Am. Midi. Nat. 153:370-377
Thermal Environment of Overwintering Ornate Box
Turtles, Terrapene ornata ornata, in Iowa
NEIL P. BERNSTEIN
Department of Biologf, Mount Mercy College, Cedar Rapids, Imva 52402
AND
ROBERT W. BLACK
Department of Biology, CortieU CoUege, Mt. Vemon, !own 52BI4
AiisrR.\{rr.—The carapace tempenuiires of oniate box liirtles {TtrruprnF ortiatn nmaUi) were
monitored by affixing tempei-ature datatoggere during two winters, 2001-2002 and 2003-2004.
The dataloggers recorded temperature eveiy 3 h to the nearest 0.5 C. Date of burrowing in the
fall, emergence time in the spring and temperatures 48 h prior to emergence were analyzed
along lAiih lhe entire temperaliire profile of the nirilcs during ihe winter. In 2003-2004 .soil
temperatures from (he surface to 0.75 m depth were also monitored.
Most turtles did not experience freezing temperannes during either winter, but Iwo unties
were below freezing for up Ui 54 consecutive days during winter 2001-2002.
Duriiig2OOI-2OO2, eight of nine turtles began burrowing within 7dof each otiierin ihe fall,
and all nine emerged within 7 d of each other in the spring. During 200,1-2004, fall burrowing
for 16 turtles occurred within 14-21 d, and emergence happened within 15 d. Ovei^wititering
periods varied between 172-201 d, which was intermediate between dates noted in studies to
the north, south and west for this species. During 2003-2004, it w;ts inferred that turtles
burrowed to a minimum of 0,.'j-(),75 m hy comparing turtle temperattucs to those recorded at
different depths in lhe soil, although we recorded a previous deprh of up to 1.67 m.
Emergence onto the surface in the spring did not relate to soil lemperature, and there was
no support for (.Mnergcnte triggered by 48 h of soil temperatures above 7 C (Grobman, 1990).
It issuggesied that spring emergence may be correlated with a more complex set of factors and
that Terrapene ornata omata can withstand prolonged freezing temperatures.
INTRODUCTION
The ornate box turtle (Tenrtperie ornata ornalu) i.s primarily a prainc species, ranging from
Indiana to the Rocky Mountains and from soutlieni South Dakota, Minnesota and
Wisconsin to Texas and Louisiana (Conant and Collins, 1991; Dodd, 2001). Within this
geographic range, especiaUy in Iowa, they are often found associated with sandy soils, which
prestimably facilitates burrowing (Dodd. 2001).
LItzgus et al. (1999) fotind (hat Cljrmmys gultata overwintered in thermally stiible environments: however, Terrapene cimata imiala burrow to avoid wintertime freezing temperatures
(Legler, 1960; DorofFand Keith, 1990). When underground, 7^ o. omatahody temperature.s
approximate soil temperatures (Legler, 1960). Tmapene amata ornata burrow deeper in the
northern porlions of their t-diige (Legler, 1960; Doroff and Keilh. 1990) and have been
reported to burrow progressively deeper into the soil as freezing temperatures descend
(Legler, 1960). Progressive btmowing correlated with decreasing winter temperattires was
Corresponding author e-mail: [email protected]
370
2005
BERNSTFIN & BLACK: ORN.VIE BOX TURTLES
S71
also reported for the three-toed box turtle (7^ Carolina triungui.s) {Carpenter, 19.57). Nevertheless, burrowed box turtles can freeze and die (Carpenter, 1957; Metcalf and Metcalf, 1979).
An increase in soil tcnipei"atiire has been suggested to trigger spring emergence.
Grobman (1990) reported that individuals emerged when subsoil temperatures were
consistently above 7 C and Curdn (1997) reported emergence correlated with an Inversion
of soil temperature that persisted for longer than 48 h. However, Dodd (2001) stated that
spring emergence was probably related to a combination of soil temperature and moisture.
Based on these obser\"atii)ns. we tested two hypotheses regarding the relationship between
temperature and winter burrowing in ornate box turtles:
1) that successfully bun-owing '/irrfl/jmi? oma/aoma/amaintain a position in lhe .soil where
the temperature i.s above freezing; and
2) that spring emergence is temperature sensitive.
To test these hypotheses, we continuously monitored the carapace temperatures of
overwintering T/nraperie omata ornata near the northern limits of the range of this species,
MeasuremenLs were made immediately prior to hurrowing in September, throughout the
\vinter and immediately following emergence in April.
METHODS
Terrapene omata omata were equipped with both radio transmitters (Advanced Telemetry
Systems, Isanti, MN) and temperature dataloggers (Thermochron ibuttons. Dallas
Semiconductors. Maxim Integrated Products, Inc., Sunnyvale, CA) in September 2001 al
tlie Hawkeye Wildlife Aiea of Johnson Co, Iowa. This complex, located along the south side
of the Iowa River, is managed by the Iowa Deparunent of Natural Resources and contains
several sand dunes that are utilized by the turtles for overwintering. Of the 2001 turtle.s,
eight females and one male were recovered after emergence in spring 2002, and of lhe 2003
turtles, len females and six males were recovered in spring 2004. To facilitate identification,
turtles" marginal carapace scutes were notched using a system adapted from Cagle (1939)
and numbers were assigned to each turtle based upon order that they were found since
1994, Both the iransmittei-s and dataloggers were gltied to the carapace with epoxy and
covered with approximately 2 mm of foam under adhesive tape.
The dataloggers were progi^imined to record temperaUires at 3-h intervals. The accuracy
of the dataloggers was tested by Angllletta and Krochmal (2003) under controlled laboratory
and field conditions, and they found them to be accurate to 0.3 ±0.1 C.
The dataloggers were also compared to cloacal temperatures of turtles. Thermocouple
wires were Inserted into the cloacas of two turtles on a tabletop under fluorescent ceiling
lights, and temperatxues were recorded to the nearest 0.01 C (CRIOX Datalogger from
Campbell Scientific Inc.). A separate thermocouple wire was suspended above the turUes to
measure ambient temperature. Both turUes were also equipped with dataloggers as described
above except that the dataloggers were programmed to record eveiy minute for this test.
.After 1 h of acclimation, data were collected for 30 min, and a one-way ANOVA was
performed to compare mean temperatures of both cloacal probes, both carapace dataloggers
and the ambient temperature. The mean, cloacal temperatures from the turtles, the ambient
temperature and tlie carapace dataloggers were significantK different (df—79, F= 16.76, P <
0,0001). The greatest difference in mean temperatures was between the two carapace
dataloggers (1.15 C), but one of the carapace dataloggers, the ambient temperature and both
cloaca! probes were within 0.44 C of each other. Differences between the ambient
temperature, the cloacal probes and the carapace dataloggers were 0.22-0.89 C, indicating
372
THE AMERICAN MIDIAND NATURALIST
153(2)
FIG. 1.—Daily mean (solid line) and maximum and minimum temperatures (dashed line) lor tunles
rtM. 18F. 32F.fiSF,199F, 242F, 487F, 2001-2002. Mean date of burrowing, 4 Oct. Mean date of
euiergcnce, 11 Apr.
thai while stadstjcatly different, the differences were too stnall to be biologically meaningful.
Wliile we did not directly measure body temperature during overwintering, past sttidies of
overwintering Chrysemys picta (Peterson, 1987) indicated that deep body temperature and
temperature on tlie plastron were similar to that of the .surrotmding mediutn.
Tunles were not disturbed during the wiiitLT, and. therefore, the depths of overwintering
turtles were not recorded. However, in 2003-2004, we buried five seLs of dataloggers jttst
below the surface as well as 0.25 m, 0.50 m and 0.75 m below the surface, We also placed
a datalogger on the surface. Only one complete set was not tampered with, leaving records of
one surface temperature, one subsurface temperature and ibree sets of 0.25 m. 0.50 m and
0.75 m depth.s. Tlie.se .soil temperature readings were used to estimate ttirtle depth when
compared to tbe temperatures recorded by their carapace dataloggers.
Because sustained subsurface soil temperatures above 7 C for 48 b has been sitggested as
a factor to stimulate emergence from winter burnjws (Grobman, 1990), mean temperatures 48
h prior to emergence were compared to 7 C by a one-sample ^test (two-tailed significance test).
RESt.it.TS
In 2001-2002, fotir ttirtles overwintered in a sand dtine. Of these, three were exposed to
.sun without shade, and one burrowed into a blackben7 tliicket along the side of a dune. The
remaining turdes overwintered on a sand prairie approximately 0.5 km away. Four of these
were exposed to direct sutilight, and one was in a grove of decidtious trees. In 2003-2004, all
ttirtles ovei^wintered on the edge of a sand dutie in either blackberties or the exposed
prairie edge of sbrubs. and all btirrows were witJiottt shade.
In graphing the winter temperature data, turtles were grouped into two categories: turtles
that experienced no temperattires between 0-0.5 C (7 turtles in 2001-'2002 and all 16 turtles
2003-2004) atid turtles that experienced temperattires well-below freezing for over 24 h (two
turtles in 2001-2002). The daily mean, daily iiuixinitim and daily minimum temperattiies of
turtles in the first category were combined into one graph for each year (Figs. 1,2), while two
turtles that were exposed to subfreezing temperatures for an extended period of time during
2001-2002 were graphed separately (Figs. 3a, b).
In 2001-2002, large daily fluctuations in temperatttre ceased between 2-9 October for
eight of the turUes, which corresponded to the dates when turUes were no longer found
2005
BERNSTEIN & BL/\CK: ORNATE B O X TURTLES
373
Date
FIG. 2.—Daily mean {solid line) and maximum and minimum lemperauires (dashed line) for turtles
during 2003-2004. Mean date of hurrowing. 2 Oct. Mean date of emergence. 8 Apr.
above ground (Figs. 1, 3b). There was little difference (maximum of 5 C) between the daily
maximum and daily minimum temperatures for these turdes from early October until they
emerged in early April, and only one datalogger recorded a temperature of 0 C between 4-6
January 2002 (Fig. I).
Extended subfreezing temperatures were recorded by the dataloggers for two turdes
(355F and 400F) during 2001-2002 (Figs. 3a, b). The October temperature fluctuadons of
400F were similar to those of the other seven turtles, whereas those of turtle 355F were
smaller than fluctuadons in ambient air temperaiiire but far greater than temperature
niittuation.s in the daily readings of tbe odier turtles (Figs. 1, 3a, b). Temperatures for tiirde
355F become less variable on 21 October. However, Turtle 335F was exposed to below
freezing temperatures from 0220-0820 on 17 October (-1.0--6.0 C) as well as freezing or
below freezing temperatures from 25 December to 16 Febmary (minimum - - 8 . 0 C) (Fig.
3a). Turtle 400 F ^vas also exposed to subfreezing temperatures from 26 December to 18
February (minimum = -5.5 C) (Fig. 3b), Both of these turtles wintered within 25 m of each
other on the edge of a sand dune.
In 2001-2002, the period of time spent underground varied from 176-193 d (Table 1).
Mean temperatures 48 h prior to emergence were significantly diflerent than 7 C (Table 1.
mean diff. = 1.90, P < 0.001). We noted a gradual warming of readings prior to emergence
(Figs. 1. 3a, b), wbicb was determined by boih increase in daily temperature flticttmtions
recorded by the dataloggers compared witb visual confirmation of uirde emergence.
Fall burrowing in 2003 was similar to obserx'ed patterns in 2001, but six turtles began
burrowing in late September (Table 2). Emergence from ovei-wintering in April 2004 also
indicated a wider range of dates wbeii compared to 2001 (Tables I, 2), Time spent
underground in 2003-2004 varied bet\veen 166-201 d. and no turtles experienced freezing
temperatures while tuiderground (Fig. 2). Mean tetnperatures 48 h prior to emergence were
significantly different tban 7 C for all but one turtle (Table 2, mean diff. = 1.53, P < 0.001)
and we again noted a gradual warming of readings prior to emergence (Fig. 2). In tbe soil,
an average of 7 C was reached at 0.75 m on 26 March, 0.50 m on 25 Marcb, 0.25 m on
23 March and just below the .surface on 20 March.
In 2002, turtles emerged during some of tbe warmest days in April with daily high air
temperatures from 9-15 April between 14.4-30.0 C, and it rained on 7-8 April 2002, Daily
high temperatures during the same time period in April 2004 were not as variable, ranging
374
T H E AMERICAN MIDLAND
153(2}
-10 ^
Date
Date
F[(;, 3,—(a)Temperature.s recorded every 3 h for turtle 355F, 2001-2002, (b) Temperatures recorded
every 3 h for turtle 400F, 2001-2002
between 17.78-27.22 C, and there was no rainfall (bttp://mesonet.agron.iastate.edu/
climodat/table.htm; http://wwwl.ncdc.noaa.gov.btml: http://www.agriculture.state.ia.us/
climatology.btm).
A minimum depth of overwintering can be inferred by comparing minimum temperatures recorded by the dataloggers on the ttirdes and the dataloggers buried in the soil.
Freezing or below freezing temperatures were only recorded at 0.5 m depdi from 26
Ianuary-5 Febmaiy 2004 and never recorded at 0.75 m depth. Therefore, although we once
recovered a detached transmitter at 1.67 m deptb after an overwintering, it is possible tbat
the 16 turtles during 2003-2004 never burrowed deeper than 0.75 m.
DISCUSSION
Doroff and Keith (1990) reported duradons of overwintering lasting 209-216 d in
Wisconsin for Terrapene omata omat/i. Altbougb Legler (1960) and Metcalf and Metcalf
(1970) did not report precise durations of overwintering in Kansas populadons, dates of
2005
375
BERNSTEIN k BLACK; ORNATE B O X TURTLES
I,—Length of ovenviiitering and Lemperature (C) prior to emergence, 2001-2002
liirtic iHiiiibn
and sc-\
6M
18 F
32 F
65 F
199 F
242 F
355 F
400 F
4S7 F
Dale of
iMiirowi.ig {OCA.)
Date (il
emergence (Apr.)
9
4
9
15
4
4
10
IS
13
9
16
13
2
9
4
3
3
21
Daw
.Mean
..v.-n.inl<-rt-d
[enipenitiire (sD)*
182
193
188
191
i92
188
176
19!
189
7.78
9.81
7.09
9.84
7,97
6.4]
13.16
10.41
7,66
(0.41)
(0.48)
(0,55)
(1,38)
(0.65)
(0.52)
(1.55)
(0.71)
(0.51)
All significantly difierent from 7 C (P < 0.001)
entering and emergence of winter burrows indicated shorter periods of overwintering than
we obsci-vcd (approximately 152-171 and 170 d, respectively). Legler (1960) did note that
some turtles remained underground into May, but he did not record the duration of lime of
overwintering for those turtles. The estimates of the overwintering period in the Nebraska
.Sandhills (Converse Pt al., 2002) were similar to those of otir study. In Texas, T. o. omatawim
found active on the surface in al! months except Januaiy and February with a few active in
November and December {Blair, 1976). Therefore, our data support the concept that length
of time below the surface correlates with latittidinal differences in winter climate.
Converse et nl. (2002) reported a mean depth for Imapfiijf omata ornata of 0.54 m in
Nebraska, which is similar to our estimates based upon comparisons of carapace datiiloggers
iind those dataloggers buried in the soil at different depths. Our e.stimates as well as the
measuremeni from the transmitter that fell off an overwintering turtle are also comparable
TABI.K
2.—Length of overwintering and temperature (C) prior to emrrgence, 2003-2004
Turtle niimlier
iiiid sex
[>ate of
liiiiMiwing
Dale of
fmerjjence (Apr.)
177 F
198 F
24.? F
248 M
253 M
292 F
319 F
345 F
365 M
378 M
414 F
442 F
476 F
.'»51 M
553 F
557 M
22 Sepi.
22 SepL
5 OcL
9OcL
2Ot!t.
12 Oct.
26 Sept.
25 SepL
3 Oct.
4 Oct.
12 Oct.
9 Oct.
21 SepL
5 Oct.
26 SepL
9 Oct.
1
1
16
9
2
1
14
13
15
Days
overwintered
6
2
2
16
s6
* All significLinily diflereiii from 7 C (P < O.OOl) except 292 F
192
192
194
183
184
172
201
201
195
166
177
176
194
194
195
180
Mt-an
U-uiperamre (SD)*
8.22
6,63
10.00
S.21
8.25
7.47
10,09
10,53
9,00
9.72
9.06
8.22
7..53
10.66
8.21
7.66
(0,44)
(1.54)
(0.00)
(0.45)
(1.30)
(1,27)
(0.33)
(1.22)
(0.00)
(0.35)
(.03.S)
(.037)
(0,30)
(O.OH)
(0.11)
(0,24)
376
T H E AMERICAN MIDLAND NATL'RALIST
153(2)
with overwintering depths between 0.5-1.8 m for the species in Wisconsin (Doroff and
Keith. 1990), and itwasassumed that most of the turtles maintained a position underground
that did not expose them to freezing tem peril tures. presumably by burrowing deeper as the
depth of the frostline increased (Legler, 1960; Dodd, 2001).
Winter exposure to freezing temperatures is a suspected cause of some mortality in box
turtles (Carpenter, 1957; Legler, 1960; Dodd, 2001), and we have found partially
decomposed turtles in the spring that possibly froze on the surface. However, few data
exist on the subsurface temperatures experienced by ovenvintering turtles. Doroff and Keith
(1990) recorded a cloacal temperature of -0.3 C in one turde excavated from winter
a burrow at in Wisconsin, biu the other 11 turtles in the study remained above freezing. Our
data demonstrated that a prolonged exposure to freezing temperatures in winter burrows
does not necessarily kill the tiudes. Costanzo et al. (1995) demonstrated freeze tolerance in
hatchling Terrapene (imatn omata, and this tolerance may extend also to adults as
documented in overwintering adult T. c. Carolina (Costanzo and Claussen, 1990; Claussen
etai, 1991).
Carapace temperatures 48 h prior to emergence did not corroborate Grobman's (1990)
finding that subsurface soil temperatures consistently above 7 C stimulated emergence.
Further, all tlie dates of when soil temperatures reached 7 C were before tlie first turtles
emerged on the surface, even when 48 b was added onto the dates. However, similar to that
reported by Crobinann (1990), we observed Terraperw ornnla (?njfi/fl just below the stirface
prior to spring emergence, and tbis bebavior wa.s also noted for T. Carolina (Congdon et al.,
1989). We often noted small boles to the surface just above the turdes" beads in early April.
Tbus, it appeared that the turtles migbt have spent several days in tbis position before
acttially emerging on tbe surface, and this corresponded witb observations by Legler (1960).
Tbe greater variability of ground temperatures recorded by tbe datalogger on turtle 355F
in October and November 2001 is noteworthy. Legler (1960) noted tbat sotne individuals in
Kansas did not bury tbemselves deeply, and coupled witb tbe fact tbat this turtle recorded
tbe lowest temperature dtiring tbe winter, we suspect that it was not deeply buried.
In addition, tbe relatively high emergence temperature in 2002 of turtle 355F is also
noteworthy. Wliile this turtle emerged on ber own witbin 7 d of tbe otber eigbt turtles in
2002, in spring 2000 turtle 355F aud another turde remained underground tbrougb tbe first
week of June, well after otber turtles bad emerged (unpubl. data). We finally dug botb
turtles out of tbe ground, and tbey botb appeared debydrated and weak. However, tbey soon
resumed normal acti\ities. As noted, tbe temperatures recorded by turtle 355F's datalogger
as well as the timing of the beginning oi overwintering were different compared to the otber
turtles during our study, possibly indicating an individual behavioral difference.
Althougb our data did not totally contradict Grobman's (1990) data for emergence
temperatures, tbey showed more variability in soil temperatures aroimd tbe turtles prior to
emergence and suggested that otber factors besides soil temperature migbt infUience
emergence in tbe spring. Wbile all tbe ttirtles emerged within 7 d of eacb other in 2002 and
within the 15 d of eacb otber in 2004, it was rare to find turtles moving above tbe surface in
early April. Turtles found during tbis period were usually totally buried or partially buried in
forms. Given the difFerent weatber conditions between the years, we suspect tbat yearly
variation iti temperature and rain could produce variadon in the absolute time of
emergence and subsequent activity above grotind.
In conclusion, tbe duration of overwintering of Iowa turtles was intermediate to tbose
reported for populations in Wisconsin, Kansas and Texas to tbe south. Most of our turdes
were not exposed to freezing temperatures in their winter burrows, but some survived
exposure to continuous freezing temperatures for up to 54 d. Emergence onto the surface
2005
BERNSTEIN & BMCK: ORNATE BOX TURTLES
377
in the spring did not relate to a specific soil temperature, bul all of the turtles in the .study
emerged within a span of 7 d in 2002 and 15 d in 2004.
Acknmnledgments.—B, Comito, R. Nieuwendaat, R. Richtsmeier and R. Slokes helped in several aspects
ui the work, P. Barieli provided the Campbell Datalogger and hi.s expertise. M. L.annoo and two
anonymous reviewers provided criricisms of earlier drafts, C. A. Rihic provided advice for some of the
statistics and tlie Iowa DNR allowed us to conduct studies at Hawkeye Wildlife Area and work with the
ihreatened species. Financial support wis received from the .Mount Mercy tk>!lege Department of
Biology and Faculty Development Program, the Cornell College Department of Biology, the Associated
Colleges of the Midwest, the Iowa Science Foundation of the Iowa Academy of Science, the Iowa College
Foundation of the R. J. McElroy Trust, the Iowa Wildlife Diversity Program of the Iowa DNR, the Iowa
Natural History Association and R. Rhodes II. We thank them all.
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3 JUNE 2004
ACCEPTED 7 SEPTEMBER 2004