AMER. ZOOL., 27:59-68 (1987)
Cardiovascular and Other Physiological Correlates of
Hibernation in Aquatic and Terrestrial Turtles'
ROBERT E. GATTEN, JR.
Department of Biology, The University of North Carolina at Greensboro,
Greensboro, North Carolina 27412
SYNOPSIS. Recent research has provided new insight into the physiology of hibernation
in freshwater, marine, and terrestrial turtles. In this paper I review what is known about
the mechanisms that permit the hearts of these turtles to withstand several months of
hypoxia or anoxia. I also report new research that indicates that a terrestrial turtle, unlike
freshwater and marine species, does not experience hypoxia in its winter burrow and thus
does not rely on glycolysis to supply ATP, at least under moderate winter conditions.
INTRODUCTION
Reptiles respond in a number of ways to
seasonally cold temperatures. Some species
remain active at the reduced winter temperatures. Other species retreat to underground or underwater sites and begin a
period of dormancy. The use of the term
"hibernation" to describe the behavioral
and physiological events accompanying
such winter dormancy in reptiles has been
questioned (Mayhew, 1965; Whittow, 1973;
Hutchison, 1979); however, it is now clear
that many winter-dormant reptiles undergo
profound physiological changes that extend
far beyond simple Q, o effects (Gregory,
1982). Because these alterations in many
ways parallel the changes in winter-dormant mammals, the term "hibernation" is
clearly applicable to reptiles as well as to
mammals (Gregory, 1982). In this paper,
I will review what is known about the cardiovascular changes that accompany hibernation in turtles. As will become clear, most
of our knowledge about the physiology of
turtles in winter has been derived from
studies of freshwater species. I will also
describe recent work I have done on the
physiology of hibernation in a terrestrial
species, Terrapene Carolina.
FRESHWATER TURTLES
Tolerance of hypoxia and anoxia
Freshwater turtles have an extraordinary capacity to survive forcible submer1
From the Symposium on Cardiovascular Adaptation
in Reptiles presented at the Annual Meeting of the
American Society of Zoologists, 27-30 December,
1984, at Denver, Colorado.
59
gence, in part because of their ability to
exchange respiratory gases directly with the
water (see Seymour [1982] for review).
However, the fact that freshwater turtles
survive long periods in 100% nitrogen or
after aerobic metabolism is blocked by the
injection of sodium cyanide clearly indicates that it is largely anaerobic metabolism (mainly glycolysis) that permits survival during prolonged submergence (see
Bennett and Dawson [1976] for review).
The great anaerobic capacity of freshwater
turtles (Belkin, 1963) has typically been
considered to be an adaptation for routine
diving. However, recent measurements
have indicated that undisturbed freshwater turtles diving and surfacing voluntarily
at moderate temperatures remain aerobic,
do not rely on glycolysis, and do not accumulate lactate (Gatten, 1981, 1984; Stockard and Gatten, 1983). If freshwater turtles do not utilize their extraordinary
anaerobic capacity during routine diving,
when do they use it? Recent work indicates
that this system is in fact employed during
aquatic hibernation when freshwater turtles may spend several months buried in
the mud of ice-covered lakes (Gatten, 1981;
Jackson and Ultsch, 1982; Ultsch and Jackson, 1982a, b; Jackson, 1987). This realization necessitates a re-evaluation of previous work that was conducted on diving
and/or anoxic turtles at moderate temperatures (typically 20-25°C). It remains
to be seen whether a response elicited from
a turtle forcibly submerged or exposed to
100% nitrogen at 25°C will occur to the
same degree (or even at all) in the same
species during voluntary underwater
hibernation at 1-5°C. It is important to
60
ROBERT E. GATTEN, JR.
note that although many aquatic turtles
overwinter beneath the mud of ponds and
lakes, these animals are not necessarily
confined to the mud for the duration of
winter; there are numerous records of turtles swimming beneath the ice and of animals emerging (perhaps to enhance aerobic metabolism) on warm winter days
(Cagle, 1950; Ernst, 1971; Ernst and Barbour, 1972; see also Ultsch and Jackson,
19826).
almost no measurements of cardiac function at the temperatures normally encountered during hibernation. The description
that follows must be viewed in that light.
When intact freshwater turtles or their
isolated hearts are switched from a welloxygenated medium to an anoxic one, the
hearts exhibit no change in their work output; they continue to function anaerobically (and generate lactate) by first utilizing
their very high stores of glycogen and then
by extracting glucose from the blood
The cardiovascular system and
(Reeves, 1963a, b\ Daw et al., 1967; Beall
aquatic hibernation
and Privitera, 1973; Clark and Miller, 1973;
The cardiovascular system plays a piv- Penney and Shemerdiak, 1973; Penney,
otal role in the survival of freshwater tur- 1974). Isolated perfused hearts continue
tles during hibernation. Circulation is crit- beating for up to 25 hr at 22°C (Penney
ical if the brain is to retain its function and Shemerdiak, 1973). In contrast to these
during anoxia (Belkin, 1968). As would be results with intact but isolated hearts, strips
expected, low temperature lowers the heart of ventricular muscle exposed to an anoxic
rate of freshwater turtles (Gatten, 1974). medium show a 57% decline in their work
Long term exposure to cold conditions output in one hour; exogenous glucose is
lowers the heart rate even further, at least not important for their continued perforin snakes (Aleksiuk, 1970; Jacobson and mance (Bing et al., 1972). The hearts of
Whitford, 1970). In hibernating painted freshwater turtles hibernating in nature
turtles at 3°C, the heart rate fell to as low exhibit a conservation of their glycogen
as 8 beats hr"' (Jackson and Ultsch, 1982). supply at the expense of liver depots (CrawCentral vascular shunting of blood, which ford, 1984). Regardless of the source of
presumably helps to meter out scarce sup- glycogen or glucose, however, glycolysis is
plies of oxygen during voluntary and forced vital to the function of the anoxic ventricle:
dives at moderate temperatures (see Sey- addition of the glycolytic poison, iodoacemour [1982] for review), may be of little tic acid, to the medium causes tension to
value to a turtle hibernating in the mud fall rapidly to zero (Bing et al., 1972). Isounder anoxic conditions. Declining tem- lated strips of turtle ventricle are much
peratures result in a decrease in the prod- more capable of maintaining tension than
uct of stroke volume and arterial-venous are strips from the ventricles of other veroxygen difference, the "oxygen pulse" tebrates when they are subjected to aci(Gatten, 1974); presumably it would dosis produced by high levels of CO2 (Gesapproach zero in a turtle hibernating under ser and Poupa, 1983). The force generated
by turtle atria is reduced by acidosis caused
anoxic conditions.
by lactic acid or by CO 2 , but this effect is
The hearts of vertebrates are typically moderated by calcium ions which are
described as highly aerobic organs, intol- released in hibernating turtles (Yee and
erant of interruption in the supply of oxy- Jackson, 1984). As anaerobiosis proceeds,
gen. The hearts of freshwater turtles, how- the oxidative capacity of turtle heart, as
ever, have a number of properties that measured by cytochrome oxidase activity,
permit continued functioning in hypoxia declines significantly (Simon and Robin,
and anoxia. It is important to note that 1970). This last observation is consistent
almost all of the studies of the physiology with recent results found by Crawford
and biochemistry of hypoxic and anoxic (1984). He studied turtles hibernating in
turtle hearts have been conducted under their natural environment. Early in the
the assumption that they encounter these winter, when oxygen levels in the water
conditions during voluntary diving at mod- may still be moderate, the turtles utilize fat
erate (>20 c C) temperatures. There are
PHYSIOLOGY OF HIBERNATION IN TURTLES
61
as an energy source; as winter progresses optimum and is strongly inhibited by ATP
and oxygen and fat supplies decline, gly- (Storey and Hochachka, 1974*). Thus, this
enzyme functions optimally under anaercolysis becomes more pronounced.
obic conditions when lactate level is high
Cardiac biochemistry
and ATP level is relatively low. Also unlike
The hearts of freshwater turtles are well the case in mammals, turtle heart PK activequipped to withstand anoxia for short ity is strongly enhanced, both directly and
periods because of their stores of ATP and indirectly, by fructose-1,6-diphosphate
phosphocreatine (PC); levels of these phos- (Storey and Hochachka, 19746). This
phagens in the heart significantly exceed metabolite increases the apparent affinity
those in brain and liver (Clark and Miller, of PK for its substrate (phosphoenolpyru1973). The cardiac ATP concentration is vate) and reverses the inhibition of PK by
well maintained during the first hour of, ATP (Storey and Hochachka, 19746). It is
anoxia at 22°C but its level and that of PC clear that the intermediate, fructose-1,6fall to near zero after 15 hr (Clark and diphosphate, by regulating both PK and
Miller, 1973; Penney and Shemerdiak, PFK, is vital in determining the rate of
1973). Subsequent maintenance of this very glycolysis in turtle heart. Finally, turtle
low level of ATP depends upon glycolysis. heart PK is unusual in that it is inhibited
Glycolysis begins with the enzyme gly- by alanine and that its rate of synthesis
cogen phosphorylase which breaks down apparently declines during anoxia (Korglycogen to yield glucose-1-phosphate. necki-Gerrity and Penney, 1974; Simon et
Although turtle hearts have a lower con- al., 1979).
centration of glycogen phosphorylase than
The activity of lactate dehydrogenase
mammalian hearts, turtle hearts have a (LDH) is vital during anaerobic glycolysis
percentage of active enzyme (phosphory- (Lehninger, 1982). Turtle heart LDH has
lase a) that is 8 times that in mammalian a high proportion of M subunits, produchearts (McNeill et al, 1971). This high per- ing a high optimal substrate concentration
centage of active enzyme presumably facil- (about twice that of rat heart) and a low
itates the rapid initiation of glycolysis dur- sensitivity to inhibition by pyruvate; these
ing hypoxia.
features promote the synthesis of lactate
Phosphofructokinase (PFK) is a major under anaerobic conditions (Miller and
regulatory enzyme in glycolysis in mam- Hale, 1968; Altman and Robin, 1969; Beall
mals (Lehninger, 1982). Reeves (1966) and Privitera, 1973).
suggested it was a likely regulatory enzyme
Exposure to cold conditions may or may
in turtle hearts as well. Unlike the case in not alter the aerobic and anaerobic capamammals, turtle heart PFK is not inhibited bilities of freshwater turtle hearts. When
by ATP; instead the regulation of this freshwater turtles were kept in 3°C water
enzyme is assumed by PC (Storey and for four weeks, either with or without access
Hochachka, 1974a; but see Jensen, 1981). to atmospheric air, there was no change in
Under anaerobic conditions, the fall in PC the affinity of cardiac LDH for its substrate
(Clark and Miller, 1973) causes a rise in the and no change in the LDH isozyme pattern
activity of PFK, so that glycolysis proceeds. from that seen in animals kept at 25°C (Beall
There is strong potentiation of this effect and Privitera, 1973). Thus, adjustment to
by the product, fructose- 1,6-diphosphate winter submergence apparently does not
(Storey and Hochachka, 1974a). The rise require the synthesis of a different LDH
in cardiac levels of ADP during anaero- isozyme. The rates of oxygen uptake and
biosis may also stimulate PFK (Penney and oxidative phosphorylation by turtle heart
Shemerdiak, 1973; Lobes and Penney, mitochondria show strong seasonal cycles
1974).
(Privitera and Mersmann, 1966). Cold
Another important regulatory enzyme exposure alters the characteristics of turtle
in the glycolytic pathway is pyruvate kinase heart mitochondrial enzymes to the extent
(PK) (Lehninger, 1982). Unlike the case in that they show reduced activities, lowered
mammals, turtle heart PK has a low pH rates of oxygen usage, and enhanced effi-
62
ROBERT E. GATTEN, JR.
ciency of phosphorylation, as judged from
P / O ratios (Kane and Privitera, 1970;
Rotermund and Privitera, 1972). Presumably such changes facilitate oxidative processes when the turtles periodically emerge
from the mud and when they finally have
access to atmospheric oxygen in the early
spring. Seasonal changes in glycolytic,
aerobic, and /3-oxidative enzymatic capacities facilitate anaerobiosis in turtle hearts
during winter (Olson, 1984).
amino acid catabolism (with the buildup of
alanine and succinate) to provide ATP
(Hochachka et al, 1975).
TERRESTRIAL TURTLES
Much less is known about the physiology
of hibernation in terrestrial than in aquatic
turtles. These animals overwinter in a variety of hibernacula ranging from self-constructed burrows in soil and leaf litter to
muskrat houses (Gregory, 1982). Whether
or not they encounter hypoxia in their
MARINE TURTLES
hibernacula is unknown. If so, survival
Sea turtles have traditionally been would be enhanced by the strong depresregarded as less tolerant of anoxia than are sion of aerobic metabolism by hypoxia
freshwater and terrestrial turtles (Belkin, (Altland and Parker, 1955) and by low tem1963). Thus, the findings that some marine perature (Q10 for standard metabolism from
turtles overwinter while buried in mud 10 to 15°C in Terrapene ornata is 54; Gatten,
(Felgeretal., 1976; Carre* a/., 1980)comes 1974). Acclimation of snakes, Thamnophis
as something of a surprise. There are no sirtalis, to winter conditions existing in
studies of the physiology of marine turtles hibernacula (4-5°C, constant darkness)
in winter at the low temperatures experi- results in a depression of in vitro heart rate
enced during hibernation, but there is evi- and of the metabolic rate of heart homogdence of their tolerance to hypoxia and enates, especially at low measurement temanoxia at warmer temperatures (Berkson, peratures (Aleksiuk, 1970; Hoskins and
1966; Bentley and Lutz, 1979). Further Aleksiuk, 1973). Aleksiuk (1976) specuwork on these animals at winter tempera- lates that the depression of both heart rate
and metabolic rate of the heart during
tures is clearly needed.
Loggerhead {Caretta caretta) heart mus- hibernation lead to a reduction in blood
cle contains three of the five possible iso- flow through the periphery and thus a
zymes of LDH; 29% of the subunits are depression in the energy metabolism of the
M-LDH, the subunit that confers the peripheral tissues with a consequent reduccapacity to function under anaerobic con- tion in overall metabolic rate and in the
ditions (Baldwin and Gyuris, 1983). Unlike depletion of energy stores.
the heart muscle LDH of freshwater turtles, the loggerhead heart LDH composed Hibernation of Terrapene Carolina
entirely of H subunits (maximally active
It is clear from the above discussion that
under aerobic conditions) is sensitive to freshwater turtles rely on glycolysis to a very
inhibition by pyruvate, especially at the significant extent as they overwinter
lower temperature expected during hiber- underwater. Because terrestrial turtles are
nation (Baldwin and Gyuris, 1983). Thus, apparently equally as tolerant of anoxia as
this enzyme at least does not seem well freshwater species (Belkin, 1963), it seems
suited to function under the anoxic con- logical to ask if they might employ their
ditions in which these animals hibernate; great glycolytic capacity during terrestrial
whether there is synthesis of more anoxia- hibernation. Therefore, in the research
tolerant isozymes before hibernation begins described below, I test the hypotheses that
is unknown (Baldwin and Gyuris, 1983). hibernating box turtles, Terrapene Carolina,
Whether all sea turtles have similar LDH 1) encounter hypoxic conditions in their
is also unknown. If so, it would be consis- burrows, 2) have a depressed metabolic
tent with the observation that forcibly sub- rate, 3) experience a depletion of blood
merged green turtles {Chelonia mydas) rely oxygen stores, 4) rely on glycolysis and as
not only on glycolysis (with the accumu- a result show elevated plasma and total body
lation of pyruvate and lactate) but also on lactate levels, 5) exhibit hemoconcentra-
PHYSIOLOGY OF HIBERNATION IN TURTLES
tion or dilution as do other hibernating
turtles (Gregory, 1982; Jackson and Ultsch,
1982), and 6) show an elevation of plasma
calcium as do hibernating aquatic turtles
(Jackson and Heisler, 1982; Jackson and
Ultsch, 1982; Jackson et ai, 1984). These
hypotheses were tested by studying hibernating turtles both in the field and in the
laboratory.
Field experiments
63
tion of the resulting suspension was
centrifuged, filtered, and frozen for later
analysis of lactate level. The concentration
of lactate in the plasma and in the whole
body homogenate was determined using
the method of Bergmeyer (1974) and a
spectrophotometer at 340 nm. All analyses
were made in duplicate. Initial tests indicated that the process of drilling the hole
in the plastron before freezing the turtle
did not result in a significant change in
total body lactate level (Mest, P > O.05).
The winter in South Carolina was exceptionally mild: mean transmitter temperature was below 5°C only twice, in midDecember and early January. As a result,
the box turtles tended to burrow to only
shallow depths: the distance from the soil
surface to the top of the transmitters ranged
from 0 to 5 cm (x ± 1 SD = 2.2 ± 1.6 cm,
n = 12). In contrast, box turtles during
very cold winters may burrow as deep as
48 cm (Cahn, 1937). The transmitter data
indicated that the turtles frequently moved
from place to place and rarely were in the
same location for more than a few days (or
weeks, during the coldest weather) (see also
Carpenter, 1957; Dolbeer, 1971). During
warm spells, the turtles moved more than
when it was colder. As the temperature
fell, turtles buried themselves more deeply
(see also Carpenter, 1957; Dolbeer, 1971).
A unique aspect of the hibernacula of turtles in the coldest weather is that there was
a tunnel the approximate diameter and
length of the turtle's neck extending
upward from the turtle through the soil to
the surface. This tunnel presumably permitted diffusion of gases between the
atmosphere and the air space around the
turtle and also presumably permitted the
turtle to breathe atmospheric air by
extending its neck upward to the air-soil
boundary.
Box turtles (n = 12, x = 371 g, SD = 41
g, range = 301-470 g) were collected in
McCormick Co., SC, and equipped on 5
November 1982 with temperature-sensitive radio transmitters (model TT-2U, J.
Stuart Enterprises, Grass Valley, CA)
attached to their carapaces. On the following day, the animals were released in a partially wooded field (85 x 90 m) on the
Savannah River Plant, Aiken Co., SC.
Ambient temperature and turtle location
were determined with a receiver (Telonics
model TR2-E) and interpulse counter
(Telonics model TDP-2) at least once a
week for the duration of the winter. On
days 12,40,61,89, 103, and 117, two turtles were extracted from their hibernacula
and used for further measurements. Using
a battery-powered drill and the frequent
application of 4% Xylocaine, I drilled a
hole in the plastron over the heart. A sample of blood from the left aortic arch was
withdrawn into a heparinized syringe,
which was immediately immersed in ice
water. I measured core temperature near
the heart with a Schultheis mercury thermometer and then immersed the turtle in
liquid nitrogen. Approximately three minutes elapsed from retrieval to immersion
in liquid nitrogen. The blood samples and
the frozen turtles were taken to the Savannah River Ecology Laboratory where the
blood oxygen content and blood oxygen
capacity were determined (Roughton and
Because ambient temperatures were
Scholander, 1943; Grant, 1947). The permild,
the body temperatures of the turtles
cent saturation of the blood with oxygen
never
fell below 6°C (range 6—16°C, x ±
was calculated from these two values. The
1
SD
= 11 ± 3, Fig. 1A). Even in the
3
min
remaining blood was centrifuged for
coldest
weather, when the turtles remained
at 16,500 rpm. A portion of the plasma
underground
continuously for longer
was frozen for later analysis of lactate conperiods
than
in
warmer
weather, blood satcentration. Each frozen turtle was homoguration
with
oxygen
remained
relatively
enized in cold, 1 N perchloric acid. A porhigh (60-100%, x ± 1 SD = 88 ± 14, Fig.
64
ROBERT E. GATTEN, JR.
NOV
DEC
DATE
Fic. 1. Core body temperature, percent saturation
of the blood with oxygen, plasma lactate concentration, and total body lactate concentration of the box
turtles in the field experiments. Each point represents
a single turtle sampled only on the date shown. The
lines connect the mean values on each date.
IB), presumably because of the tunnel and
air space around the animals. As a consequence, the turtles apparently did not rely
upon glycolysis to provide a supply of
energy; plasma and body lactate concentrations remained low (plasma: 43-99 ng
ml-', x ± 1 SD = 62 ± 20, Fig. 1C; body:
57-169 Mg ml" 1 , x = 109 ± 32, Fig. ID).
Total body lactate levels were very low in
comparison with values seen in aquatic turtles during underwater hibernation (Gatten, 1981; Ultsch and Jackson, 1982a).
Laboratory experiments
It seemed possible that the lack of reliance on glycolysis by box turtles hibernating in the field was due to the mild weather
and subsequent shallow depth of burrowing that may have permitted them to
remain aerobic. Therefore, I tested the
responses of box turtles subjected to constant cold in the laboratory. Box turtles
(n = 12, x = 326 g, SD = 109 g, range =
181-581 g) were collected during late summer in Guilford Co., NC, and kept at 25°C
under a 12:12 light: dark cycle centered at
1200 hr. A basking light was on from 0800
to 1600 hr. I fed the turtles three times a
week with fruit, vegetables, and dog food.
They drank twice a week from a stream of
water flowing through their holding tank
for 8 hr. The turtles were fasted for two
weeks, and on 2 November 1983 placed in
individual containers (1 x w x h = 30 x
20 x 35 cm) filled to a depth of 30 cm with
potting soil. The containers were kept in
constant temperature cabinets set at 5°C
with a light that was on from 0800 to 1600
hr. After 2, 16, 31, 44, 64, and 78 days,
two turtles were removed from their containers. Just prior to removing them from
the soil, I took a 10-ml sample of air from
the soil at the level of the turtle with a
slender plastic tube attached to a syringe.
The oxygen concentration of the sample
was measured by passing it through Drierite and Ascarite (to remove water vapor
and CO 2 , respectively) and an Applied
Electrochemistry oxygen analyzer. As in
the field experiments, 1 drilled a hole in
the plastron over the heart with the frequent application of 4% Xylocaine and took
blood samples. The turtles were then frozen in liquid nitrogen and used in determinations of total lactate concentration as
described above. The blood samples were
used in determinations of blood oxygen
content, blood oxygen capacity, percent
saturation with oxygen, and plasma lactate
level (as described above) and in determinations of hematocrit (centrifugation in
capillary tubes for 3 min) and total plasma
calcium concentration (Sigma kit 585-A).
All analyses were carried out in duplicate.
The box turtles hibernating at 5°C in the
laboratory were buried 1 to 16 cm below
the soil surface (x ± 1 SD = 9 ± 6 cm,
n = 12). Gases in the soil around them were
apparently in equilibrium with atmospheric air; the oxygen concentration was
20.92 ± 0.03% (x ± SD) and ranged from
20.87 to 20.95%.
Some hibernating turtles experience
hemoconcentration whereas others exhibit
hemodilution (Gilles-Baillien, 1974). In the
present study, the box turtles hibernating
in the laboratory experienced no change
in hematocrit over the winter (regression
of hematocrit on time, P > 0.05), although
their mean hematocrit value (x ± 1 SD =
36 ± 7%, range 25-50%, Fig. 2A) was
above the values normally found in this
65
PHYSIOLOGY OF HIBERNATION IN TURTLES
species (Wintrobe, 1933; Gaumer and
Goodnight, 1957; Altland and Thompson,
1958; Horton etal., 1972). Dehydration of
the animals was unlikely because the soil
was well saturated with water and the
humidity of the air in the air space around
the turtles was presumably high. Box turtles in nature hibernate only in moist soil
(Carpenter, 1957) and do not dehydrate
during this period (Brisbin, 1972).
The percent saturation of the blood with
oxygen remained high (x ± 1 SD = 71 ±
12%, range 56-92%, Fig. 2B), presumably
because the air around the turtles was in
equilibrium with the atmosphere. Plasma
and body lactate values were relatively low
and constant with the exception of two animals, one sampled on day 31 and another
on day 78 (Fig. 2C, D). These animals had
lactate values similar to those of hibernating aquatic turtles (Gatten, 1981; Ultsch
and Jackson, 1982a). Perhaps the great
anaerobic capacity of these animals is utilized only once every few years when the
winters are sufficiently severe to drive them
deeper into the soil (see Carpenter, 1957).
In aquatic turtles hibernating for several
months in anoxic water at 3°C, total plasma
calcium levels rise to very high values,
either in response to or in compensation
for the large increase in plasma lactate
(Jackson and Ultsch, 1982; Jackson and
Heisler, 1982). Hutton and Goodnight
(1957) reported that box turtles hibernating in the laboratory exhibit a decrease in
plasma calcium level, but Minnich (1982)
found these reported values to be unusually low and of questionable reliability.
Total plasma calcium concentration did not
change during the period of induced hibernation in this study (regression of calcium
level on time, P > 0.05; Fig. 2E), perhaps
because plasma lactate levels did not
increase. The mean value found here (136
fig ml"1 = 3.4 mM) is near the mean value
for terrestrial and freshwater turtles (Minnich, 1982, Table 1).
Acclimation of metabolic rate
Some hibernating reptiles exhibit a
reduction in their rate of oxygen consumption (Gregory, 1982). Therefore, I
tested the possibility that cold temperature
0
10
20
30
40
50
TIME (DAYSl
60
70
80
FIG. 2. Hematocrit, percent saturation of the blood
with oxygen, plasma lactate concentration, total body
lactate concentration, and plasma calcium concentration of the box turtles in the laboratory experiments.
Symbols as in Figure 1.
acclimation depresses the metabolic rate of
box turtles. Specimens (n = 5, x = 287 g,
SD = 92 g, range = 176-402 g) were collected and housed as described above. After
they had been at 25°C for at least one
month, I considered them to be acclimated
to that temperature. They were fasted for
one week and used in determinations of
resting metabolic rate at 5°C. The values
derived from these measurements thus
represent the resting energy use of warm
acclimated turtles measured at a temperature normally encountered only during
winter. Following these measurements, the
turtles were acclimated to cold conditions
for one month by placing them in containers of potting soil in constant temperature cabinets set at 5°C as described above.
After this period of cold acclimation, their
resting metabolic rate at 5°C was again
determined; the values derived in these
measurements thus represent the energy
usage of cold acclimated animals at a normal winter temperature. The metabolic
rates of the turtles were measured using a
66
ROBERT E. GATTEN, JR.
closed-system method (Gatten, 1985). In
order to determine if cold acclimation
caused a change in resting metabolic rate
from that seen in warm acclimated turtles,
I compared the data from the acclimation
groups with a paired /-test. There was no
difference in the metabolic rates of warm
and cold acclimated animals (x ± 1 SD =
0.00105 ± 0.00096 ml O 2 g" 1 h" 1 for
warm, 0.00069 ± 0.00031 ml O 2 g"1 h"1
for cold; P > 0.05). Thus, these box turtles
in winter experienced no reduction in resting oxygen usage, perhaps because an adequate supply of oxygen and energy reserves
were available.
This study has revealed that box turtles
hibernating in their natural environment
during a comparatively mild winter and in
the laboratory under constant cold can
extract sufficient oxygen from the air space
surrounding them to keep their blood fairly
well saturated with oxygen. There is no
acclimation of metabolism. The continued
supply of oxygen to the tissues eliminates
the need to rely on glycolysis for the provision of ATP. Thus, box turtles do not
employ their extensive capacity for glycolysis during hibernation under the conditions employed here. Neither do they show
an increase in plasma calcium or experience hemoconcentration or dilution during this period. Thus all six hypotheses must
be rejected. The great anaerobic capacity
of terrestrial turtles may be utilized during
emergencies when they withdraw into the
shell to escape a predator, during brief
bursts of intense locomotion (Gatten,
1974), and perhaps during cold winters at
northern latitudes when they must retreat
far underground.
CONCLUSIONS
Recent research has provided a number
of new insights into the physiological correlates of hibernation in turtles. However,
much more work is needed before we have
an adequate understanding of the mechanisms that permit the heart, brain and other
organs to survive so long without oxygen.
There is a great need for field studies in
which data are collected on freshwater,
marine, and terrestrial animals in winter
in their normal habitats. Furthermore, lab-
oratory studies of animals under ecologically-relevant conditions should provide
further elucidation of winter physiology.
It seems especially important to examine
animals in the laboratory at temperatures
pertinent to hibernation and only after the
animals have become acclimatized to winter conditions.
ACKNOWLEDGMENTS
It is a pleasure to express my gratitude
to J. Congdon and the other herpetologists
at the Savannah River Ecology Laboratory, especially J. W. Gibbons, R. Semlitsch, S. Morreale, and J. Pechmann, without whose cooperation and enthusiastic
support this research would not have been
possible. Funding for this project came
from the Savannah River Ecology Laboratory (via contract DE-AC09-76SR00819
between the Department of Energy and
the University of Georgia's Institute of
Ecology), the Department of Biology and
the Research Council of the University of
North Carolina at Greensboro, and a travel
contract from Oak Ridge Associated Universities. I am grateful to W. Burggren, J.
Congdon, H. Lillywhite, H. Pough, and an
anonymous reviewer for helpful comments
on an earlier version of the manuscript.
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