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ORIGINAL PAPER
R. S. Wilson á R. S. James á I. A. Johnston
Thermal acclimation of locomotor performance in tadpoles
and adults of the aquatic frog
Xenopus laevis
Accepted: 28 October 1999
Abstract Among amphibians, the ability to compensate
for the eects of temperature on the locomotor system
by thermal acclimation has only been reported in larvae
of a single species of anuran. All other analyses have
examined predominantly terrestrial adult life stages of
amphibians and found no evidence of thermal acclima-
tory capacity. We examined the ability of both tadpoles
and adults of the fully aquatic amphibian Xenopus laevis
to acclimate their locomotor system to dierent tem-
peratures. Tadpoles were acclimated to either 12 °Cor
30 °C for 4 weeks and their burst swimming perfor-
mance was assessed at four temperatures between 5 °C
and 30 °C. Adult X. laevis were acclimated to either
10 °Cor25°C for 6 weeks and their burst swimming
performance and isolated muscle performance was de-
termined at six temperatures between 5 °C and 30 °C.
Maximum swimming performance of cold-acclimated
X. laevis tadpoles was greater at cool temperatures and
lower at the highest temperature in comparison with the
warm-acclimated animals. At the test temperature of
12 °C, maximum swimming velocity of tadpoles accli-
mated to 12 °C was 38% higher than the 30 °C-accli-
mation group, while at 30 °C, maximum swimming
velocity of the 30 °C-acclimation group was 41% faster
than the 12 °C-acclimation group. Maximum swimming
performance of adul t X. laevis acclimated to 10 °C was
also higher at the lower temperatures than the 25 °C
acclimated animals, but there was no dierence between
the treatment groups at higher temperatures. When
tested at 10 °C, maximum swimming velocity of the
10 °C-acclimation group was 67% faster than the 25 °C
group. Isolated gastrocnemius muscle ®bres from adult
X. laevis acclimated to 10 °C produced higher relative
tetanic tensions and decreased relaxation times at 10 °C
in comparison with animals acclimated to 25 °C. This is
only the second species of amphibian, and the ®rst adult
life stage, reported to have the capac ity to thermally
acclimate locomotor performance.
Introduction
Temperature in¯uences the reaction rates of most bio-
logical processes and can signi®cantly aec t the physio-
logical performance and ®tness of ectotherms (Christian
and Tracy 1981; Kingsolver and Watt 1983; Lynch and
Gabriel 198 7). An acute decrease in body temperature
reduces power output (Johnson and Johnston 1991), re-
laxation rates (Fleming et al. 1990), and the contraction
velocity (Johnston et al. 1985) of muscles and, conse-
quently locomotor performance (Christian and Tracy
1981; Bennett 1990). Short-term responses to a decrease
in temperature include the compression of motor neurone
recruitment, with fast muscles recruited at lower speeds
(Rome et al. 1984). Longer-term acclimatory responses
in ®sh include an increase in maximum speed for a ®xed
swimming period (Fry and Hart 1948) and improved
acceleration and higher maximum velocity during fast
starts (Johnson and Bennett 1995; Beddow et al. 1995).
The physiological mechan isms underlying such acclima-
tory changes in ®sh locomotion often include a major
restructuring of the muscle phenotype, although species
dier in their response (see Johnston 1993 for review).
For carp, changes in myosin heavy chain composition
(Imai et al. 1997), ratio of light to heavy chains (Crock-
ford and Johnston 1990) and myo®brillar ATPase
J Comp Physiol B (2000) 170: 117±124 Ó Springer-Verlag 2000
Communicated by I.D. Hume
R. S. Wilson (&)
Physiological Ecology Laboratory,
Department of Zoology and Entomology,
The University of Queensland, St. Lucia QLD 4072 Australia
e-mail: rwilson@zoology.uq.edu.au,
Fax: +61-7-3365-1655
R. S. James
School of NES, Coventry University,
D Block, Priory Street, Coventry, CV1 5FB, UK
I. A. Johnston
Gatty Marine Laboratory,
School of Environmental and Evolutionary Biology,
University of St. Andrews, St. Andrews,
Fife, KY16 8LB, Scotland, UK
activity (Johnston et al. 1975) are associated with changes
in maximum tension and velocity of the muscle. A char-
acteristic feature of the ®sh acclimation response is the
long time period required (2±3 weeks; Heap et al. 1986).
Earlier studies of the thermal acclimatory capacity of
the amphibian locomotor system, both in terms of whole-
animal and muscle contractile properties (Putnam and
Bennett 1981; Renaud and Stevens 1983; Rome 1983;
Whitehead et al. 1989; Knowles and Weigl 1990) indi-
cated that the locomotor system of the predominantly
terrestrial adult amphibians shows limited phenotypic
plasticity to seasonal temperature change. In the ®rst
study of larval amphibians, Wilson and Franklin (1999)
found tadpoles of the striped marsh frog (Limnodynastes
peronii) could compensate for the rate-limiting eects of
low temperature on locomotor performance by thermal
acclimation. Thus, among the amphibians, the ability to
thermally acclimate locomotor performance may change
with life stage. However, ontogenetic shifts in thermal
physiology of ectotherms are usually associated with
changes in habitat. For example, newly emergent drag-
on¯ies (Libellula pulchella) have a lower lethal tempera-
ture and broader thermal range of ¯ight performance
than adults, which corresponds with dierences in habi-
tat and lower and more variable thoracic body temper-
ature of the younger dragon¯ies (Marden 1995; Marden
et al. 1996). Temple and Johnston (1998) found the
ability to thermally acclimate locomotor performance in
the ®sh Myoxocephalus scorpius was manifest following
the migration from inshore habitats to the deeper and
more thermally stable but seasonally ¯uctuating habitat.
For most amphibians, metamorphosis from the larval to
adult stage parallels a shift from an aquatic to a pre-
dominantly terrestrial lifestyle. Consistent with both this
pattern and data from previous amphibians studies is the
hypothesis that the capacity to thermally acclimate lo-
comotor perfo rmance among amphibians is possessed by
only the fully aquatic life stages.
In this study, we investigated the ability of both
tadpoles and adults of the fully aquatic African clawed
frog, Xenopus lae vis, to acclimate its locomotor system
to cool temperatures. Adult X. laevis usually inhabit
permanent bodies of water, and emergence onto land is
restricted to occasional migrations between adjacent
ponds during heavy rain. The distribution of X. laevis is
restricted to the temperate and sub-tropical regions of
southern Africa, with both the tadpoles and adults often
active over dierent seasons. We predicted that both life
stages of X. laevis would show improvements in loco-
motor and muscle performance following thermal ac-
climation because it is a fully aquatic amphibian that
experiences signi®cant dierences in environmental
temperature between summer and winter.
Materials and methods
African clawed frogs (X. laevis) were obtained from a breeding
colony in the Gatty Marine Laboratory, St. Andrews, Scotland.
The colony was maintained in aquaria kept at 25 °C and fed twice
weekly on beef heart and ®sh pellets.
Thermal acclimation of burst locomotor performance in tadpoles
Four pairs of X. laevis were induced to breed by injecting both
males and females with human chorionic gonadotrophin (HCG).
Female X. laevis were injected with 0.35 ml and males with 0.15 ml
of a 500 U ml
)1
solution of HCG. All fertilised eggs were trans-
ferred to well aerated aquaria and maintained at 25 °C until they
developed into free-swimming tadpoles. After approximately
4 weeks, tadpoles were divided randomly into two temperature-
acclimation groups; 12 °C and 30 °C. Tadpoles from all treatments
were maintained at high densities (15 animals per litre) to ensure
that development rate was slowed. It is well documented that
amphibian larvae kept at high densities have slower growth and
developmental rates (Newman 1987; Tejedo and Reques 1994).
Slow development ensured any dierences in growth rates between
larvae maintained at 12 °C and 30 °C did not result in any signi-
®cant body size dierences between treatment groups after 4 weeks.
Tadpoles were fed vegetable matter and ®sh pellets and exposed to
a 12 h light:12 h dark light regime.
Following an exposure period of at least 4 weeks, 12 individuals
from both acclimation groups were selected for measurement of
their burst swimming performance at four temperatures between
5 °C and 30 °C. The temperature of the tadpoles was changed at a
rate of 5 °Ch
)1
. The burst swimming performance of both ex-
perimental groups was assessed in the order of 20 °C, 12 °C, 30 °C
and 5 °C, and then repeated at 20 °C. As there was no dierence in
swimming performance for any experimental group between the
initial and ®nal testing at 20 °C, it was concluded that the swim-
ming performance of the tadpoles did not change over the course of
the experiment due to fatigue, experience or some other in¯uence
and was not a confounding factor in the analysis. Moreover, a
positive correlation was observed between initial and ®nal measures
of burst swimming performance for tadpoles from each acclimation
group; although this correlation was non-signi®cant, more repli-
cates may have resulted in statistical signi®cance.
Burst swimming performance was measured by videotaping at
least ®ve `startle responses' for each individual at each temperature.
Startle responses were elicited in the tadpoles by touching them
with a ®ne wire on the front of the head. These burst swimming
sequences were videotaped in a 30 cm ´ 30 cm ´ 5 cm clear plastic
aquarium suspended in a temperature-controlled water bath
(0.5 °C). A silhouette of the tadpole was produced by underneath
illumination of the aquarium through frosted glass. Swimming
sequences were videotaped by recording the image on a mirror
suspended at an angle of 45° above the ®lming arena using a video
camera (Panasonic F10, Japan) at 50 frames s
)1
. A U-matic video
recorder (Model CR-6600 E) was used to play back the recorded
sequences frame-by-frame, with the image projected onto a ®lm
motion analyser digitising tablet (NAC, Japan). The digitising
tablet was linked to the MOVIAS (NAC, Japan) ®lm analysis
software package which calculated the X,Y co-ordinates of the
animals every 20 ms. Velocity of the tadpoles was determined by
calculating the distance moved between each frame (i.e. 20 ms).
Each burst swimming sequence was analysed frame-by-frame to
determine the maximum velocity (U
max
), the time taken to reach
maximum velocity (T-U
max
) and the distance travelled during the
initial 200 ms (D
200
). The fastest of the swimming sequences ana-
lysed for each individual at each temperature was used as a measure
of their maximum burst swimming performance.
Thermal acclimation of burst locomotor performance in adults
Thirty-eight adult X. laevis were divided randomly into two tem-
perature-acclimation groups; 10 °C and 25 °C. Frogs were fed
twice weekly on beef heart and exposed to a 12:12 photoperiod.
After an exposure period of at least 6 weeks, the burst swimming
performance or isolated muscle performance of the frogs was as-
sessed at six dierent temperatures between 5 °C and 30 °C. Ten
118
frogs from each acclimation group were used for the swimming
performance experiments, while nine animals from each acclima-
tion group were used in the study of muscle mechanics.
Burst swimming performance
Burst swimming performance of the adults was measured by ®lm-
ing at least ®ve startle responses for each individual at each tem-
perature. Adults were stimulated to swim by tapping them lightly
on the urostyle. Only startle responses that produced swimming
along the bottom of the aquarium from a stationary position were
analysed. Burst swimming performance for both experimental
groups was assessed in the order of 20 °C, 25 °C, 10 °C, 15 °C,
30 °C and 5 °C, and retested again at 20 °C. As with the tadpoles,
there was no dierence in swimming performance for any experi-
mental group between the initial and ®nal testing at 20 °C. Fur-
thermore, a positive (but non-signi®cant) correlation was also
observed between initial and ®nal measures of burst swimming
performance for the adults from each acclimation group. Burst
swimming sequences were ®lmed in a 1.5 m ´ 0.5 m ´ 0.2 m
aquarium with underneath illumination provided by ®ve 75-W
¯uorescent strip lights through frosted glass. Filming and analysis
of adult swimming performance was done by using the same
methods outlined for the tadpoles. The temperature of the aquar-
ium was changed at a rate of 5 °Ch
)1
.
Isolated muscle performance
Frogs were killed by a blow to the head, pithing and transection of
the spinal cord. The gastrocnemius muscle was removed from the
right hind-leg of each frog and a smaller bundle of ®bres (approx.
10% of the original muscle volume) was separated from the main
body of muscle. Dissection was carried out on a cooled microscope
stage (5 °C) in Ringer's solution with the following composition (in
mmol l
)1
): NaCl, 115; KCL, 2.5; Na
2
HPO
4
, 2.15; NaH
2
PO
4
, 0.85;
sodium pyruvate, 5.0; CaCl
2
, 1.8; pH 7.2 at 20 °C. Aluminium foil
clips were attached to the ends of the ®bre bundles and mounted on
stainless steel hooks, with one end attached to a force transducer
(AME 801, SenSonor, Norway; sensitivity of 0.5 mN V
)1
). The
muscle preparation was mounted inside a temperature controlled
Perspex bath with circulating Ringers solution.
The muscle was stimulated via two parallel platinum wire
electrodes placed on each side of the muscle preparation. Stimu-
lation frequency was adjusted to maximise tetanus height (95±
120 Hz), while stimulation amplitude, pulse width and muscle
length were adjusted to produce a maximum twitch. Data were
collected and analysed using Labview software (National Instru-
ments). The order of temperatures was randomised for each muscle
preparation. Tetani were routinely repeated at the initial test tem-
perature during each experiment to monitor variation in each
muscle's ability to produce force over time. Any changes in per-
formance of each muscle throughout the experiment were corrected
for to ensure dierences in force were due to temperature alone. A
7- to 10-min recovery period was allowed between each tetanic
contraction. Maximum twitch (F
tw
) and tetanic force (F
tet
), time to
maximum twitch force (TPT), and the time from last stimulus to
50% tetanic relaxation (RT
1/2
) were all recorded.
Morphological measurements of each tadpole and adult were
recorded. All length measurements were made with Mitutoyo cal-
ipers (Japan; 0.1 mm) and mass was recorded using an analytical
balance (0.1 mg). For tadpoles, snout-vent, tail and total length,
and mass were recorded, while for the adults, snout-to-urostyle
length and mass were measured.
Statistical analyses
The eects of acute temperature change and acclimation eects on
all experimental variables following exposure to dierent temper-
ature regimes were analysed using Multivariate Analysis of Vari-
ance (MANOVA). The MANOVA was followed by a multiple
comparisons test to isolate the speci®c dierences in treatment
eects. Body size and developmental stage data, and Q
10
and R
10
values were compared between the treatment groups using Stu-
dent's t-tests or Mann-Whitney U-tests. Correlative analyses
between initial and ®nal measures of burst locomotor performance
for each individual from both tadpole and adult acclimation groups
were performed using least square regression techniques. As the
mass of each muscle preparation was not recorded, the isolated
muscle performance data were normalised and then arcsine trans-
formed to account for variation in size of muscle bundles used in
experiments. All results are presented as mean SE. Signi®cance
was taken at the level of P<0.05.
Results
Thermal acclimation of burst locomotor performance
in tadpoles
Maximum swimming performance of cold-acclimated
X. laevis tadpoles was greater at cool temperatures and
lower at the highest temperature in comparison with the
warm-acclimated animals (Fig. 1). Tadpoles acclimated
to 30 °C did not exhibit a startle response at 5 °C; thus
Fig. 1 Eect of temperature on A maximum swimming velocity
(U
max
), B time taken to reach maximum swimming velocity (T-U
max
),
and C the distance moved after the initial 200 ms (D
200
), in tadpoles of
the African clawed frog (Xenopus laevis) acclimated to either 12 °Cor
30 °C for a period of 4 weeks (n 12). An asterisk denotes a
statistically signi®cant dierence between acclimation groups at a
particular test temperature. Signi®cance level at P<0.05
119
all comparisons between the two experimental treat-
ments were made at the temperatures of 12 °C, 20 °C
and 30 °C. At the test tem perature of 12 °C, the U
max
of
12 °C-acclimated tadpoles was 38% higher than the
30 °C-acclimation group (P<0.01; Fig. 1A). There
was no dierence in T-U
max
between the two treatment
groups at 12 °C, but D
200
for the 12 °C-acclimation
group (2.81 0.18 cm) was 32% greater than the
30 °C-exposure group (2.13 0.20 cm; P<0.001;
Fig. 1C). At 20 °C, there were no signi®cant dierences
between the 12 °C- and 30 °C-acclimation groups for all
swimming performance parameters (Fig. 1A±C). How-
ever, at the test temperature of 30 °C, U
max
of the 30 °C-
acclimation group (5.96 0.60 m s
)1
) was 41% faster
than the 12 °C-acclim ation group (4.80 0.30 m s
)1
;
P<0.001). Similarly, at 30 °C, the 30 °C group
reached U
max
after 91.1 10.5 ms, while T-U
max
for
the 12 °C group was signi®cantly slower at 137 14 ms
(P<0.01). D
200
for the 30 °C group was
5.96 0.60 cm at 30 °C, which was 24% further than
the 12 °C group (P<0.01).
There was no signi®cant dierence in mass or total
body length between the 12 °C- (8.7 1.2 mg,
26.6 1.4 mm) and 30 °C-acclimation group
(9.1 1.5 mg, 28.0 1.6 mm). Furthermore, there
was no dierence in developmental stage between the
two acclimation groups.
The Q
10
and R
10
values for the swimming performance
parameters between the test temperatures of 12 °C and
30 °C were compared betwe en the acclimation groups
(Table 1). Only the Q
10
s for U
max
were signi®cantly dif-
ferent between the two acclimation groups , with a value
of 1.96 0.16 and 1.30 0.05 for the 30 °C- and
12 °C-acclimation groups, respectively (P<0.01).
Thermal acclimation of the adult locomotor system
The eect of temperature on burst locomotor perfor-
mance and isolated muscle performance was compared
between adult X. laevis acclimated to 10 °C and 25 °C.
Acute changes in tem perature aected all kinematic
variables of whole-animal performance and isolated
muscle performance in animals from both acclimation
groups (MANOVA; P<0.05). Body mass or snout-
vent length did not signi®cantly dier between the 10 °C
group (48.9 3.4 g, 75.9 1.4 mm) and the 25 °C
group (46.8 3.0 g, 76.6 1.3 mm) .
Burst swimming performance
Maximum swimming performance of adult X. laevis
acclimated to 10 °C was higher at the cooler tempera-
tures than the 25 °C acclimated animals (Fig. 2A±C).
However, there was no dierence in swimming perfor-
mance for any measured parameter between the two
acclimation groups when tested between 20 °C and
30 °C (Fig. 2A±C). At the test temperature of 10 °C,
U
max
of the 10 °C-acclimation group (0.98 0.05 m
s
)1
) was 67% faster than the 25 °C group (P < 0.001)
(Fig. 2A). Similarly, the T-U
max
for the 25 °C group was
66% slower than the 10 °C group when tested at 10 °C
(P<0.001; Fig. 2b). D
200
for the 10 °C group was
14.5 0.80 cm, which was 96% further than the ani-
mals exposed to 25 °C for 6 weeks (P<0.01; Fig. 2c).
There was no signi®cant dierence in mass or snout-vent
length between the 10 °C- (26.8 1.9 g, 61.2
1.8 mm) and 25 °C-acclimation group (20.2 3.6 g,
55.5 3.8 mm).
The Q
10
and R
10
values for all kinematic parameters
between the test temperatures of 10 °Cand25°C were
signi®cantly dierent between the acclim ation groups
(Table 1). The Q
10
s for U
max
in the 10 °C- and 25 °C-
acclimation groups were 1.11 0.02 and 1.61 0.04,
respectively (P<0.001).
Isolated muscle performance
Acclimation of X. laevis adults to 10 °C for 6 weeks
increased the performance of several isolated muscle
Fig. 2 EectoftemperatureonA U
max
, B T-U
max
,andC D
200
,in
adult African clawed frogs (X. laevis) acclimated to either 10 °Cor
25 °C for 6 weeks (n 10). An asterisk denotes a statistically
signi®cant dierence between acclimation groups at a particular test
temperature. Signi®cance level at P<0.05
120
parameters at the cooler temperatures relative to the
warm-acclimated animals (Figs. 3±4). Although relative
twitch force produced by the isolated gastrocnemius
muscle did not dier between acclimation groups be-
tween the test temperatures of 5 °C and 25 °C, the
25 °C-acclimation group possessed a signi®cantly higher
F
tw
than the 10 °C group at the highest test temperature
of 30 °C (Fig. 3A; P<0.05). At 10 °C, the F
tet
of
muscle from the 10 °C-acclimated animals was
84.6 1.8% of it s maximum, which was signi®cantly
higher than the 25 °C group with only 77.6 2.3% of
its maximum (P<0.05). Similarly , at a test tempera-
ture of 5 °C, the 10 °C-acclimated animals produced
72.6 3.2% maximum relative twitch force, while the
25 °C group only produced 61.9 2.7% of maximum
(P < 0.05). There was no signi®cant dierence between
the acclimation groups in F
tet
produced by the isolated
muscle when tested at temperatures greater than 10 °C.
The mass of several of the isolated muscle preparations
was recorded to determine the tetanic stress production
of the muscles, which was approximately 250±
275 kN m
)2
at 25 °C.
There was no signi®cant dierence between the two
experimental groups in the eect of temperature on TPT
(Fig. 4A; P>0.05). TPT was greater than 120 ms for
both groups at 5 °C and less than 30 ms when tested at
30 °C. Relaxation rates were greater in the 10 °C group
than the 25 °C-acclimation group at the lower test
temperatures, with RT
1/2
signi®cantly greater for the
25 °C-acclimated animals between the test temperatures
of 5 °Cand15°C(P<0.001; Fig. 4B). There was no
dierence in RT
1/2
between the two acclimation groups
between the temperatures of 20 °Cand30°C.
The Q
10
and R
10
values for the isolated muscle per-
formance parameters between the test temperatures of
10 °C and 25 °C were compared between the 10 °C- and
25 °C-acclimation groups (Table 1). Only the R
10
for
RT
1/2
diered between the acclimation groups, with
Table 1 Q
10
and R
10
values for burst swimming parameters be-
tween the test temperatures of 12 °C and 30 °CinXenopus laevis
tadpoles acclimated to either 12 °Cor30°C for 4 weeks (n = 12),
and values for burst swimming (n = 10) and isolated muscle per-
formance (n = 9) between 10 °C and 25 °C in adult X. laevis
acclimated to either 10 °Cor25°C for 6 weeks. Burst swimming
parameters include maximum swimming velocity (U
max
), time
taken to reach maximum velocity (T-U
max
), and distance moved
after 200 ms (D
200
), while isolated muscle performance parameters
include maximum relative twitch (F
tw
) and tetanic (F
tet
) force, and
the time from last tetanic stimulus to half relaxation (RT
1/2
)
Tadpoles acclimation group Adults acclimation group
12 °C30°C10°C25°C
U
max
1.30 0.05** 1.96 0.16 1.11 0.02* 1.61 0.04
T-U
max
1.02 0.09 0.76 0.10 0.88 0.04* 0.65 0.03
D
200
1.32 0.08 1.86 0.20 1.19 0.06* 1.69 0.1
F
tw
1.11 0.02 1.17 0.02
F
tet
1.04 0.02 1.05 0.03
RT
1/2
0.53 0.01* 0.46 0.02
An asterisk denotes a statistically signi®cant result between accli-
mation groups; *P<0.05, **P<0.01
Fig. 3 EectoftemperatureontheA relative twitch force, and B
relative tetanic force, of isolated gastrocnemius muscle from adult
X. laevis acclimated to either 10 °Cor25°C for 6 weeks (n 9). An
asterisk denotes a statistically signi®cant dierence between acclima-
tion groups at a particular test temperature. Signi®cance level at
P<0.05
Fig. 4 Eect of temperature on A thetimetopeaktwitchforce
(TPT), and B time from last stimulus to 50% tetanic relaxation (RT
1/2
),
of isolated gastrocnemius muscle from adult X. laevis acclimated to
either 10 °Cor25°C for 6 weeks (n 9). An asterisk denotes a
statistically signi®cant dierence between acclimation groups at a
particular test temperature. Signi®cance level at P<0.05
121
values of 0.53 0.03 for the 10 °C group and
0.46 0.03 for the 25 °C-acclimation group
(P<0.05).
Discussion
We found that both tadpoles and adults of the African
clawed frog (X. laevis) possessed the ability to acclimate
their locomotor performance to low temperatures. After
acclimation to 12 °C for 4 weeks, tadpoles of X. laevis
had a U
max
that was 38% greater at 12 °C than tadpoles
acclimated to 30 °C. Furthermore, X. laevis adults ac-
climated to 10 °C for 6 weeks had a U
max
that was 67%
faster at 10 °C than animals acclimated to 25 °C. This is
the ®rst study to report an adult amphibian possessing
the capacity to compensate for low temperatures by
thermal acclimation of locomotor performance. Wilson
and Franklin (1999) reported that tadpoles of the striped
marsh frog (L. peronii) acclimated to 10 °C for 6 weeks
had a U
max
that was 47% greater and a maximum
acceleration 53% greater at 10 °C than tadpoles accli-
mated to 24 °C. Unlike tadpoles of L. peronii, warm-
acclimated X. laevis tadpoles also improved their
performance at higher temperatures in comparison with
the cold-acclimat ed animals.
Improved escape speed following thermal acclimation
re¯ects phenotypic modi®cations of the muscle proper-
ties (see Johnston 1993). Several parameters of muscle
performance were altered following thermal acclimation
in adult X. laevis. Isolated gastrocnemius muscle ®bres
from adult X. laevis acclimated to 10 °C produced
higher relative tetanic tensions and decreased relaxation
times at 10 °C in comparison with animals acclimated to
25 °C. Isometric muscle tension was also aected by
temperature acclimation in carp (Johnston et al. 1985),
gold®sh (Langfeld et al. 1991), and the short-horned
sculpin (Beddow and Johnston 1995). Although muscle
activation rates were unaected by temperature accli-
mation in X. laevis, half-relaxation times were reduced
by approximately 30% at low temperatures after long-
term exposure to 10 °C. Half-relaxation times were also
reduced by thermal acclimation in gold®sh (Langfeld
et al. 1991), but unchanged in the short-horned sculpin
(Beddow and Johnston 1995). Changes in relaxation
rates in gold®sh were associated with an increase in
Ca
2+
-ATPase activity of the sarcoplasmic reticulum
(Fleming et al. 1990). Changes in relaxation rates asso-
ciated with thermal acclimation in adult X. laevis also
probably aected maximum stroke frequency during
burst swimming. However, due to the enormous be-
havioural variation exhibited by adults in the length of
the gliding phase during swimming (following propul-
sive leg extension and before the onset of active leg
¯exion), an accurate measure of maximum stroke fre-
quency could not be obtained.
Although few modi®cations in isolated muscle per-
formance were observed following thermal acclimation
in adult X. laevis, these changes most likely underlie at
least some of the alterations in whole-animal locomotor
performance. Another possible mechanism underlying
modi®cations in swimming performance with cold ac-
climation is a change in the proportion of muscle ®bre
types that dier in their thermal sensitivity, i.e. aerobic
to anaerobic. However, maximum escape speed is pow-
ered by the recruitment of anaerobic ®bre types
(Wakeling and Johnston 1998). Aerobic ®bres with their
slower contraction speeds will not make a signi®cant
contribution to power output under conditions of
maximal eort as the in vivo speed will exceed or at least
be a signi®cant proportion of the Vmax of the red ®bres.
Thus, even if there was a shift in ®bre types this would
not result in a change in the thermal dependence of
maximum locomotor performance or be relevant to the
behaviour measured in this study. A more detailed
analysis of the changes in muscle properties associated
with thermal acclimation in X. laevis would involve an
examination of the changes in the maximum power
output of the hindlimb muscles. Total muscle power
output limits the hydrodynamic power output in ®sh
(Wakeling and Johnston 1998) and is also likely to be
the most critical determinant of maximum swimming
performance in amphibians. Johnston et al. (1995)
measured the changes in average power output per
stimulation cycle in short-horned sculpin (M. scorpius)
muscle following thermal acclimation with the work-
loop technique. At 15 °C, isolated muscle from short-
horned sculpin acclimated to 15 °C for extended periods
produced average power outputs of 23.8 W kg
)1
, while
animals acclimated to 5 °C produced only 6.3 W kg
)1
.
Thermal acclimation of locomotor performance
in amphibians
Clearly, some amphibians possess the ability to accli-
mate their locomotor performance to dierent temper-
atures, but why do some species and not others? As
thermal acclimatory capacity has been recorded in both
larval and adult stages of amphibians (Table 2), it is not
likely to be directly related to ontogenetic dierences.
Similarly, as both the inability and capacity to acclimate
locomotor performance has been recorded in a diverse
array of amphibians (Table 2), it is not likely due to
phylogenetic eects alone. Moreover, it is also unlikely
due to the type of locomotor performance assessed in
each study. Altho ugh several modes of locomotion were
assessed across these studies (i.e. jumping, swimming, or
running), `burst' type locomotor performance was used
in each study and is thus unlikely to underlie dierences
in acclimatory ability. However, an examination of the
relationship between acclimatory capacity of amphibi-
ans and their aquatic or terrestrial habits reveals a clear
division between aquatic life stages that possess the
capacity to acclimate their locomotor performance and
the predominantly terrestrial life stages that do not
(Table 2). Both Renaud and Stevens (1983) and Rome
(1983) found no evidence for thermal acclimation of
122
locomotor performance in the predominantly terrestrial
adult Rana pipiens to low temperatures. Similarly, adult
spotted marsh frogs (L. tasmaniensis) did not exhibit an
acclimatory response in maximum jump distance to
dierent constant-temperature regimes (Whitehead et al.
1989). The thermal dependence of the muscle and lo-
comotor performance in the terrestrial adult stage of the
salamander, Ambystoma tigrinum nebulosum, was also
unchanged by exposure to low temperatures (Else and
Bennett 1987). Furthermore, the locomotor perfor-
mance of the toad Bufo americanus (Putnam and Ben-
nett 1981) and the frogs R. sylvatica and Pseudacris
triseriata (Knowles and Weigl 1990) is unaected by
long-term exposure to dierent temperatures. However,
tadpoles of the frog L. peronii (Wilson an d Franklin
1999), and tadpoles and fully aquatic adults of X. laevis,
possess the ability to acclimate their locomotor system
to cool temperatures. Thus, thermal acclimatory
capacity may be directly related to the aquatic habits of
amphibians. A further test of this hypothesis would be
an examination of the thermal acclimatory capacity of
adults of the striped marsh frog (L. peronii). As adults of
L. peronii are predominantly terrestrial, we would pre-
dict that unlike the tadpoles of this species the adults
would not possess the ability to acclimate their loco-
motor system to dierent temperatures.
Large daily ¯uctuations in temperature are often as-
sociated with reduced thermal acclimatory abilities of
ectotherms (Sidell et al. 1983; Temple and Johnston
1998). Thermal acclimatory ability of amphibians may
also be dependent on whether they come from an envi-
ronment that experiences large daily ¯uctuations in
temperature. Aquatic habitats are generally more ther-
mally buered and stable than terrestrial environments
and thermal acclimation of locomotor performance has
only been found among the aquatic amphibian life
stages. Fish from habitats with large daily ¯uctuations in
temperature possess a reduced, or no thermal acclima-
tory response of their locomotor performance. For ex-
ample, Temple and Johnston (1998) found long-spined
sea scorpions (Taurulus bubalis) and juvenile short-
horned sculpins (M. scorpius), that occur in habitats
with large daily ¯uctuations in temperature, do not
exhibit an acclimatory response of their locomotor
system to dierent temperatures. However, adult
M. scorpius that live in deep thermally stable environ-
ments that vary in temperature only across seasons ex-
hibited an acclimatory response over a temperature
range of 5±15 °C (Beddow et al. 1995; Temple and
Johnston 1998). Similarly, the killi®sh (Fundulus he-
teroclitus) which lives in salt-marsh habitats that expe-
rience rapid daily changes in temperature of up to 14 °C
in an hour (Sidell et al. 1983) exhibit a reduced accli-
matory response of their locomotor performance in
comparison to gold®sh (Carassius auratus) that often
live in more thermally stable environments (Johnson and
Bennett 1995). Although signi®cant shifts occurred in
the expression of the myosin heavy chain (MHC) iso-
forms with thermal acclimation in the gold®sh, MHC
expression in the killi®sh was unaected (Johnson and
Bennett 1995). The killi®sh also did not exhibit any ac-
climation of the myo®brillar ATPase activity between
5 °C and 25 °C (Sidell et al. 1983).
Why ectotherms from environments with large daily
¯uctuations in temperature possess a reduced acclima-
tory capacity has not been directly investigated. One
suggestion is that the acclimatory capacity of ectotherms
from these environments has been reduced, or lost be-
cause these environments do not provide a stable cue for
acclimation (Temple and Johnston 1998). Alternatively,
a thermally variable environment may lead to selection
for thermally independent physiological traits which
may in turn result in a reduced acclimatory ability.
Clearly, if a physiological trait is thermally independent
over the range of temperatures experienced throughout
dierent seasons then there would be no necessity for
acclimation. A correlation between a reduced acclima-
tory capacity and low thermal sensitivity of performance
may be evidence for a trade-o between a thermal gen-
eralist strategy (reduced acclim atory capacity and low
thermal sensitivity) and specialist (readily undergoes
acclimation and highly sensitive to temperature). If this
trade-o exists then it could be predicted that a reduc-
tion in acclimatory capacity would be associated with an
increase in the relative thermal independence of a trait.
Interestingly, killi®sh not only possess a reduced accli-
matory capacity of their locomotor performance in
comparison to gold®sh but their locomotor performance
is more thermally independ ent (Johnston and Bennett
1995). A narrower thermal performance range was also
Table 2 The pattern of thermal acclimatory capacity of locomotor and muscle performance in amphibians and its relationship to life
stage, and aquatic or terrestrial habits
Species Life stage Aquatic or terrestrial Acclimation Study
Ambystoma tigrinum nebulosum Adult Terrestrial No Else and Bennett (1987)
Bufo americanus Adult Terrestrial No Putnam and Bennett (1981)
Pseudacris triseriata Adult Terrestrial No Knowles and Weigl (1990)
Limnodynastes peronii Tadpole Aquatic Yes Wilson and Franklin (1999)
Limnodynastes tasmaniensis Adult Terrestrial No Whitehead et al. (1989)
Rana pipiens Adult Terrestrial No Renaud and Stevens (1983); Rome (1983)
Rana sylvatica Adult Terrestrial No Knowles and Weigl (1990)
Xenopus laevis Tadpole Aquatic Yes Present study
Xenopus laevis Adult Aquatic Yes Present study
123
associated wi th an increase in the magnitude of an acc-
limatory response in tadpoles of the striped marsh frogs
(L. peronii) when acclimated to 10 °C for an extended
period (Wilson and Franklin 1999). However, evidence
for such a trade-o is scant and based on poor corre-
lations rat her than direct experimental evaluation. Fur-
ther study of the role of short-term ¯uctuations in
temperature on the acclimatory capacity of ectotherms
will clearly improve our understanding of the ecological
and evolutionary signi®cance of physiological plasticity
in nature, both in terms of the limitations imposed by
the ¯uctuating environment on the expression of accli-
mation and also the trade-os associated with dierent
strategies.
Acknowledgements RW was a recipient of an Australian Post-
graduate Scholarship. We thank Keith Sillar for providing many of
the animals used in this study and Eleanor Hartis for her generous
assistance with many aspects of Xenopus husbandry. We also thank
Mr. Iain Johnston, G. Temple, J. McDearmid, N. Cole and S. Butt
for their invaluable assistance in the lab.
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