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First published online February 20, 2004
Journal of Experimental Biology 207, 1183-1192 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00870
Locomotor performance of closely related Tropidurus species: relationships with physiological parameters and ecological divergence
1 Departamento de Fisiologia, Instituto de Biociências, Universidade
de São Paulo, Rua do Matão, Travessa 14, No. 321, 05508-900,
São Paulo, SP, Brazil
2 School of Science and the Environment, Coventry University, James
Starley Building, Priory Street, Coventry CV1 5FB, UK
3 Department of Zoology and Entomology, The University of Queensland, St
Lucia, QLD 4072, Australia
4 Departamento de Morfologia, Instituto de Biociências,
Universidade Estadual Paulista, 18618-000, Botucatu, SP, Brazil
* Author for correspondence (e-mail: navas{at}usp.br)
Accepted 9 January 2004
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Summary |
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Key words: locomotion, Tropidurus, muscle physiology, metabolism, enzyme activity, habitat divergence, evolution
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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;
Garland and Losos, 1994).
Because different suites of morphological and physiological characters
increase behavioral performance in specific ecological contexts, one can
expect that natural selection favors traits that improve performance in an
individual's current habitat (Garland and
Losos, 1994). The hypothesis of a relationship between structural
habitat, morphology and locomotor performance is supported by studies with
Anolis lizards (Losos,
1990,
1992;
Losos et al., 1998;
Beutell and Losos, 1999) but is
not unambiguously supported by data from other lizard taxa
(Miles, 1994;
Vanhooydonck and Van Damme,
1999). For example, the sub-family Tropidurinae (including genera
Plica, Strobilururs, Eurolophosaurus and Tropidurus)
exhibits remarkable ecological diversity but minimal morphological
differentiation compared with that reported for Anolis. Species from
forested and more open environments differ in limb and tail morphology, but
species from sand dunes, rock outcrops, semiarid grasslands and other habitats
exhibit similar body shape and size
(Kohlsdorf et al., 2001).
The limited morphological diversity among Tropidurinae lizards from
non-forested environments is surprising given the striking ecological
diversity that exists among these species. For example, rock and sand are
common substrates for Tropidurus but contrast dramatically in
friction and force restitution coefficients. Sand involves a higher energetic
cost of locomotion and limits acceleration in comparison with non-granular
surfaces (Hawkey, 1991;
Lejeune et al., 1998; Kerdork
et al., 2002). In addition, the greater exposure to predators and increased
risk of overheating in sand dunes environments
(Rocha, 1998) may favor high
sprint speeds. In the context of Arnold's paradigm
(Arnold, 1983), therefore, sand
species should exhibit traits that improve performance on this demanding
substrate. However, given the absence of remarkable morphological
differentiation within this taxon, physiological divergence may be responsible
for relative changes in the locomotor performance of tropidurines from sandy
habitats. This hypothesis is the focus of the present paper.
Various aspects of muscle physiology and energy metabolism are known to
enhance the locomotor performance of lizards exhibiting contrasting behaviors
and ecologies. For example, an increase in proportion of fast-glycolytic
fibers associated with high limb cycle frequency appears to enhance sprint
speed in phrynosomatid species that locomote on sand
(Bonine et al., 2001).
Improved endurance capacity, involved in the large distances covered by
lizards that forage actively, is associated with high proportions of oxidative
fibers in the leg muscles (Mutungi,
1992), high field metabolic rates
(Anderson and Karasov, 1981)
and high metabolic scopes (Frappell et
al., 2002). Muscle fibers of a glycolytic nature contract quickly
but fatigue rapidly, whereas slow-oxidative fibers, aerobic in nature, exhibit
high endurance (Brooks et al.,
1996) but require improved oxygen delivery to the muscle. Because
of this functional link, positive correlations are expected among the
proportion of aerobic fibers in leg muscles, the activity of oxidative
enzymes, the muscle fatigue resistance, the aerobic scope and the degree of
sustained activity of lizard species. The daily activity patterns demonstrated
by a species should be related to the ecological pressures affecting that
species (Irschick and Garland,
2001). The metabolic design of animal muscle tissue varies
according to daily locomotor activity patterns
(Kernell et al., 1998) and
would consequently affect muscle power output and fatigue resistance
(Rome, 1998). Therefore,
species that use different habitats and locomote on substrates with distinct
mechanical and energetic demands should exhibit variation in locomotor
performance supported by differences in organismal aerobic capacity and muscle
functioning, which is influenced by the proportion of fiber types and the
activity of specific enzymes from glycolytic and oxidative pathways.
In the present study, we predict that (1) Tropidurus species that
inhabit sandy environments will exhibit higher sprint speeds but will be less
prone to jump, since their specific substrate does not favor activities that
demand very high propulsive forces, (2) physiological differentiation of the
leg muscles, specifically in the proportion of fiber types and enzyme
activity, will be associated with the improved sprint speed performance in
sand species and (3) variation in aerobic capacity (indicated by metabolic
scopes) to sustain activity will be associated with an increased capacity to
sustain jumping activity in rock species. We focus on two sister-species in
the genus Tropidurus – T. itambere and T.
psamonastes – that, despite systematic proximity and overall
morphological similarity, exploit the two contrasting habitats in question.
Tropidurus itambere is a rock-outcrop specialist found in Brazilian
Cerrados (Fig. 1A; a habitat
characterized by scattered trees and bushes and exposed rocks; for a
description, see Van Sluys,
1991), while T. psamonastes exclusively utilizes the sand
dunes of the Caatinga (Fig. 1B;
a northeastern Brazilian habitat characterized by sandy soil, scattered
shrubs, high substrate temperatures and seasonal and limited rainfall; for a
description, see Rocha, 1998).
As an outgroup for additional comparison (see
Garland and Adolph, 1994), and
to represent a species that uses more than one substrate type, we included
T. oreadicus, a generalist species typical of Cerrados that moves
amongst the rocks, fallen branches and bushes
(Colli et al., 1992). We
quantified the maximal sprinting capacity on rocky and sandy surfaces and the
mean number of jumps produced for each of the three Tropidurus
species. This latter variable was considered a behavioral indicator of
propensity to jump in undisturbed animals and a physiological index of
endurance in tests with uninterrupted stimulation. We also examined aerobic
metabolic scopes as a whole-organism measurement of the ability to sustain
activity aerobically. Additionally, to explore the underlying physiological
basis of any interspecific variation in performance, we compared fiber type
composition, power output and maximal activities of key metabolic enzymes
(citrate synthase, pyruvate kinase and lactate dehydrogenase) of the
iliofibularis muscle.
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Materials and methods |
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Locomotor performance tests
Sprint speed and jumping capacity were measured at 35°C in eight to 10
individuals of each species. One hour before the onset of experiments, animals
were placed inside a climatic chamber set at the test temperature.
Sprint speed
Sprint speed was measured using a racetrack with five sets of photocells at
intervals of 50 cm, connected to a computer using customized software. The
substrate of this track could be changed to either rock or dry, white,
fine-grained sand. Animals were stimulated to run by hand. Three trials on
each substrate were conducted for each individual, and only the shortest time
was used in computations. Races in which individuals ran against the walls of
the racetrack or turned back before the end of the track were discarded and
new trials were conducted an hour later. Both absolute and body
length-independent sprint speeds were analyzed. Absolute sprint speed was
considered indicative of the speed ecologically relevant for the lizards,
since it indicates the time necessary to cross a specific distance whilst
escaping from a predator. Relative sprint speed (corrected by body length) is
indicative of the speed physiologically relevant for the lizards, since it
indicates how fast muscles can contract and impulse the body towards the
movement, independently of the animal size. To verify whether or not
performance had changed with captivity, the sprint speed of T.
oreadicus and T. psamonastes was measured again prior to
sacrifice, two and four months after the first trials; no changes were
observed (t-test, P=0.735, d.f.=36).
Jumping capacity
Jumping capacity was tested on a jumptrack (50 cm width;
Fig. 2A) with barriers of
different heights. Tests were conducted with one individual at a time. First,
undisturbed animals inside the jumptrack were filmed over 6 h, and jumping
behavior and performance were quantified from the videotapes
(Fig. 2A). In a second test
series, individuals were stimulated to jump each barrier by tail tapping,
starting with the lowest barrier, which was replaced by the next highest every
time that the individual successfully jumped
(Fig. 2B). The experiment was
finished when the individual was exhausted and refused to move even when
turned with the abdomen up. Jumps were classified as `successful' when lizards
crossed the barrier and as `attempts' when lizards jumped but did not cross
the barrier. Jumping success ratio for each species was calculated as the
fraction of individuals that crossed the barriers in relation to the total
number of individuals tested.
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Respirometry
Metabolic scopes were calculated as the difference between activity
metabolic rates (AMR), measured on a treadmill at horizontal level and
inclined level, and resting metabolic rates (RMR). RMR and AMR were estimated
from oxygen consumption in 9–10 individuals of T. itambere, T.
oreadicus and T. psamonastes using open-flow respirometry, as
described in detail by Withers
(1977). Both experiments were
conducted at 35°C using a PA-1 O2 Analyzer (Sable Systems,
Henderson, NV, USA) and a flow of 42.45 ml O2
min–1 for resting experiments and 150.95 ml O2
min–1 for activity experiments, with lizards in a
post-absorptive state. Resting experiments were conducted with the lizard
inside a metabolic chamber, and the oxygen consumption was recorded overnight
(12 h). The lowest nadir of the signal generated by the gas analyzer (duration
of at least 15 min) was used to calculate RMR. AMR were measured using a latex
mask around the animal's head and stimulating the lizard to run on a treadmill
set at 1.2 m s–1 at two inclinations (0° and 20°).
AMR was calculated from the peak of oxygen consumption (usually three points,
less than 1 min, since the animals don't run at a steady speed and present
episodic ventilation). Three races were conducted at each incline, and the
maximal value at each level of AMR was considered. All metabolic measurements
were divided by body mass and corrected for standard conditions of pressure
and temperature.
Histochemistry
Histochemistry was used to determine the proportion of different fiber
types in the iliofibularis muscle of the three Tropidurus species.
Four individuals of each species were weighed and then anesthetized and killed
with an intraperitoneal injection of 0.5 ml of hypnol. The right legs were
dissected and iliofibularis muscles removed without damaging the tissues.
Muscles were dehydrated in talcum, frozen in liquid nitrogen and stored at
–85°C. Serial transverse sections (9 µm thickness) were obtained
in a cryostat at –20°C and stained with hematoxylin–eosin for
morphological examination. Subsequent sections were also stained for NADH-TR
(nicotinamide adenine dinucleotide tetrazolium reductase) and myofibrillar
ATPase (alkaline preincubation pH 10.4 or acid preincubation pH 4.5) to
evaluate oxidative activity and relative contraction speed (fast or slow),
respectively (Bancroft and Stevens,
1982). Digital images from histological laminae were obtained from
a light microscope connected to a computer by a video camera using the
software program Stereo Investigator 2000 (MicroBrightField Inc., Colchester,
VT, USA). Five to six fields of each cut were obtained using a 10x
magnification. The frequency of fiber types was determined from digital
images, and muscle fibers were classified as fast-glycolytic fibers (FG),
slow-oxidative fibers (SO) or fast-oxidative and glycolytic fibers (FOG),
according to Bancroft and Stevens
(1982). We used only the
NADH-TR results to perform statistical analyses because NADH-TR and ATPase
reactions are complementary, which was confirmed by preliminary results. This
complementarity is explained by the characteristic of oxidative fibers to
sustain activity aerobically, contracting slower than glycolytic fibers, which
contract fast but use mainly glycolytic pathways and fatigue quickly.
Muscle mechanics
Experiments of muscle mechanics were conducted to determine the ability of
iliofibularis muscle both to generate power at different contraction
velocities and to resist fatigue. Femur and tibia lengths were measured on
5–7 lizards of each species. Body mass was measured to the nearest 0.01
g. Lizards were then killed using a guillotine. The right iliofibularis
muscles were used for muscle mechanics and the left ones used for biochemical
assays. Iliofibularis muscle was isolated at room temperature (25°C) in
oxygenated (95% O2; 5% CO2) Ringer solution for reptiles
(NaCl 145 mmol l–1, Na2HPO4 2.15 mmol
l–1, NaH2PO4 0.85 mmol
l–1, KCl 4 mmol l–1, glucose 10 mmol
l–1, CaCl2 solution 2.5 mmol l–1,
pH 7.60 at room temperature pre-oxygenation). Iliofibularis muscle is a flexor
of the superior part of the leg and pulls the hind leg behind, pushing the
body towards the movement (Romer,
1985). It is active during the swing phase of running or walking,
when the femur is abducted and the knee is bent
(Jayne et al., 1990). For each
muscle, a small section of bone was left at the end of both proximal and
distal tendons. Aluminum foil clips were wrapped around the tendons at either
end of the muscle, leaving both sections of bone unwrapped to prevent tendon
slippage in the clips.
Isometric studies
The muscle preparation was attached via the foil clips to a load
cell (UF1; Pioden Controls Ltd, Canterbury, UK; calibrated to 77.6 mN
V–1) at one end and a motor arm (V201; Ling Dynamics Systems,
Royston, UK) attached to an LVDT (linear variable displacement transformer;
DFG 5.0; Solartron Metrology, Bognor Regis, UK; calibrated to 1.35 mm
V–1) at the other. The muscle was then maintained at
35±0.5°C in circulating oxygenated Ringer solution for reptiles.
The preparation was stimulated via parallel platinum electrodes while
held at constant length to generate a series of twitches (stimulus amplitude,
pulse width and muscle length were adjusted to determine the stimulation
parameters and muscle length corresponding to maximal isometric twitch force
production). A tetanic force response was then elicited by subjecting the
muscle to a train of stimulation (250 ms). After 5 min, a twitch response was
then initiated to confirm that there had been no slippage of the tendons in
the foil clips. In the rare cases where twitch force had dropped, the muscle
was reclipped and twitch force was again maximized before tetanic stimulation
was resumed. Stimulation frequency (180–240 Hz) was then altered to
determine maximal tetanic force. A rest period of 5 min was allowed between
each tetanic response.
Work loop studies
After optimization of stimulation parameters, the muscle was then subjected
to cycles of three sinusoidal length changes (work loops;
Josephson, 1985). Muscle
stimulation and length changes were controlled via a D/A board and an
in-house program produced using Testpoint software (CEC, Bedford, NH, USA).
Data were collected at a rate of 1500 points per work loop cycle. For each
work loop cycle, muscle force was plotted against muscle length to generate a
work loop, the area of which equated to the net work produced by the muscle
during the cycle of length change
(Josephson, 1985). The net
work produced was multiplied by frequency of length change cycles to calculate
net power output.
The total strain of length change cycles was maintained at 0.12 (i.e.
±6% of resting muscle length), as in Swoap et al.
(1993). The cycle frequency of
length change was altered at random from 4 Hz to 14 Hz to ensure that a power
output cycle frequency curve could be generated to encompass the range of
lizard limb cycle frequencies measured in previous studies
(Swoap et al., 1993;
Nelson and Jayne, 2001).
During these length changes the muscle was usually subjected to phasic
stimulation (active work loop cycle) but sometimes these length changes were
performed without stimulation (passive work loop cycle) to monitor the net
work done on the muscle during the length change cycle. For passive work loop
cycles, the net passive power (net passive work multiplied by cycle frequency
of length change) was used to indicate the power input required (the power
absorbed) to move the unstimulated muscle through length change cycles. The
passive power input values were also used to indicate the relative stiffness
of the muscle, as increased passive power input was accompanied by an increase
in muscle stiffness (indicated by an alteration in the shape of the work
loop).
Every 5 min, the muscle was subjected to a further set of three work loop cycles with stimulation parameters being altered until maximum net work was achieved. Before the fatigue run, a set of control sinusoidal length change and stimulation parameters were imposed on the muscle every 3–4 sets of work loops to monitor variation in the muscles ability to produce power/force. Any variation in power was found to be due to a matching change in ability to produce force. Therefore, the power produced by each preparation, prior to the fatigue run, was corrected to the control run that yielded the highest power output, assuming that alterations in power-generating ability were linear over time. All muscles still produced over 70% of maximal control run power by the end of each experiment, i.e. prior to the fatigue run.
Iliofibularis muscles were then subjected to a fatigue run consisting of 32 work loop cycles. Recovery from fatigue was monitored by regularly subjecting the muscle to a set of three control active work loop cycles.
Muscle dimension measurements and calculations
At the end of muscle mechanics experiments, the aluminum foil clips, bone
and tendons were removed from iliofibularis muscles and each muscle was
blotted on absorbent paper to remove excess Ringer solution. Wet muscle mass
was determined to the nearest 0.0001 g using an electronic balance (FA 1604;
Shangping Inc., Shanghai, Jiangsu, China). Mean muscle cross-sectional area
was calculated from muscle length and mass assuming a density of 1060 kg
m–3 (Mendez and Keys,
1960). Maximum isometric muscle stress was then calculated as
maximum tetanic force divided by mean cross-sectional area (kN
m–2). Normalized muscle power output was calculated as power
output divided by muscle mass (W kg–1).
Biochemistry
Measurements of the maximal activities of pyruvate kinase (PK) and lactate
dehydrogenase (LDH), from the glycolytic pathway, and citrate synthase (CS),
from the TCA cycle, were made using spectrophotometric techniques based on
absorbance changes of cofactors after substrate addition. The differences
between absorbance changes of control (without substrate) and after substrate
addition were used to estimate maximal enzyme activity. Iliofibularis muscle
samples from the left leg of T. psamonastes, T. itambere and T.
oreadicus were collected when animals were killed for the muscle
mechanics experiments. Muscle samples were immediately frozen in liquid
nitrogen and stored at –85°C. As the iliofibularis muscle from these
species is so small, it is difficult to quickly separate the regions
corresponding to red and white fibers prior to freezing. For this reason, a
transversal section of the muscle, with both regions, was used in enzymatic
assays.
Muscle samples were homogenized using a Marconi homogenator (Marconi Ltd,
Piracicaba, São Paulo, Brazil) in ice-cold 20 mmol l–1
imidazol (pH 7.4) buffer with 2 mmol l–1 EDTA, 20 mmol
l–1 NaF, 1 mmol l–1 phenylmethylsulfonyl
fluoride (PMSF) and 0.1% Triton X-100. The homogenates were then submitted to
sonication using a U-200S control unit (IKA-Labor Technik, Staufen, Germany)
for three 10 s intervals and directly used in the assays. Measurements were
performed at 35°C with a Beckman DU-70 spectrophotometer, following the
changes in the absorbance of NADH at 340 nm or
5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) at 412 nm, under substrate
saturation and in the absence of inhibitory conditions. All reactions were
performed at a final volume of 1 ml in duplicate. Results were expressed in
µmol of substrate converted to product per min per g wet mass. Enzyme
protocols followed Bergmeyer
(1983), with minor
modifications as follow: PK (E.C. 2.7.1.40): 100 mmol l–1
imidazol (pH 7.0), 10 mmol l–1 MgCl2; 100 mmol
l–1 KCl; 2.5 mmol l–1 ADP; 0.02 mmol
l–1 fructose-1,6-biphosphate; 0.15 mmol l–1
NADH; 12 U ml–1 LDH, 5 µl of muscle sample homogenate
(diluted 1:10) and 3.6 mmol l–1 phospho(enol)pyruvate
(omitted for control). LDH (E.C. 1.1.1.27): 100 mmol l–1
imidazol (pH 7.0); 5 mmol l–1 DTT; 0.15 mmol
l–1 NADH, 5 µl of muscle sample homogenate (diluted 1:30)
and 1 mmol l–1 pyruvate (omitted for control). CS (E.C.
4.1.3.7): 50 mmol l–1 Tris (pH 8.0); 0.1 mmol
l–1 DTNB; 0.2 mmol l–1 acetyl-CoA, 28 µl
of muscle sample homogenate (diluted 1:10) and 0.9 mmol l–1
oxalacetate (omitted for control).
Statistical analysis
Body mass and length and iliofibularis muscle mass and length were compared
by conventional analysis of variance (ANOVA) between the three species. An
arcsine transformation was conducted on fiber proportions
(Zar, 1996). One-way ANOVA or
Kruskal–Wallis (according to normality) were used to analyze sprint
speed, mean total amount of jumps (with and without stimulus), metabolic
scope, activities of CS, LDH and PK, fiber type proportion (FG, SO and FOG)
and power output at each cycle frequency (4, 6, 8, 10, 12 and 14 Hz). From
muscle fatigue experiments, power output at 10th, 20th and 30th loops was
calculated as percentage of power output of 1st loop. Frequencies were arcsine
transformed and compared in a two-way ANOVA. Passive power input was analyzed
using one-way ANOVA.
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Physiological parameters
Fiber type proportion and CS activity differed among the three species:
whereas T. psamonastes exhibited almost 20% more FG fibers than
T. itambere (Table 1),
T. itambere exhibited a marginally higher proportion of SO fibers
than T. psamonastes (Table
1). Tropidurus oreadicus presented intermediate values of
fiber type proportion and CS activity
(Table 1). The proportion of
FOG fibers did not significantly differ among the species
(Table 1). Variation in muscle
composition was mainly related to proportion, and not number, of fiber types,
since the total amount of fibers did not differ among species (ANOVA,
F2,3=0.35, P=0.714). In relation to enzymatic
activities, the three species possessed similar values for PK and LDH, but
T. psamonastes exhibited lower activity of CS than T.
oreadicus and T. itambere
(Table 1).
Maximal isometric stress did not differ among the species
(Table 1) and was within the
range (85–180 kN m–2) previously reported for the
iliofibularis of other lizard species at this temperature
(Putnam and Bennett, 1982).
The relationship between peak power output and cycle frequency did not vary
between species (ANOVA, P>0.50, G.L.=2,
Fig. 3A). The fatigue
resistance of the iliofibularis was also similar between species (ANOVA,
P>0.05, G.L.=2; Fig.
3B). The iliofibularis muscle of T. psamonastes was
longer (ANOVA, F2,3=11.73, P<0.001, after
Bonferroni correction) and thinner (ANOVA, F2,3=4.39,
P=0.029, after Bonferroni correction) in comparison with T.
itambere and T. oreadicus, resulting in a lower calculated
cross-sectional area. The net passive power inputs differed between the three
species at the cycle frequency of 14 Hz (ANOVA, F2,3=6.83,
P=0.008; Fig. 3C),
which suggests differences in muscle stiffness.
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The three species studied did not differ significantly in the metabolic scopes attained at either horizontal or inclined races (Table 1) and exhibited values close to 0.2 ml O2 g–1 h–1 in all situations.
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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). Absolute speeds are of particular ecological relevance
because they define how fast a lizard can flee from a potential predator.
Differences in absolute sprint speed may be partially explained by the larger
body length of T. psamonastes in comparison to T. itambere,
allowing greater speeds due to the increased limb length while still
maintaining the contractile properties of the muscle. However, the
interspecific variation in locomotor performance could also be related to
differences in stride frequency (Biewener
and Gillis, 1999; Irschick and
Jayne, 1999; Jayne and
Irschick, 1999) and/or differences in parameters of
muscle–tendon unit design, such as tendon dimensions
(Biewener, 1998;
Alexander, 2002). Additionally,
subtle differences in foot morphology in sand species may elicit alterations
in absolute sprint performance (Carothers,
1986; Arnold, 1995;
Melville and Swain, 2000).
Although only one hind limb muscle was examined, which limits our ability to
evaluate the extent to which muscle physiology explains differences in
locomotor performance, it is possible to suggest that physiological traits are
involved as well, because T. psamonastes possesses a higher
proportion of FG fibers in the iliofibularis (see below).
The three species studied exhibited the same rank (T. itambere > T.
oreadicus > T. psamonastes) in both types of jumping test (i.e. with
undisturbed and constantly stimulated individuals). The number of jumps for
undisturbed animals was considered to be a measurement of an individual's
propensity or motivation to jump, while the number of jumps for lizards
stimulated until exhaustion was considered to be an index of jumping
endurance. The interspecific differences observed suggest a contrast in
jumping endurance that might have an underlying basis in muscle and metabolic
physiology. The tendency towards a higher proportion of SO fibers in the
iliofibularis of T. itambere (high jump endurance species) supports
this prediction, although this trend is not statistically significant and not
supported by either enzymatic or whole-animal metabolic analyses. The
magnitude of differentiation in jumping performance among the three species
does not parallel their taxonomic distances. The differences between the
sister-species T. itambere and T. psamonastes are greater
than those between them and the congeneric T. oreadicus, the more
distantly related species (Frost et al.,
2001). The strong divergence in jumping performance between the
two sister-species suggests that Tropidurus species exhibit
significant phenotypic plasticity of behavioral, physiological and locomotor
traits and that this lability might rapidly lead to enhanced performance and
fitness in their specific habitats.
The fiber type and enzyme profile of Tropidurus are probably
related to ecological demands. Tropidurus psamonastes exhibits high
absolute sprint speeds on sand and a high proportion of FG fibers in
comparison with T. oreadicus and T. itambere. The latter
frequently locomotes on vertical surfaces and exhibits a comparatively high
proportion of oxidative fibers, a trait that may be required for the increased
work against gravity. Overall, our results support the relationship between
fiber type proportion and locomotor performance that has been reported in
other lizard genera (Putnam et al.,
1980; Gleeson and Harrison,
1988; Mutungi,
1989,
1992;
Young et al., 1990;
Mirwald and Perry, 1991).
These studies suggest high proportions of FG fibers in fast species
(Putnam et al., 1980;
Gleeson and Harrison, 1988;
Bonine et al., 2001), while
slower and more aerobic species exhibit high proportions of SO fibers
(Mutungi, 1989;
Bonine et al., 2001). Because
of spatial restrictions, and given that an increase in number of fibers
implies an increment in metabolic costs, an increase in proportion of one
fiber type necessarily implies a decrease in the amount of the other fiber
types (Alexander, 2000). SO
fibers generally consume less ATP per unit force generated and use mainly
oxidative pathways such as the TCA cycle, while FG fibers use mainly energy
derived from oxygen-independent pathways
(Hochachka and Somero, 1984;
Hochachka, 1994). Dominant
fiber-type might also be correlated with typical limb cycle frequency during
locomotion because slow fibers are designed to produce power at slow speeds
whereas fast fibers work optimally at the frequencies used during fast speeds
(James et al., 1995).
In addition to a high proportion of FG fibers, T. psamonastes
exhibits low values for CS activity in comparison with the other two
congeneric species and with the same muscle in other lizard species
(Guppy and Davison, 1982;
John-Alder, 1984;
Garland and Else, 1987;
Gleeson and Harrison, 1988).
The values of CS in T. psamonastes are comparable to the red and
white muscles of sluggish species of fishes
(Hochachka and Somero, 1984;
Suarez et al., 1986;
Moon and Mommsen, 1987;
Moyes et al., 1992) and to the
leg muscles of amphibians (Taigen et al.,
1985). It is possible that there were no strong selective
pressures for increasing the oxidative capacity of the iliofibularis muscle in
T. psamonastes, since this species relies predominantly on short
bursts of running to escape from predators or explore the environment. By
contrast, T. oreadicus and T. itambere may have evolved
higher oxidative capacities in their muscles associated to frequent use of
jumps and vertical locomotion to explore higher portions in the habitat.
The exposed habitat of sand dunes, with greater distances between refuges,
might favor lizards that exhibit high sprint speeds when escaping from
predators, and sand species would probably need increased muscle power output
at higher limb cycle frequencies to achieve higher sprint speeds. By contrast,
the species from rocky environments, T. itambere and T.
oreadicus, should possess greater muscle fatigue resistance correlating
with their increased jumping endurance. However, both iliofibularis maximum
muscle power output at different cycle frequencies and fatigue resistance are
comparable among the three Tropidurus species, despite differences in
locomotor performance and muscle morphology. Peak power output was between
40.5 and 44.0 W kg–1 for each species, which is somewhat
lower than the values for the iliofibularis of Dipsosaurus dorsalis
(Swoap et al., 1993). Despite
the similarity in muscle contractile properties between the
Tropidurus species studied, it is still possible that the in
vivo performance of the iliofibularis differs among species, as
performance of skeletal muscle fibers during movement is affected by length
trajectory, amplitude and frequency of stimulation, initial fiber length and
velocity of contraction (Marsh,
1999; Nelson and Jayne,
2001). Our study also found that species differed in the power
required to lengthen the iliofibularis, given by the higher passive power
input observed in T. itambere and T. oreadicus
(Fig. 3C), which indicates
greater muscle stiffness. This increased stiffness could feasibly occur as a
result of more frequent jumping, which involves a lot of eccentric muscle
activity (i.e. the muscle is active while it is being stretched), leading to
muscle damage. Muscle damage of this type tends to lead to changes in collagen
type or increased collagen content
(Williams et al., 1988) acting
as a protective mechanism to increase the forces subsequently required to
stretch the muscle. The high jumping success ratio of T. oreadicus is
possibly linked to this high net power input observed at high cycle
frequencies.
The lack of differences in power output and fatigue resistance of
Tropidurus iliofibularis muscles could also be supplemented by
changes in other features not analyzed, such as tendon or limb bone compliance
(not measured in in vitro work loops with isolated muscles), which
may amplify in vivo muscle power output
(Biewener and Roberts, 2000;
Blob and Biewener, 2001).
Alternatively, it is also possible that the iliofibularis is not the primary
muscle limiting in vivo performance
(Nelson and Jayne, 2001) or
that in vitro measurements of power output may not reflect
interspecific differences in in vivo conditions
(Marsh, 1999). Animals may
recruit different muscle groups or change activation patterns to vary
contractile function (Biewener and Gillis,
1999) in response to shifts in environmental parameters (e.g.
changes in substrate inclination) or activity mode (e.g. shift from running to
jumping). In addition, kinematic adjustments may also play an important role
in differences observed in jumping performance
(Toro et al., 2003) and
absolute sprint speed (Irschick and Jayne,
1999; Jayne and Irschick,
1999). It has been argued that natural selection and learning by
experience may favor changes in kinematic parameters, such as step length,
stance duration and limb cycle frequency, which maximize speed or minimize
energetic costs (Alexander,
2000).
In conclusion, the present study shows remarkable divergence in absolute
sprint speed on sand between closely related Tropidurus species that
is correlated with physiological differentiation in the proportion of fiber
types in the iliofibularis muscle. Additionally, propensity and endurance for
jumping activity vary among Tropidurus species, with greater
performance observed for those species exploiting more complex structural
habitats. Differences in absolute performance on sand appear to be explained
by some organismal traits, including body size, but are better predicted by
proportion of glycolytic fibers in the iliofibularis than by any other
physiological parameter measured. The biomechanical and biochemical profiles
of the iliofibularis muscle appear to be poor predictors of performance, but
this trend may not apply to other muscles underlying locomotor performance.
Evolutionary changes in behavior seem to be equally important for locomotor
differentiation and may undergo more rapid divergence than morphology or
muscle physiology (Blomberg et al.,
2003). Further investigation of other physiological parameters
that may constrain performance and/or the endocrinal basis for motivation may
prove productive.
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Acknowledgments |
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References |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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