daily from 24 May to 23 June
in 1994 and from 14 May to 3 July in 1995. In each year,
daily searches began once nesting activity was detected. In
1994, | arrived at nesting areas at about 1000 h (all times are
central daylight savings time) and completed surveys by 1300
h. In 1995, surveys began at 0700 h and ended by 1400 h on
most days. Although the entire day was not spent searching
for nesting turtles, all nesting areas were visited at least once
daily in each year during the respective time intervals. In both
years, nesting areas in Jersey County were visited after 1800 h
on days with extensive turtle activity.

Hatchlings were collected from 18 April to 19 May 1995,
in part with a drift fence. However, the drift fence used in this
study was not completed until 1 May. As a result, analysis of
drift fence data was restricted to the period between | May
and 19 May 1995, during which time I caught 158 hatchlings.
Four hatchlings were caught on the incomplete fence between
16 April and 28 April 1995. Two nests were discovered with
hatchlings emerging on 20 April 1995. In all, I caught 179
hatchlings. No hatchlings were collected in 1994.

The drift fence (see Tucker 1995 for details on fencing)
was 180 m long (Figure 4). The habitat at this site consisted
of a disked agricultural field which had contained millet,
sunflowers, weedy grasses, and other forbs. The fence was

placed between the location of nests and the nearest body of

2 Illinois Natural History Survey Biological Notes

me

Figure 2. Back view of the turtle shown in Figure 1. Note the angle

assumed by the turtle which allows greater nest depth.

water (i.e., down slope from where nests were located, Figure
4). Turtles could have missed the fence at either end. How-
ever, there was no indication that turtles attempted to avoid the

fence because all pits along the length of the fence caught

No. 140

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hatchlings. The purpose of the fence was to sample hatchlings,

not to catch them all. The fence was checked at least twice
daily, once between 0600-0630 h and again between 1800-
1830 h.

Female plastron length and nest depth were measured with
a millimeter rule to | mm. Eggs were carefully removed from
each nest prior to measuring the nest’s depth. Hatchling
carapace and plastron lengths were measured with vernier
calipers to 1 mm. Hatchlings were weighed on a Sartorius
electronic balance to 0.01 g.

I collected environmental parameters (rainfall and daily
high and low temperatures) at Nutwood, Illinois (Jersey
County), at the Mark Twain National Wildlife Refuge at Swan
Lake (Calhoun County), and at the Illinois Department of
Natural Resources offices at Rosedale (Jersey County)
(Figure 3). Because gaps occurred in the data sets when
observers were absent, observations for all three sites were
combined and averages used for analysis. Rainfall amounts
were measured in inches and temperatures were measured in
degrees F. Both were converted to metric equivalents prior to
analysis. I also recorded subjective impressions daily about
the weather that are mentioned when appropriate. Soil
temperatures at the Stump Lake site were measured with a
Reotemp” Bi-metal thermometer (to 1°C) two or more times
monthly until hatchlings were first found emerging. On each

date, four sites were measured with three separate measure-

January 1997 Tucker: Natural History Notes on the Red-eared Slider Turtle 3

Nesting areas: Location of weather observers
Gilbert Lake 5 A = Nutwood
Pohiman Slough B = Illinois DNR Rosedale

Swan Lake C = Mark Twain NWR office
Stump Lake

Gilead Slough

aran—
oun nw

Lock & Dam 25

Missouri

Calhoun County,

Iinois N
Missouri 1

Mississippi River = 3 kilometers

Figure 3. Location of nesting areas (numbers) visited during the current
study along with locations (letters) from which weather data were

collected.

ments taken at each site at 100, 150, 200, and 250 mm soil
depth as measured from the soil surface. Therefore, soil
temperatures for each observation date at each depth were a
mean of the 12 measurements.

All statistical analyses were performed with the SAS (SAS
Institute 1988). I used Spearman’s rank correlation coeffi-
cients to judge associations between data classes. I selected
the Kruskal-Wallis (= KW) test (the NPAR1I WAY procedure
in SAS) to compare means for hatchlings collected at various
periods during emergence because some variables were not
normally distributed.

For multiple comparisons, I used the sequential Bonferroni
procedure (Rice 1989). In this publication, I performed 10
correlation analyses and 2 partial correlation analyses. I
considered the analyses for females in 1994 and 1995
independent of each other. In each of these analyses, three
correlation analyses and one partial correlation analysis were
performed. Because each of these has an overall probability
of a type I error of 0.05, the lowest value of p must be less
than 0.0125 (= 0.05/4). Values of p between 0.05 and 0.0125,
while possibly supporting a hypothesis, cannot exclude the
possibility of a type I error at the 0.05 level when the lowest
value was greater than the Bonferroni sequential value (Rice
1989). I considered the hatchling analysis to be independent

of the female analyses. In this instance, the lowest value of p

for the correlations among hatchling numbers and the three
environmental measures must be less than 0.0167 (= 0.05/3).
The final comparison between female plastron length and
nest depth was considered independent of hatchling and
female correlations with environmental measures. Here, p
must be less than 0.05 for statistical significance. I did not
include the correlation analyses on daily high and low
temperatures because these were methodological require-
ments to determine whether partial correlation was needed.
Daily high and low temperatures showed significant
positive correlation (rho = 0.56, p = 0.0026, n = 27 days) in
1994. Likewise, in 1995, daily high and low temperatures
were positively correlated (rho = 0.74, p = 0.0001, n = 44
days). In other words, relatively warm nights usually
accompanied relatively warm days. This complicated
analysis because correlation between female numbers and
high or low temperatures could be the result of one or the
other variable or both in combination. Therefore, I used
partial correlation along with unadjusted correlation to
correct for the interaction between daily high and low
temperatures where correlation between female numbers and

daily high or low temperatures was found.

- = 36 meters

ne

Illinois Power line
Route 100
Millet
@ Nest 1 |
@ Nest 2 Sunflower |
se] Creek
= Drift fence
~ \
8 |
5 {
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é Borrow pit |

Stump Lake Ditch
+
Figure 4. Detail of the Stump Lake nesting area showing placement of

the drift fence and locations of two nests found emerging during the

study in 1995

4 Illinois Natural History Survey Biological Notes

RESULTS

In 1994, the number of females collected between 24 May
and 19 June was positively correlated with daily high
temperature (rho = 0.40, p = 0.0398, n = 27 days) and with
daily low temperature (rho = 0.22, p = 0.2752, n = 27 days).
However, neither was significant at the level (p < 0.0125)
needed to reduce the probability of type I errors to less than
0.05. With daily low temperature factored out, the signifi-
cance of the relationship (rho = 0.34, p = 0.0880) was further
reduced. Thus, turtle activity, while possibly correlated with
daily high temperature, also was influenced by warm nights.
As a result, each peak in turtle numbers was preceded by a
period of rising temperatures (Figure 5).

The relationship between temperature and female nesting
activity was clearer in 1995 (see Figure 6). Like 1994, the
number of females found between 14 May and 27 June was
positively correlated with daily high temperature (rho = 0.41,
p = 0.0059, n = 44 days) and with daily low temperature (rho
= 0.25, p = 0.1005, n = 44 days). The correlation for daily
high temperature was significant (p < 0.0125). However,
removing the effect of daily low temperature reduced the
significance of the correlation (rho = 0.34, p = 0.0244). The
larger sample from 1995 provided statistically sound support
for the hypothesis that the number of females found was
related to daily high temperature and that the relationship was
strengthened when daily low temperature was relatively
warm.

The relationship between rainfall amount and the number
of females found was complex. In 1994, daily rainfall
between 24 May and 19 June and number of females found
were positively correlated (rho = 0.47, p = 0.0126). In 1995
(between 14 May and 27 June), they were negatively
correlated (rho = -0.31, p = 0.0419). Neither was significant
at the level (p < 0.0125) necessary to reduce the probability of
a type I error to 0.05 or less.

The somewhat contradictory results for rainfall may reflect
the difficulty in quantifying rainfall. Nesting turtles seemed
to respond more to the timing of rainfall than to the amount.
For instance, the rainfall on 24 May 1994 and | June 1995
occurred overnight and had stopped before morning on each
day (Figures 5 and 6). The days were warm and sunny to
partly cloudy. Turtles were very active on both days despite
or possibly because of the overnight rainfall. In contrast,
rainfall on 2-3 June 1994 and 6, 8, and 9 June 1995 came

during the morning hours and appeared to suppress turtle

Number of females

Number of females

No. 140
30 x = rainfall > 2.5 cm 40
* = rainfall < 2.5 cm
25
* * 30
20
High
115 20
10
Low 10
| |
; Halll [. | Hh
14) 49) 245 29") 3 Ome Ome OmeeG

May June

Figure 5. Relationships between the number of females collected,
rainfall, and daily low and high temperatures in 1994.

50 40

* = rainfall < 2.5 cm
x = rainfall > 2.5 cm

~x< x

Low 10
10
0 0
14 23 1 10 19 28
May June

Figure 6. Relationships between the number of females collected,

rainfall, and daily low and high temperatures in 1995.

(9) eunyesedwe)

(9D) aunyesodway

January 1997

nesting activity (Figures 5 and 6). As a consequence, simple
comparisons of rainfall amounts and the number of nesting
turtles collected failed to elucidate the complex relationships
between the two. My subjective impression from 2 years of
observations was that rainfall stimulates nesting activity on
the following day.

The length of the nesting season varied between 1994 and
1995. In 1995, turtles began nesting earlier and continued
later than in 1994. Temperature patterns for the 2 years did
not obviously differ (compare Figures 5 and 6). In part, the
extended nesting season in 1995 may reflect collecting effort,
because I was able to reach the nesting areas earlier in the day
in 1995 than in 1994. Since I found females of T. s. elegans
nesting in the morning hours in the study area, I may have
missed females nesting prior to 1000 h in 1994. Missing
females may be critical in determining the early and late
portions of the nesting season when few females were nesting.

Turtles that I studied moved varying distances to nest
depending on the site and river levels at the time. In 1994, the
river did not flood during the nesting season to any appre-
ciable extent. Consequently, turtles at Stump Lake traveled at
least 300 m from the ditch on the site to reach the area where
most turtles nested (i.e., the millet field, Figure 4).

At Swan Lake, turtles left the lake and crossed the Deer
Plain Terrace (Figure 3). Upon reaching the edge of the
Calhoun County uplands, they climbed a large sand ridge to
nest in apple orchards and adjoining agricultural fields, a
minimum distance of 700 m. No turtles were seen nesting in
the intervening Deer Plain Terrace. Some turtles at Pohlman
Slough also crossed the Deer Plain Terrace and nested on the
Calhoun County uplands, a distance of 1.2 km. However,
many nested along the Illinois River road shoulder some 60 m
from Pohlman Slough. Turtles at Gilbert Lake either nested
on the shoulder of Illinois Route 100, some 30 m from Gilbert
Lake, or crossed the road and nested at the base of the bluffs
some 50 m from Gilbert Lake.

In 1995, the Mississippi and Illinois rivers crested after a
significant flood event at about the same time nesting began in
earnest. Flooding increased the size of the aquatic habitat and
shortened the travel distances to the nesting areas (Figure 7).

During the nesting season, females were found in drainage
ditches and flooded areas in agricultural fields close to nesting
areas. For instance, on 27 May 1994, I caught 43 females in a
puddle behind a levee and in a nearby flooded drainage ditch
on the Deer Plain Terrace near Swan Lake (Figure 8). None

of these females had palpable oviductal eggs at the time they

Tucker: Natural History Notes on the Red-eared Slider Turtle 5

Figure 7. The flooded Deer Plain Terrace in 1995.

border the Pohlman Slough complex. Swan Lake is to the left and not

© Sei

Photo by John Tucker, INHS Center for Aquatic Ecology

Trees in distance

visible. Many turtles emerged along the tree line in the left foreground

and nested in the cultivated field that the photographer was standing in.

Photo by Charles Theiling, National Biological Service

Figure 8. A flooded drainage ditch near Swan Lake on the Deer Plain

Terrace from which females were collected in 1994. The author and
assistant (Moynell M. Tucker) are shown in the process of collecting
females for marking.
were caught and marked for later recognition; however, six
were later recaptured nesting in June. In 1995, I caught 10
females at a small flooded area in an agricultural field. Two
of these 10 were later (on 18 and 21 June) recaptured nesting.
Females of this thoroughly aquatic turtle were never observed
at such locations except during the nesting season. The
recaptured turtles suggested that not all females returned to
their nonbreeding habitat after nesting but instead used these
temporary sites to reduce energy expended in moving to nest
sites to lay subsequent clutches

A total of 25 females was found in 1995 that had com-

pleted nest cavities (i.e., one or more eggs deposited in the

6 Illinois Natural History Survey Biological Notes

nest). Nest depth and female plastron length were positively
correlated (rho = 0.45, p = 0.0223, n= 25). The regression
model for nest depth vs. female plastron length (Figure 9) was
significant (R? = 0.23, F = 6.96, p = 0.0147). The 25 females
averaged 209 mm in plastron length (range = 187-225 mm,
SD = 10.9 mm), and their nests averaged 138 mm in depth
(range = 115-167 mm, SD = 13.9 mm).

Hatchling activity (Figure 10), as measured by numbers of
hatchlings caught on the drift fence (Figures 4 and 5) between
1 and 19 May, was positively correlated with daily high
temperature (rho = 0.50, p = 0.0291, n = 20 days) and with
daily low temperature (rho = 0.41, p = 0.0820), but negatively
correlated with rainfall (rho = -0.17, p = 0.4810). However,
none of these correlations were statistically significant. A p
value of less than 0.0167 was needed to reduce the possibility
of a type I error to 0.05 or less.

It may be that hatchling activity was more closely related
to soil temperatures at various depths in the soil. At the
Stump Lake site in 1995, soil temperatures at all depths from
the soil surface showed a steep increase beginning 15 Febr-
uary and continuing through 20 April when the two emerging
nests were found. At first, soil temperature was lower closer
to the soil surface than soil temperature deeper in the soil.
However, by 5 April, soil temperature at 100 mm depth
surpassed soil temperatures at greater depths. Plots of 100
mm-depth soil temperatures and 250 mm-depth soil tempera-
tures best illustrated the relationship (Figure 11). Hatchlings
may have responded to the changing soil temperature gradient
as well as the absolute temperature at a particular depth.

However, temperature measurements were made only in
1995. Several years of measurements would be required to
accumulate enough data to test for an association between
hatchling emergence date and soil temperatures. Even data of
this sort would only establish the approximate range of soil
temperatures at which hatchlings emerge. It would not
determine the proximal cause for the emergence of individual
nests. Only continuous tracking of the conditions within
individual nests would accomplish that goal.

Hatchlings apparently emerged and moved during daylight
hours. Except for four found after 1800 h on 14 May, all
hatchlings were recovered at the 1800 h fence check. On this
particular day, I decided to check the fence several times
during the day to get some idea of the distribution of
hatchlings on the fence in time (Figure 12). I chose this day
because subjectively it was a fine turtle day (i.e., warm and

sunny) and because the previous day’s captures suggested that

Number of hatchlings

Depth of nest (mm)

No. 140

180

Nest depth = 0.616*Female plastral length + 9.06

170

160

150

140

130

120

110

00
180 190 200 210 220

Female plastron length (mm)

230

Figure 9. Scatter plot of female plastron length vs. nest depth show-
ing correlation between the two variables for 25 completed nests from
1995.

x = rainfall > 2.5 cm
* = rainfall < 2.5 cm

16

1 6 ant
Day (May)

(9) auntesodway

Figure 10. Relationships between numbers of hatchlings caught in 1995

and weather variables.

January 1997

—10cm =—25cm

Temperature (C)

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Meneses t,o. = = 0.0. .o
Aa ove pre yO) | ie Soe
= we eee =
we Di | CON OO St
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Figure 11. Soil temperatures measured at 100 mm and 150 mm
below the soil surface from January until hatchlings were found

emerging from nests on 20 April 1995.

hatchlings were actively moving. On that particular day, 38 of
the 60 hatchlings captured were caught before 1330 h. I also
monitored the drift fence several times per day on 15 and 16
May but few turtles were caught on those days.

I compared the sizes (mass, carapace, and plastron length)
of hatchlings caught between | and 10 May inclusive and
those caught after 10 May (Figure 10) to see if hatchling size
varied with emergence time (i.e., early, n = 52 hatchlings or
late, n = 106 hatchlings). Early hatchlings were slightly
heavier (mean = 6.83 g, range = 5.37-8.30 g, SD = 0.70 g)
than late hatchlings (mean = 6.71 g, range = 4.53-8.59 g, SD
=0().91 g). Early hatchlings had plastron (mean = 29 mm,
range = 26-32 mm, SD = 1.39 mm) and carapace (mean = 31
mm, range = 28-33 mm, SD = 1.15 mm) lengths identical to
those of late hatchlings (plastron length: mean = 29 mm,
range = 25-33 mm, SD = 1.64 mm; carapace length: mean =
31 mm, range = 27-34 mm, SD = 1.59 mm). Not surprisingly,
all these comparisons were statistically insignificant (KW’s <
0.28, p > 0.59). The 179 hatchlings caught at the Stump Lake
nesting area by all collecting methods averaged 6.79 g in
weight (range = 4.53-8.59 g, SD = 0.82 g), 29 mm in plastron
length (range = 25-33.0 mm, SD = 1.52 mm), and 31 mm in

Number of hatchlings caught

Tucker: Natural History Notes on the Red-eared Slider Turtle 7

carapace length (range = 27-34.0 mm, SD = 1.43 mm).

Even though I caught many hatchlings at the drift fence,
observations of actual hatchling emergence were limited to
two nests found on 20 April 1995. The nests were found in
the field adjoining the one with the drift fence (Figure 4).
This particular field had been planted to millet in 1994. The
millet was burned off in March of 1995. Consequently,
visibility at ground level was excellent. The area was
purposefully searched when time allowed because turtles were
known to have nested there in 1994.

The first nest found at 1100 h contained at least seven
hatchlings, two of which were found about | m directly down
slope from the nest. Because these turtles were alerted to my
presence, I removed the remaining hatchlings in order to
measure the nest. The exit hole measured 32 X 41 mm. The
hatchlings were in a chamber that was 114 mm long and 76
mm wide. The maximum depth of this chamber was 105 mm.
Temperature within the chamber was 12°C, whereas air
temperature immediately above the nest was 14°C. The
chamber containing the hatchlings likely was not the original
location of the eggs. Egg fragments were located at 170 mm
depth as measured from the soil surface. At this level, soil
temperature also was 12°C as measured immediately adjacent
to the egg fragments. I estimated that the nest originally
contained about 14 eggs. Most of the hatchlings that
remained in the nest faced the exit hole.

20

15

10

Oo Oo
|
oOo —
~~

1230
1330
1430

Figure 12. Number of hatchlings caught each hour at the drift fence
on 14 May 1995.

8 Illinois Natural History Survey Biological Notes

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(FSS SSS
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Egg shells

Figure 13. Diagram showing dimensions of the second nest
discovered on 20 April 1995 during emergence of the hatchlings
along with positioning of the hatchlings within the nest cavity. A. Top
view. B. Cross section showing position of egg shells in relation to

hatchlings.

The second nest was discovered at about 1130 h (Figure
13). At this location, soil temperature was 12°C, and air
temperature was 16°C. I discovered the nest after finding a
hatchling 6 m down slope from the nest opening. Because I
saw the nest opening from a distance of about 5 m, I ap-
proached and observed it for some time before disturbing it.
During this time, heads of three hatchlings appeared at the
exit hole. These three hatchlings exited the nest one at a time
with one to a few minutes between exits. Each traveled a
different path immediately upon exiting the nest (Figure 14).
I terminated observations at this time and recovered the three
hatchlings. Four other turtles were discovered in the nest at

this time. Total elapsed time was 40 minutes. Hatchlings

No. 140

moved steadily except just before course changes, at which
time they stopped and elevated their heads as though looking
for clues as to what to do next.

DISCUSSION

Nesting season is apparently quite variable for T. scripta.
Cagle (1950) suggested that Illinois 7: s. elegans nested from
the end of April until 17 July, whereas in Louisiana the same
subspecies nested between 20 April and the last week of July.
Cahn (1937) gave from mid-June to the end of the first week
in July as the nesting season in Illinois, whereas Smith (1961)
listed June and July as the nesting season for this region.
Webb (1961) found enlarged ovarian follicles from 19 May to
21 July in T: s. elegans in Oklahoma. Mitchell (1994) found
T. s. scripta nesting between 18 May and 25 June in Virginia,
whereas Gibbons and Greene (1990) reported that 7: s. scripta
nested in April-July in South Carolina. Palmer and Braswell

| N

Nest 2

Hatchling 2
Hatchling 1
a
|
Stump Lake
Oe iy
HS i = |
Hatchling 3
Tucker
Legend ry ine
a — t meter

Figure 14. Detailed map (see Figure 4) showing approximate paths
followed by three hatchlings emerging from the second nest found on

20 April 1995 while it was under observation.

January 1997

(1995) observed nesting among 7. s. scripta between | May
and 29 June in North Carolina

Although the length of the nesting season may vary from
year to year, the time of day that females nested showed little
variation in my study. With few exceptions, all females
nested prior to 1330 h. My observations are consistent with
Cagle’s (1937) that nesting females avoid the “heat of the
day." Cagle (1937) also noted that T. s. scripta prefers to lay
its eggs in the morning and found only three turtles nesting
during midday. However, he noted that it would also nest in
the late evening as did Mitchell (1994) for 7. s. scripta. I did
not observe late evening nesting at my study sites. Although I
did not visit the Illinois sites that I studied each evening, I
found no nesting turtles at the Stump Lake site in 12 visits
made to the site after 1800 h during 1995.

Nests were identical to the descriptions published by Cagle
(1937, 1950), as all had flask-shaped chambers below a
narrower neck and opening through which the eggs had been
deposited. Completed nests had a damp soil plug as noted by
Cagle (1937, 1950). My observations also agree with those of
Cagle (1937, 1950) in that the interior of the nest cavity
contained very little soil mixed with the eggs.

The orientation of hatchlings in nests during emergence
was similar to that of overwintering hatchlings of C. picta
(Breitenbach et al. 1984). The hatchlings that I found
emerging from nests varied in retention of the caruncle. Two
of seven turtles recovered from one nest retained the caruncle,
whereas seven of eight from the other nest retained it. Some
observers use the presence of a caruncle to identify juvenile
aquatic turtles found in terrestrial habitats as hatchlings
(reviewed by Ernst et al. 1994). Since turtles such as T.
scripta actually hatch the previous fall and overwinter in the
nest, it might be more correct to refer to them as recently
emerged turtles rather than as hatchling turtles. Regardless of
the terminology adopted, it is noteworthy that only 9 of 15
turtles known to have just emerged from the nest after
overwintering actually retained the caruncle. The absence of
a caruncle in a small aquatic turtle without evidence of growth
found in a terrestrial habitat does not mean that the turtle is
not a hatchling.

Along with the considerable life history information
reported above, an important conclusion of my study is that
female nesting activity was correlated with temperature
variation at least in 1995. The correlation with temperature,
which was only statistically significant in | of the 2 years of

the study, could be completely spurious. It could be the result

Tucker: Natural History Notes on the Red-eared Slider Turtle

of the generally rising temperatures characteristic of May and
June at this latitude, coinciding with increased turtle activity
due to some other environmental variable such as photope-
riod. Nonetheless, Sexton (1959) found movements of the
painted turtle (Chrysemys picta marginata) strongly associ-
ated with temperature.

However, reliance on environmental cues to trigger nesting
may be advantageous if it synchronizes nesting. The apparent
result at my study site was that many females nested on the
same days (i.e., 25 May and | June of 1994 and | June of
1995, Figures 5 and 6), reminiscent of well-known arribada
nesting among some sea turtles (reviewed by Pritchard 1969).
Eckrich and Owens (1995) showed that first-night mammalian
predation was significantly higher for solitary nests as
compared to arribada nests for the olive Ridley sea turtle
(Lepidochelys olivacea). Consequently, nesting synchrony in
T. s. elegans might also serve to reduce first-night predation
on nests. If so, natural selection would favor responses to
environmental cues that fostered nesting synchrony.

The bimodality apparent in the distribution of turtles
caught in 1995 coincided with passages of cold fronts that
produced rain and lowered temperatures (1.e., Figure 6, 8-14
June). Bimodality might also be explained by invoking the
number of clutches produced per year (= clutch frequency) as
a causative agent. In fact, Iverson and Smith (1993) sug-
gested that three nesting peaks they observed for C. picta in
Nebraska reflected clutch frequency at their study site. They
did not, however, compare their data on nesting females to
daily temperature variation.

Mean clutch frequency for 7: s. elegans in Illinois is
between 2 and 2.7 clutches per year (Cagle 1950; Thornhill
1982: Tucker unpublished). Production of about two clutches
per year may be fairly widespread in this species considering
that Mitchell and Pague (1990) found that 71% of Virginia
turtles produced two clutches. If clutch frequency accounts
for uneven distribution of females through the nesting season,
then for 7. scripta in Illinois, nesting activity should be
bimodal reflecting the underlying clutch frequency of about
two in this species.

Unfortunately, it is difficult to decide which of the
hypotheses accounts for the patterns that I report and those
reported by Iverson and Smith (1993), because daily variation
of environmental cues in relation to turtle nesting behavior
has received no previous attention. It may be that both
selection for nesting synchrony and the underlying clutch
frequency play a role in the nesting patterns found in

freshwater turtles.

10 Illinois Natural History Survey Biological Notes No. 140

Another important finding is the near absence of overlap
between the nesting season for females and the time during
which hatchlings were found emerging (Figure 15). Since the
drift fence was on site until 28 May, it is likely that few if any
hatchlings emerged after 19 May when the last of them were
caught. Prior to 28 May, a single female nested. It may be
that the drift fence discouraged nesting at the Stump Lake
site. However, nesting behavior was not observed at the other
sites, and the turtle nesting on the 14th did so at the Stump
Lake nesting area.

70 50
(Number of hatchlings
60 | @ Number of females
40
% 50
&
LS
2 30
= 40
Hes
xe)
2 4 20
S
=
Zz 20
10
10
0
| 7 We) WS) 2a sal & Wi ae 2s

May June

Figure 15. Number of hatchlings and number of females found in
1995 showing possible segregation of hatchling emergence and

female nesting.

Timing of hatchling emergence so that it does not coincide
with the nesting season may not be accidental. It could
benefit both mothers and hatchlings. If hatchlings emerged
during the peak of the nesting season, they could be more
easily found and eaten by predators attracted to the nesting
area by the presence of freshly laid eggs. Because little is
known about the timing of hatchling emergence and timing of
nesting beyond generalities, it is unclear whether emergence
and nesting periods differ for other species whose hatchlings
overwinter in the nest. However, DePari (1996) found that
painted turtles (C. picta) in northern New Jersey spent an

average of 314 days in the nest from oviposition to emer-

SO|EWA] JO JOQUINN

gence. Thus, hatchlings emerged prior to the next year’s
nesting season in this species as well.

The possibility that the observations I made in 1995 for T-
scripta are atypical cannot be ignored. However, between 6
and 8 May 1993, I collected 27 recently emerged turtles on
the road to the Natural History Survey field station office
when it was located adjacent to Ellis Bay in St. Charles
County, Missouri (Tucker et al. 1995). At that time I traveled
this road daily. Even so, I observed no further activity by
recently emerged turtles on the road after 8 May. The first
nesting female was found crossing the road on 22 May of that
year. Unfortunately, the Mississippi River also crossed the
road shortly thereafter, terminating observations for 1993.
The 1993 observations, though incomplete, suggest that the
temporal segregation of hatchling emergence and female
nesting found in 1995 is not an isolated occurrence.

If hatchlings do benefit from emergence at times when
females are not nesting, then some sort of environmental cues
detectable by hatchlings in the subterranean nest must be
important in the timing of emergence. Even though a large
number of hatchlings were captured during the study, the
environmental cues used by them to time their emergence
remain unclear. DePari (1996) associated emergence in the
painted turtle (C. picta), a species that overwinters in the nest
similar to T. scripta, with softening of the nest plug by rainfall
along with increasing hatchling activity due to increased soil
temperatures in the spring.

Soil temperature is known to have a profound effect on
turtle hatchlings and salamanders. In sea turtles where
nocturnal emergence of hatchlings is the norm, hatchling
emergence is related to the diurnal fluctuations of soil
temperatures (Witherington et al. 1990; Hayes et al. 1992:
Gyuris 1993, for examples and reviews of older literature).
Previously, Sexton et al. (1990) showed that springtime
reversal of soil temperature gradients similar to those that I
found predicted emergence of a salamander better than other
environmental parameters. More specifically, Bleakney
(1963) suggested that soil temperature gradients in the fall
and winter (i.e., October through March) encouraged
hatchlings to remain in the nest cavities because soil above the
nest cavity was relatively colder than soil near the bottom of
the nest cavity. During April and May when soil temperature
gradients reverse, hatchlings emerge in response to this
reversal according to Bleakney’s (1963) hypothesis.

If turtles continually attempt to move towards areas of

highest temperatures within the nest cavity, such movements

January 1997

would, of necessity, result in turtles gradually approaching the
soil surface once the temperatures in the upper layer of the
soil become greater than temperatures deeper in the soil
(DePari 1996). Hatchling emergence among the T. scripta
that I studied in 1995 commenced after upper soil layers
warmed compared to lower ones, consistent with Bleakney’s
hypothesis and DePari’s findings. However, hatchling
emergence for species that normally overwinter in the nest
cannot be completely ascribed to the hatchlings’ responses to
rainfall and temperature gradients. If so, hatchlings would
emerge during wet warm periods during the fall and winter,
which they do not generally do in species such as T. scripta
and C. picta. Thus, hatchling emergence also must have an
unexplored developmental component in species whose
hatchlings overwinter in the nest. Studies of soil temperature,
their diurnal fluctuations, and nature of soil temperature
gradients in relation to hatchling activity within the nest
chamber are needed.

SUMMARY

The results of this study suggest that environmental cues,
such as temperature and rainfall, are important in the timing
of nesting in 7. s. elegans. Nesting synchrony in response to
detectable environmental cues could reduce the likelihood of
nest predation by swamping predators. Temporal segregation
of hatchling emergence and female nesting found in this study
have not been reported previously for other turtles whose
hatchlings overwinter in the nest. A new hypothesis states
that natural selection would favor such segregation due to
decreased hatchling predation if hatchling emergence occurs
at times when nesting does not. Except for the observations
presented herein, this hypothesis is untested. Hatchling
emergence was not correlated with any environmental
parameter measured. However, soil temperature and the
reversal in soil temperature gradient from winter to spring
may be important. I also found that recently emerged
hatchlings may or may not retain the caruncle, which
complicates the use of the presence or absence of a caruncle
as an indicator of hatchling age. Although a great deal of
effort was expended during the 2 years that this study
spanned, many questions about nesting and hatchling

emergence remain to be answered.

Tucker: Natural History Notes on the Red-eared Slider Turtle 11

ACKNOWLEDGMENTS

I thank James Hatcher, James B. Camerer, Charles H.
Theiling, and Moynell M. Tucker for assistance in the field.
James Hatcher took the photographs used for Figures | and 2.
Charles H. Theiling took the photograph used in Figure 8.
Neil Booth and Kim Postlewait (Illinois Department of
Natural Resources) at the Mississippi River State Fish and
Wildlife Area in Rosedale, Illinois, allowed free access to the
Stump Lake nesting area. I greatly appreciate their allowing
the placement of the drift fence at the site even though it
complicated their management duties. Kim Postlewait
collected weather data for that site. Karen L. Drews (U.S.
Fish and Wildlife Service) allowed access to drainage ditches
and levees near Swan Lake. Ron Fisher (U.S. Fish and
Wildlife Service) provided weather data for the Swan Lake
area, which is part of the Mark Twain National Wildlife
Refuge-Brussels District. Ken Hagen allowed me unlimited
access to his apple orchards. Robert Myer permitted access to
his property near Swan Lake as well. Fredric J. Janzen, Gary
L. Paukstis, Chris Phillips, and Don Moll read the manuscript,
and I appreciate their criticisms. I particularly appreciate
Chris Phillips’ suggestions and discussions on the importance
of soil temperature gradients to springtime emergence in

salamanders.

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-=—a6 =")

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Tucker; Natural History Notes on the Red-eared Slider Turtle 13

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Illinois Natural History Survey
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