Aerobic characteristics of red kangaroo skeletal muscles: is a high aerobic capacity matched by muscle mitochondrial and capillary morphology as in placental mammals?
School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia
* Author for correspondence (e-mail: t.dawson{at}unsw.edu.au)
Accepted 27 May 2004
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Summary |
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Red kangaroo skeletal muscle morphometry matched closely the general
aerobic characteristics of placental mammals. The relationship between total
mitochondrial volume in skeletal muscle and
O2max during
exercise was identical to that in quadrupedal placentals, and differed from
that in bipedal humans. As for placentals generally, red kangaroo
mitochondrial oxygen consumption at
O2max was 4.7 ml
O2 min1 ml1 of mitochondria.
Also, the inner mitochondrial membrane densities were 35.8±0.7
m2 ml1 of mitochondria, which is the same as for
placental mammals, and the same pattern of similarity was seen for capillary
densities and volumes.
The overall data for kangaroos was equivalent to that seen in athletic placentals such as dogs and pronghorns. Total skeletal muscle mass was high, being around 50% of body mass, and was concentrated around the pelvis and lower back. The majority of the muscles sampled had relatively high mitochondrial volume densities, in the range 8.810.6% in the major locomotor muscles. Again, capillary densities and capillary blood volumes followed the pattern seen for mitochondria. Our results indicate that the red kangaroo, despite its locomotion and extreme body form, shows fundamental aerobic/muscular relationships that appear common to both marsupials and placentals. The evolution of such metabolic relationships apparently predates the divergence of the therian groups in the early Cretaceous, and perhaps evolved in the mammal-like reptiles during the Triassic (220 million years ago) before the actual evolution of the mammals.
Key words: kangaroo, marsupial, muscle, mitochondria, capillary, aerobic capacity
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Introduction |
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The red kangaroo Macropus rufus has the highest aerobic capacity
(O2max) yet
measured for a marsupial (Kram and Dawson,
1998
). It has a
O2max comparable
to that of the most `athletic' of placentals such as dogs
(Kram and Dawson, 1998
;
Dawson et al., 2003
). Given
the long independent evolution of the two therian groups, do kangaroos achieve
their high aerobic capacity using the same structural and functional
mechanisms as (athletic) placentals? Of note, kangaroos and their relatives
have unusual locomotory energetics; they travel above moderate speeds with
lower relative cost than quadrupedal placentals
(Baudinette et al., 1992
;
Dawson and Taylor, 1973
;
Kram and Dawson, 1998
;
Webster and Dawson, 2003
).
The links between the aerobic capability of an animal and the supporting
structural elements in the oxygen cascade from the lungs to the muscles have
been a matter of conjecture (see discussion in
Hoppeler et al., 1981a).
Studies initiated by Taylor and Weibel
(1981
) examined the design of
the mammalian (placental) respiratory system. Body size dependent, or
allometric, variations in aerobic capacities
(
O2max) were
correlated with variation in structural and functional aspects of the
cardiorespiratory system such as pulmonary diffusing capacity and capillary
density, and with the densities of mitochondria utilizing oxygen in selected
muscles (Gehr et al., 1981
;
Hoppeler et al., 1981b
;
Mathieu et al., 1981
;
Taylor et al., 1981
).
Phylogenetic variation in aerobic capacity across vertebrates generally
appears to follow a similar pattern. Differences in the aerobic potentials of
reptiles, mammals and birds are primarily associated with the amount of
mitochondria in organs and skeletal muscle, although some differences in
packing density of the inner mitochondrial cristae membranes occur (Else and
Hulbert, 1981,
1983
,
1985a
;
Suarez, 1996
). However, within
the mammals no large differences in total mitochondrial capacity were noted
between placental, marsupial and monotreme species
(Else and Hulbert, 1985b
).
Further studies of placental mammals have shown that differences in
metabolic capacity may be associated with induced and adaptive variation
(Weibel et al., 1987). Induced
variation of
O2max is due to
training, as seen in man (Hoppeler and
Lindstedt, 1985
). Adaptive variation results from different
evolutionary pressures; `athletic' species may have a two- to threefold
greater oxidative capacity than more `sedentary' species of similar body mass
(Taylor et al., 1981
). When
athletic species such as dogs and horses are compared with more sedentary
species like goats and cattle, the athletic species have larger hearts and a
bigger mass of muscle, together with higher muscle mitochondrial densities and
capillary volumes (Weibel et al.,
1991
; Hoppeler and Weibel,
1998
). This quantitative overall match between design and
functional parameters, such as in the pathway for oxygen from the lung to the
mitochondria in the muscle cells, has been referred to as `symmorphosis'
(Taylor and Weibel, 1981
;
Weibel et al., 1991
;
Weibel, 2000
).
Aspects of the cardiorespiratory system of marsupials are not the same as
those of placentals. They have resting heart rates less than half of those of
placentals (Kinnear and Brown,
1967; Dawson and Needham,
1981
), but hearts that are generally bigger
(Dawson et al., 2003
). A
similar pattern is also seen with breathing, marsupials having very low
resting respiratory rates but large tidal volumes
(Dawson and Needham, 1981
;
Cooper and Withers, 2003
).
Despite these differences, the mechanisms by which the kangaroos achieve high
aerobic capacity may be similar to that seen in athletic placentals. The large
heart of the red kangaroo has the same proportionality to
O2max as seen in
the athletic dog, and the same is true for the volume of cardiac muscle
mitochondria (Dawson et al.,
2003
). The principal aim of this study was to determine whether
similar relationships are seen in skeletal muscles.
Our study was also concerned with the potential for muscular energy output
throughout the body of the kangaroo. Hopping is associated with a distinctive
body shape, including a concentration of muscle mass in the hind limbs and a
large tail. Studies of the functional properties of kangaroo musculature have
generally concentrated on the lower leg, specifically the Achilles tendon and
the gastrocnemius muscle, because of considerable interest in the role of
energy conservation by elastic recoil (e.g.
Dennington and Baldwin, 1988;
Bennett and Taylor, 1995
).
However, we know little about the actual sites of power generation and the
role of the large tail. An associated question is whether bipedal locomotion
and the upright stance at times adopted by kangaroos leads to similarities
with humans, who diverge from the usual (placental) relationship between body
mass and
O2max
(Hoppeler, 1990
).
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Materials and methods |
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Muscle sample collection and preparation
To assess the mitochondrial and capillary characteristics of the skeletal
muscle of the whole body we followed a sampling procedure comparable to that
of Hoppeler et al. (1984). The
kangaroo musculature was divided into seven functional units, head and neck,
foreleg, trunk, back, upper hindleg, lower hindleg and tail
(Fig. 1). After being shot the
kangaroos were weighed to ±0.05 kg and then eviscerated, with the mass
of the large forestomach and its contents determined by weighing on
appropriate calibrated electronic balances. The mass of the intestines,
including caecum and contents, were similarly determined. Empty body mass was
then calculated. The skin was removed and muscle was dissected from one half
of the body and weighed. The dissection was carried out in an air conditioned
room and evaporation from the muscles was contained by towels dampened with
physiological saline. The separation of regions was as follows: head and neck
was separated anterior to the first thoracic vertebrae; foreleg included all
muscles attached to foreleg; trunk included the m. erector spinae plus rib and
abdominal muscles and the diaphragm; back musculature comprised the m.
multifidi lumborum and m. sacrocaudalis dorsalis, which run between the
spinous and mamillary processes; upper hindleg included all muscles attached;
lower hindleg included the m. gastrocnemius and associated muscles; tail was
separated immediately behind the pelvis. Diagrams of the principal muscles of
the hindleg and back of the red kangaroo are provided in
Fig. 2A,B.
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The mitochondrial and capillary characteristics of the skeletal muscle of
the regions and hence the whole body was determined by sampling from muscles
from each region. For four animals, one muscle in each body region was
selected by the throw of dice, except in the upper hindleg region, where three
muscles were selected (by throw of dice) because of the large mass of this
region. Overall, the muscles sampled represented 31.5% of the total
musculature. Data was also collected from the diaphragm, so a total of ten
muscles were sampled. A concurrent study of the heart was also undertaken
(Dawson et al., 2003).
Each muscle was removed and weighed to ±0.1 g. Three sample blocks per muscle (per animal) were cut from random locations (superficial or deep, and proximal, central or distal; again selected by throw of dice). Samples were placed immediately into a fixative solution of 6.25% glutaraldehyde in 0.1 mol l1 sodium cacodylate buffer (pH 7.4), then kept refrigerated at 4°C prior to preparation for electron microscopy.
Preparation for electron microscopy
In total, 30 muscle sample blocks (10 muscles x 3 blocks) per animal
were prepared for electron microscopy; three blocks each from the head/neck,
foreleg, back, lower hindleg and tail regions, six blocks from the trunk and
nine blocks from the upper hindleg. Where required, blocks were trimmed into
smaller pieces. Blocks were rinsed in 0.1 mol l1 sodium
cacodylate buffer and left overnight in fresh buffer. Blocks were post-fixed
in a buffered solution of osmium tetroxide for 4 h, rinsed in 2% sodium
acetate solution and placed in 2% uranyl acetate for 1 h. The samples were
dehydrated in a series of 15 min ethanol washes, using solutions from 50% to
100% ethanol. After a final 30 min wash in 100% ethanol, the samples were
washed twice for 15 min in 100% dry acetone. Spurr's low-viscosity epoxy resin
(slow cure) was used to embed the samples. The blocks were placed in a 1:1
acetone:resin mixture for 1 h, then refrigerated for 34 days in a 1:9
mixture. This long infiltration period was selected to reduce tearing of the
muscle tissue during sectioning. The samples were placed in 100% resin for 30
min at 60°C, transferred into embedding moulds filled with fresh resin,
and cured at 60°C for 48 h.
A Reichart-Jung Ultracut ultramicrotome (Vienna, Austria) was used to section the embedded samples. 510 ultra-thin sections showing silver (6090 nm) or gold (90150 nm)interference colours were cut from each block and mounted on 200 mesh copper grids. Orientation of the sections was generally transverse or oblique to the muscle fibre axis. Grid-mounted sections were stained with 2% uranyl acetate in 50% ethanol for 10 min and then rinsed in distilled water.
Mitochondrial volume
Grids were viewed using a Hitachi 7000 (Tokyo, Japan) transmission electron
microscope (TEM) at a magnification of 12 000x. For each sample block,
ten grid squares were selected using a systematic random sampling method
(Howard and Reed, 1998, pp.
25-29) and a digital image was obtained of the top left corner of each of the
grid squares, using an Olympus SQ (Tokyo, Japan) digital camera attached to
the TEM and AnalySIS software. For each animal, 30 images were obtained per
muscle (10 images x 3 blocks); these were imported into the image
analysis software Adobe Photoshop. A human operator selected all mitochondria
in an image using a selection tool, coloured the mitochondria black and
filtered the background to plain white. The percentage area covered by
mitochondria (`mitochondrial area fraction') was thus represented by the area
of black in each image, which was calculated with reference to the total image
area (51.2 µm2 for images obtained at 12 000x) using a
suite of Photoshop plugins (Image Processing Toolkit, Reindeer Graphics,
Asheville NC, USA). The mitochondrial area fraction is equivalent to the
mitochondrial volume fraction or volume density, VV(mt,f).
The total mitochondrial volume V(mt,m) for each muscle (in
cm3) was calculated from:
![]() | (1) |
Surface density of the inner mitochondrial membranes
The inner mitochondrial membrane surface density (S(im,m)) was
estimated in the m. multifidi lumborum, one of the larger skeletal muscles.
Grid squares were selected as for mitochondrial density measurements. A
mitochondrion at the centre of the field of view (at magnification 12
000x) was then viewed at a magnification of 30 000x or 40
000x so that the inner membranes could be seen clearly. Twenty
mitochondria from each animal were examined. A cycloidal line grid (grid C1;
Howard and Reed, 1998, p. 210)
was used to estimate the surface density of inner mitochondrial membranes per
unit volume of mitochondria, SV(im,mt) in m2
cm3 using equation 6.4 of Howard and Reed
(1998
). An overall estimation
of the total surface area of inner membranes in each multifidi lumborum muscle
was given by:
![]() | (2) |
Capillary length and volume
Both transverse and longitudinal sections were sliced from muscles and were
used to estimate the tortuosity factor c(K,0) of the capillary
network, using the shortcut estimation method of Mathieu et al.
(1983). The tortuosity factor
was determined for the diaphragm, m. multifidi lumborum, m. gastrocnenius, m.
vastus lateralis and m. semitendinosus; the mean was
c(K,0)=1.37±0.07 (N=5). Grids were viewed using the
TEM at a magnification of 1500x. Grid squares were selected using a
systematic random sampling method. Ten digital images (of area approximately
equal to one grid square) of transverse sections were taken and used to
estimate the number of capillaries per unit area (numerical capillary density,
NA(c,f), in mm2). Capillary length
density JV(c,f) was calculated from numerical capillary
densities according to:
![]() | (3) |
![]() | (4) |
The cross-sectional areas A(c) of capillary profiles in transverse
sections were estimated using the same procedure as for mitochondrial area
densities, except that the absolute area covered by capillaries was obtained
(rather than a percentage). As in previous studies
(Conley et al., 1987), we
assumed that capillary profiles were approximately circular and estimated the
mean capillary radius rc from A(c):
![]() | (5) |
The capillary blood volume V(c) in each muscle was calculated
from:
![]() | (6) |
In this study we examine the relationship between muscle mitochondrial
volume and O2max
in M. rufus. To do this requires certain assumptions, which have been
accepted in the extensive studies of placental mammals undertaken by Hoppeler,
Weibel, Taylor and coworkers (for a discussion of these assumptions, see
Hoppeler, 1990
). We have
accepted the same assumptions for comparative purposes, but also because the
resultant errors are likely to be small. For example, assuming that the
measured muscle volume consists entirely of muscle fibres results in an
overestimation of the muscle fibre volume of less than 10%
(Hoppeler et al., 1987
).
Furthermore, some assumptions lead to opposite errors and cancel each other
out (Hoppeler, 1990
); assuming
all oxygen is consumed by muscle mitochondria at
O2max
overestimates mitochondrial activity (again of the order of 10%), so that
estimates of the maximum oxygen consumption rate per volume of muscle
mitochondria will be little affected.
Statistical methods
Comparisons between muscles were analysed using one-way analyses of
variance (ANOVAs). A StudentNewmanKeuls (SNK) multiple-range
test was applied when significant differences were indicated by the ANOVA
(using Statistica/Mac software). Values are given as means ± standard
error (S.E.M.). Regression analyses were carried out using
Microsoft Excel.
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Results |
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Mitochondrial volume density, VV(mt,f), varies between muscles (Table 2). Those from the back and trunk, including the diaphragm, together with the large m. gluteus medius of the upper hindleg, have VV(mt,f) in the range 8.810.8%, which is significantly higher than in the other muscles measured. On the other hand, muscles in the foreleg and neck have VV(mt,f) in the range 3.73.8%, significantly lower than in other muscles. The area of the inner mitochondrial surface, SV(im,m), was measured for the m. multifidi lumborum and was 35.8±0.7 m2 cm3 (N=4).
Capillary characteristics also differ between muscles from different
regions (Table 2). The
capillary length density, JV(c,f), is considered a good
estimate of the capillary supply to muscles
(Conley et al., 1987); it
incorporates the tortuosity factor, c(K,0), which in this case was
determined for five muscles from each animal. However, no significant
difference was found between these muscles and the overall mean tortuosity
(1.37±0.07, N=5) was used generally. Muscles that have
significantly higher JV(c,f) tended to have a high
VV(mt,f). Large muscles of the upper hindleg, m. gluteus
medius, and the trunk, the m. erector spinae, have JV(c,f)
that are significantly higher than in other muscles. However, the pattern is
not clearcut. While the JV(c,f) of the m. triceps of the
foreleg is significantly lower than in most other muscles, this is also the
case for the m. gastrocnenius and the m. vastus lateralis. The pattern in
JV(c,f) was also reflected in the capillary volume per g
of muscle, V(c)/gMm, because no significant
differences in capillary diameter were seen between muscles. The mean
capillary diameter was 4.51 µm, with the range of muscle means being
4.434.57 µm. Consequently, the ratio V(c)/V(mt,m)
also indicates the capillary blood supply to mitochondria in various muscles.
The muscle with highest capillary volume per unit of mitochondria was the m.
trapezius of the neck. This was followed by the m. triceps, m. coccygeus and
the m. erector spinae. Overall there was a significant negative correlation
(r=0.65, P<0.05) between V(c)/V(mt,m)
and the VV(mt,f) of various muscles
(Fig. 3A).
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The distribution of muscle in the regions of the body is shown in Table 3. The upper hindleg had significantly more muscle than other regions, containing 44.3% of the total skeletal muscle mass. This is followed by the trunk (25.6%), back (10.1%) and the lower leg (9.1%). The fore part of the body was lightly muscled. Notably, the foreleg carried only 3.9% of skeletal muscle. These patterns were also largely followed in the distribution of mitochondria and capillaries throughout the skeletal muscles, although the low VV(mt,f) of the foreleg and the head and neck resulted in these regions containing only 12% of the total volume of skeletal muscle mitochondria. With respect to capillary volumes, the trunk appears to have a higher V(c) relative to its muscle mass than other sections.
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Discussion |
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The core unit of metabolic capacity is the area of the inner mitochondrial
surface, SV(im,m); it has been consistently correlated
with the activity of the terminal respiratory chain enzyme in several
vertebrate groups (Else and Hulbert,
1981). In placental mammals the SV(im,m) of
muscle mitochondria appears relatively constant between muscles and across
species, with a value of
35 m2 cm3 (Else
and Hulbert, 1985; Hoppeler et al.,
1981a
; Schwerzmann et al.,
1989
). Consequently, mitochondrial volume has been used to assess
the aerobic potential of species
(Hoppeler, 1990
; Hoppeler and
Weibel, 1998
,
2000
;
Kayar et al., 1989
). The
SV(im,m) of a red kangaroo skeletal muscle was the same as
in placentals, 35.8±0.7 m2 cm3.
Essentially similar values (3538 m2 cm3)
occur in the skeletal muscles of other marsupials
(Webster, 2003
) and in the
heart muscles of the red kangaroos and other marsupials
(Dawson et al., 2003
).
Assuming the aerobic capacity per unit of membrane surface area is relatively
constant, the consistent SV(im,m) between the two therian
groups implies that we can use mitochondrial volume to assess the aerobic
potential of kangaroos.
The volume densities of mitochondria VV(mt,f) in the
muscles of red kangaroos are variable, 3.710.6%, with the
VV(mt,f) for the diaphragm (10.8%) generally being
significantly higher (Table 2).
Generally, the VV(mt,f) of red kangaroo muscles are higher
than in comparable muscles of placental mammals
(Else and Hulbert, 1985b;
Hoppeler et al., 1981b
;
Mathieu et al., 1981
) when
body size is accounted for because VV(mt,f) generally
increases with decreasing mass. There are placental mammals that do have
muscle VV(mt,f) comparable to kangaroos; these are
athletic species with a high aerobic capacity, i.e. a high
O2max. This is
reflected in the total body mitochondrial patterns (see
Table 4), but also in
VV(mt,f) of comparable muscles such as the diaphragm
(Hoppeler et al., 1987
). This
pattern of difference between athletic and sedentary placentals has been
clearly demonstrated in several paired comparisons
(Hoppeler and Weibel, 1998
;
Weibel et al., 1991
;
Weibel, 2000
).
Across different regions of the body of red kangaroos variation in the
VV(mt,f) of muscles is obvious
(Table 2). The muscles from the
proximal parts of the body, the neck and foreleg had a significantly lower
VV(mt,f); those with significantly higher
VV(mt,f) were the large muscles of the back and trunk, the
m. multifidi lumborum and m. erector spinae, together with the m. gluteus
medius of the upper hindleg. However, other large muscles of the hindleg, such
as the m. vastus lateralis, m. semitendinosus and m. gastrocnemius, had
intermediate values. Such a variable pattern was also found by Hoppeler et al.
(1981a) in two wild African
bovids, wildebeest and dik-dik. The amount of mitochondria in a muscle
presumably determines its longer term aerobic capacity rather than its total
capacity to do work over short periods. Consequently, variation in
VV(mt,f) between muscles may reflect the mix of fibre
types in muscles. Hoppeler et al.
(1981a
) found that
fast-twitch-glycolytic fibres (type IIb) typically had low
VV(mt,f), about 1%, whereas in oxidative fibres, both
slow-twitch-oxidative (type I) and fast-twitch-oxidative-glycolytic (type IIa)
had VV(mt,f) in the range 515%. Marsupial limb
muscles also can express slow and fast myosin proteins, resulting in fibres of
types I, IIa, IIx and IIb (Zhong et al.,
2001
). Dennington and Baldwin
(1988
) found that in the m.
gastrocnemius of the western grey kangaroo (Macropus fuliginosus) a
large majority of the muscle fibres were type IIa. The muscles of the trunk,
back and hindleg of the red kangaroo which have higher
VV(mt,f) are similarly deep red in colour and presumably
have large proportions of type IIa fibres. Data for the muscles of the foreleg
and head and neck are lacking, apart from the jaw-closing muscles, which
express a relatively slow cardiac myosin
(Hoh, 2002
).
Because of their fundamental role in aerobic metabolism it was anticipated
that mitochondria in muscles would have a matching supply of oxygen
via a proportional volume of capillaries and that the capillary
network of the body would match aerobic capacity, i.e.
O2max. However,
while previous studies with quadrupedal placental mammals have shown this to
be broadly the case, there is much variability
(Conley et al., 1987
;
Hoppeler et al., 1981b
). A
similar pattern also emerges in the red kangaroo
(Table 2). The m. erector
spinae and m. gluteus medius have a significantly higher volume of capillaries
per g of muscle (V(c)/gMm) but the large aerobic
m. multifidi lumborum of the back has a V(c)/gMm
similar to the m. trapezius. There was only a trend toward a positive
correlation between V(c)/gMm and
VV(mt,f) for red kangaroo muscles (r=0.58,
P=0.1) (Fig. 3B),
though when expressed as capillary density the values overlapped closely the
data of Hoppler et al. (1981b), who report a statistically significant
correlation from a larger data set.
Per unit volume of muscle mitochondria, the capillary supply was on average
16 km of capillaries, or 0.27 ml of capillary blood; this was similar to that
in muscles of several placental quadrupeds, the values being 14 km and 0.22
ml, respectively (Conley et al.,
1987). However, in both kangaroo muscles and the muscles of the
placentals the capillary supply is variable. For kangaroos the range was
1129 km ofcapillaries and 0.170.47 ml of capillary blood per ml
of mitochondria; the equivalent values for placentals being 1725 km and
0.180.45 ml. In fact, for the kangaroo muscles there was a significant
negative correlation between V(c)/VV(mt,m) and
VV(mt,f) (Fig.
3A). The mitochondria in m. trapezius had the highest blood supply
while that to the mitochondria in the large muscles of back and hindleg was
significantly lower (Table 2).
Conley et al. (1987
) found a
tendency for the muscles of aerobic species to have a lower capillary supply
than those of less aerobic species. They explain this apparent paradox by the
muscles of more aerobic (athletic) species being supplied with blood with a
higher hematocrit, i.e. a higher hemoglobin concentration. This explanation
would not seem to apply for the muscles in a single species. Does the answer
lie in a modulation of the hematocrit as an animal increases its aerobic
metabolism? As in placentals, the spleen in marsupials contains a large
reserve of erythrocytes (Dawson and Denny,
1968
). Could these be metered into the circulation as a mammal
increases energetic output, such that when the large mitochondria-dense
locomotor muscles are fully active they are supplied with hemoglobinrich
blood? An explanation based on differential fibre types
(Hoppeler et al., 1981a
) would
not seem to be appropriate if the kangaroo locomotor muscles have few type IIb
(fast-glycolytic) fibres.
How do these data on the mitochondrial and capillary characteristics of red
kangaroo muscles translate into a picture of the aerobic capacity of the whole
animal? Red kangaroo females shot in the field comprised 46.8±1.7%
skeletal muscle. Values of 4752% have been reported previously for red
kangaroos; variability can occur due to variable fill of the large foregut
(Table 1) and sexual dimorphism
(Grand, 1990;
Hopwood, 1981
;
Hopwood and Griffiths, 1984
;
Tribe and Peel, 1963
). A value
near 50% places kangaroos among the most muscular of mammals
(Table 4)
(Grand, 1990
). The muscle mass
of the red kangaroo is concentrated around the pelvis, particularly in the
upper hindleg (Fig. 1;
Table 3). Not only does the
upper hindleg make up 44.3% of all skeletal muscle, when the trunk and back,
which have their bulk posteriorly located
(Fig. 1), are included some 80%
of skeletal muscle is so positioned. This pattern does not just reflect the
energetic needs of a kangaroo's hopping. During slow-speed locomotion, often
called pentapedal locomotion because the tail is used as a `fifth leg', the
hindlegs are moved forward in unison when the body is supported by the tail
and the forelegs. J. M. Donelan, S. Rodoreda, A. Grabowski, R. Kram and T. J.
Dawson (unpublished observations) examined pentapedal locomotion and showed
that the small forelegs act only as a brake, while the hindlegs and the tail
successively provide the propulsive forces. Apparently the muscles of the tail
and back provide the propulsive forces during both forms of kangaroo
locomotion.
The volume of mitochondria in the body regions is primarily determined by regional muscle masses (Table 3). However, the upper hindleg, trunk and back regions also generally have high VV(mt,f) and contain 86% of all skeletal muscle mitochondrial volume. Thus, the majority of the aerobic power output associated with locomotion will be generated in this region. By contrast, the foreleg has only 2% of the total muscle mitochondrial volume. The lower hindleg, with its large Achilles tendon, has been the focus of interest in relation to the elastic conservation of energy during hopping, but it contains only 7% of muscle mitochondria. The distribution of capillary blood broadly follows the distribution of muscle mass, with the trunk being more endowed with capillaries (Table 3). The V(c) of the lower hindleg and the foreleg are relatively low. Combining these regional data establishes the overall aerobic characteristics of the red kangaroo (Tables 3 and 4).
To examine the possible phylogenetic and adaptive influences on muscle
morphometry in the red kangaroo, we compared our data and the
O2max data of
Kram and Dawson (1998
) with
data from placental species of similar size
(Weibel, 2000
), both sedentary
(goat) and athletic (dog and pronghorn)
(Table 4). The use of animals
of similar size removes effects due to allometry
(Weibel et al., 1987
). It is
obvious that the marsupial red kangaroo closely resembles the dog and
pronghorn in its
O2max, muscle
mass and the morphometry of its muscle mitochondria and capillaries
(Table 4). However, the ratio
of
O2max to
VV(mt,m) is similar for the red kangaroo and all the
placental species, whether athletic or sedentary
(Table 4). The close
relationship between
O2max and total
volume of skeletal muscle mitochondria holds for a broader range of placental
mammals, including those of different sizes (Hoppeler et al., 1990), as well
as the kangaroo (Fig. 4). The
mitochondrial oxygen consumption at
O2max calculated
for the red kangaroo was 4.7 ml O2 min1
ml1 of mitochondria, which is the same as the average for
placentals in general (Hoppeler,
1990
; Hoppeler and Weibel,
1998
). This feature is apparently conserved among therian mammals.
Because SV(im,mt) is the same in both therian groups
(
35 m2 ml1 mitochondria), the maximum oxygen
consumption per unit area of inner mitochondrial membrane appears also
invariant at
0.13 ml O2 min1
m2.
|
The whole body capillary volume of the red kangaroo is also related to
O2max in the
manner seen in placental quadrupeds (Table
4). As indicated by Conley et al.
(1987
), to appreciate the full
picture of the oxygen supply to muscle mitochondria the concentration of
haemoglobin in the blood has to be considered. The difference in
V(c)/Mb between sedentary species and athletic
species (including the kangaroo) is not equivalent to the difference in
O2max. However,
when hematocrit during exercise is taken into account
(Table 4) the capillary
erythrocyte volume V(ec)/Mb matches the various
levels of
O2max.
This is a pattern seen generally in placental mammals
(Hoppeler and Weibel, 1998
;
Jones et al., 1989
;
Weibel et al., 1991
). The
heart is intimately involved with the flux of oxygen through the body and the
red kangaroo's heart is also matched to a high
O2max in a
manner comparable to that seen in placental mammals
(Dawson et al., 2003
).
Bipedal humans diverge most from the relationship between
VV(mt,m) and
O2max
(Fig. 4). Humans are capable of
reaching
O2max
with a subset of their body musculature. Bergh et al.
(1976
) found that
O2max obtained
by concurrent arm and leg work did not result in a
O2max higher
than during running, and the
O2max was lower
than would be predicted by adding separately measured arm and leg
O2max. Unlike
humans, the red kangaroo involves almost its whole musculature to achieve
O2max during
exercise, despite its bipedal posture (Fig.
4).
In summary, the red kangaroo has characteristics fundamental to highly
aerobic mammals. The heart is large and levels of muscle mitochondria,
capillaries and hematocrit are essentially the same as those of athletic
placental mammals like dogs, horses and pronghorn antelope
(Weibel, 2000). These features
support the symmorphosis model of Taylor and Weibel
(1981
), Weibel et al.
(1991
) and Weibel
(2000
). The red kangaroo has
considerable muscle mass, about 50% of total body mass Mb,
which is largely situated around the pelvis and the upper hindlegs. The
locomotor force for both forms of locomotion, `walking' and hopping, is
provided by this region. Although its predominate gait is bipedal, and its
locomotor characteristics are unusual
(Dawson and Taylor, 1973
),
this marsupial seems to be just an extension of a basic mammalian
skeletal/muscular energetic pattern. The evolution of such a pattern obviously
predates the divergence of the marsupials and placentals before 125 million
years ago. Of note, it has been suggested that the basic mammalian aerobic
pattern was set with the mammal-like reptiles in the Triassic (220 million
years ago), before the actual evolution of the mammals
(Ruben, 1995
). Recent findings
regarding monotreme evolution (Musser,
2003
) would support this contention, monotremes having heart and
muscle mitochondrial characteristics which are essentially mammalian
(Else and Hulbert, 1985b
).
List of symbols and abbreviations
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Acknowledgments |
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References |
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