The scaling of postcranial muscles in cats (Felidae) II: Hindlimb and lumbosacral muscles

Abstract

In quadrupeds the musculature of the hindlimbs is expected to be responsible for generating most of the propulsive locomotory forces, as well as contributing to body support by generating vertical forces. In supporting the body, postural changes from crouched to upright limbs are often associated with an increase of body mass in terrestrial tetrapods. However, felids do not change their crouched limb posture despite undergoing a 300-fold size increase between the smallest and largest extant species. Here, we test how changes in the muscle architecture (masses and lengths of components of the muscle-tendon units) of the hindlimbs and lumbosacral region are related to body mass, to assess whether there are muscular compensations for the maintenance of a crouched limb posture at larger body sizes. We use regression and principal component analyses to detect allometries in muscle architecture, with and without phylogenetic correction. Of the muscle lengths that scale allometrically, all scale with negative allometry (i.e. relative shortening with increasing body mass), whereas all tendon lengths scale isometrically. Only two muscles' belly masses and two tendons' masses scale with positive allometry (i.e. relatively more massive with increasing body mass). Of the muscles that scale allometrically for physiological cross-sectional area, all scale positively (i.e. relatively greater area with increasing body mass). These muscles are mostly linked to control of hip and thigh movements. When the architecture data are phylogenetically corrected, there are few significant results, and only the strongest signals remain. None of the vertebral muscles scaled significantly differently from isometry. Principal component analysis and manovas showed that neither body size nor locomotor mode separate the felid species in morphospace. Our results support the inference that, despite some positively allometric trends in muscle areas related to thigh movement, larger cats have relatively weaker hindlimb and lumbosacral muscles in general. This decrease in power may be reflected in relative decreases in running speeds and is consistent with prevailing evidence that behavioural changes may be the primary mode of compensation for a consistently crouched limb posture in larger cats.

Full text

The scaling of postcranial muscles in cats (Felidae) II:
hindlimb and lumbosacral muscles
Andrew R. Cuff,
1,2
Emily L. Sparkes,
2
Marcela Randau,
1
Stephanie E. Pierce,
2,3
Andrew C. Kitchener,
4,5
Anjali Goswami
1,
* and John R. Hutchinson
1,2,
*
1
Department of Genetics, Evolution and Environment, University College London, London, UK
2
Structure and Motion Lab, Department of Comparative Biomedical Sciences, The Royal Veterinary College, Hatfield, Hertford-
shire, UK
3
Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, Cambridge,
MA, USA
4
National Museums Scotland, Edinburgh, UK
5
Institute of Geography, University of Edinburgh, Edinburgh, UK
Abstract
In quadrupeds the musculature of the hindlimbs is expected to be responsible for generating most of the
propulsive locomotory forces, as well as contributing to body support by generating vertical forces. In supporting
the body, postural changes from crouched to upright limbs are often associated with an increase of body mass in
terrestrial tetrapods. However, felids do not change their crouched limb posture despite undergoing a 300-fold
size increase between the smallest and largest extant species. Here, we test how changes in the muscle
architecture (masses and lengths of components of the muscle-tendon units) of the hindlimbs and lumbosacral
region are related to body mass, to assess whether there are muscular compensations for the maintenance of a
crouched limb posture at larger body sizes. We use regression and principal component analyses to detect
allometries in muscle architecture, with and without phylogenetic correction. Of the muscle lengths that scale
allometrically, all scale with negative allometry (i.e. relative shortening with increasing body mass), whereas all
tendon lengths scale isometrically. Only two muscles’ belly masses and two tendons’ masses scale with positive
allometry (i.e. relatively more massive with increasing body mass). Of the muscles that scale allometrically for
physiological cross-sectional area, all scale positively (i.e. relatively greater area with increasing body mass). These
muscles are mostly linked to control of hip and thigh movements. When the architecture data are
phylogenetically corrected, there are few significant results, and only the strongest signals remain. None of the
vertebral muscles scaled significantly differently from isometry. Principal component analysis and MANOVAs showed
that neither body size nor locomotor mode separate the felid species in morphospace. Our results support the
inference that, despite some positively allometric trends in muscle areas related to thigh movement, larger cats
have relatively weaker hindlimb and lumbosacral muscles in general. This decrease in power may be reflected in
relative decreases in running speeds and is consistent with prevailing evidence that behavioural changes may be
the primary mode of compensation for a consistently crouched limb posture in larger cats.
Key words: anatomy; biomechanics; effective mechanical advantage; locomotion; mammal; morphometrics.
Introduction
In terrestrial tetrapods, where there are evolutionary
increases in body masses there tend to be changes in limb
posture from crouched to upright to avoid potential
increases in stresses within the supportive tissues, whose rel-
ative strengths tend not to vary (Biewener, 1989, 1990,
2005). Extant felids are unusual in that they maintain the
same crouched posture from the smallest species to the lar-
gest (Day & Jayne, 2007) throughout their ~1–300 kg range
of body masses (Cuff et al. 2015). In addition, felids mostly
capture prey using ambushes and short, high-speed pur-
suits. Larger felids (above cheetah, Acinonyx jubatus,size)
seem to suffer from reduced locomotor performance
relative to their smaller relatives (e.g. range of speeds: Gar-
Correspondence
Andrew R. Cuff, Department of Genetics, Evolution and Environ-
ment, University College London, Darwin Building, Gower Street,
London, WC1E 6BT, UK. E: Andrew.Cuff@ucl.ac.uk
*Joint senior authors.
Accepted for publication 1 March 2016
©2016 Anatomical Society
J. Anat. (2016) doi: 10.1111/joa.12474
Journal of Anatomy

land, 1983; Day & Jayne, 2007), which may be emphasised
more strongly in Felidae than in some other mammals due
to their conserved limb postures. Previous work on the scal-
ing of the limb bones in felids shows that long bone lengths
in both the hind- and forelimbs scale isometrically with
body mass (Anyonge, 1993; Christiansen & Harris, 2005;
Doube et al. 2009). However, diameters and cross-sectional
areas of those bones scale with positive allometry, meaning
long bones become relatively more robust (and stiffer and
stronger as a consequence) in larger felid species (Doube
et al. 2009; Meachen-Samuels & Van Valkenburgh, 2009,
2010; Lewis & Lague, 2010). Similar patterns have been
found for vertebral dimensions in felids, indicating that
some degree of skeletal allometry may help to support
loads on the spine that might otherwise incur greater stres-
ses as body mass increases. However, the lumbar region
tends to show relatively weaker allometry than is observed
in the cervicothoracic regions (Jones, 2015; Randau et al. in
press).
Muscles generate greater moments around joints partly
by increasing moment arms (i.e. by lengthening the dis-
tance of muscle action from the joint), increasing the
mechanical advantage of the muscles; e.g. as potentially
present for the M. gastrocnemius on felid calcanei (G
alvez-
Lop
ez & Casinos, 2012). Although larger animals might not
forestall increases in tissue stresses if they do not straighten
their limbs to increase their limbs’ effective mechanical
advantage (EMA) (Biewener, 1989, 1990, 2005), maintaining
a crouched posture at larger body sizes may otherwise
increase the ability to generate horizontal (as opposed to
vertical) forces, needed in accelerations and manoeuvring.
As the hindlimbs generally are the main propulsive drivers
in the locomotion of felids, their muscles must be able to
provide forces and power that are capable of generating
the required forward movement and acceleration. Across
mammalian quadrupeds, this force requirement tends to be
largely achieved through an increase of the volume of hip
extensor musculature (Alexander et al. 1981; Usherwood &
Wilson, 2005; Williams et al. 2008, 2009). The same or simi-
larextensor(e.g.antigravity)musclesmustalsobeableto
support the animal’s body weight. The impulse (force-time
integral) required for this support is equivalent to the pro-
duct of the animal’s body weight and stride time (Alexan-
der & Jayes, 1978). At faster speeds the foot is in contact
with the ground for a shorter period of time (shorter stance
time) and a smaller proportion of the stride (decreasing
duty factor). Therefore, peak limb force must increase
(Witte et al. 2004) and the muscles must be able to gener-
ate larger amounts of forces and joint moments to sustain
this limb force.
In addition, during the swing phase the hindlimbs must
be protracted quickly enough to reposition them in time
for the next stance phase. This capacity for limb protraction
is limited by the limbs’ inertia (Lee et al. 2004), the internal
muscle architecture (including maximal contraction velocity
of the muscle fibres), and the moment arms of the muscles
(Hudson et al. 2011a,b). In fast-running tetrapods there
tends to be a reduction in muscle mass towards the distal
ends of limbs, in which the distal muscles transmit their
forces down long tendons (Alexander et al. 1981; Alexander
& Jayes, 1983; Payne et al. 2005; Smith et al. 2006, 2007;
Hudson et al. 2011a,b). This tapering of the limbs reduces
their inertial properties and therefore reduces the amount
of power that would otherwise be required from the mus-
cles to swing the limb (Hudson et al. 2011b). Additional
energy savings are achieved by using long tendons to store
elastic strain energy, contributing to the bouncing dynamics
of locomotion and enabling the muscles to remain closer to
optimal isometric activity during steady-state locomotion
(Alexander, 1984; Alexander & Maloiy, 1989). In addition to
the limbs, the vertebral musculature is important for loco-
motion in quadrupeds, whether being used in active
dynamic flexion and extension of the spine, or for stabilisa-
tion of the spine in larger taxa (Boszczyk et al. 2001).
Here we measure the architecture of the musculature of
the hindlimb and lumbosacral vertebrae in a range of
felid species, spanning almost their full spectrum of body
sizes, to quantify patterns of musculoskeletal scaling and
interpret their biomechanical consequences. This work fol-
lows that of Cuff et al. (in press) on scaling of the fore-
limb, cervical and thoracic musculature across extant
felids. We hypothesise that, as in the forelimbs (Cuff et al.
in press), many of the muscles involved in limb and body
support scale with positive allometry such that the muscles
are more adept at supporting the increasing body masses.
We further hypothesise that muscle fascicles scale with
negative allometry (i.e. shortening), whereas tendons scale
with positive allometry (i.e. lengthening), as is common in
other cursorial tetrapods (Alexander, 1977; Pollock & Shad-
wick, 1994a,b). We finally predict that, as with the cervico-
thoracic vertebral muscles (Cuff et al. in press), the lum-
bosacral musculature scales indistinguishably from isome-
try.
Methods
Muscle data collection
The methodological protocol used here is identical to that
described in detail in Cuff et al. (in press). In brief, the spe-
cies studied in this study were the black-footed cat (Felis
nigripes: NMS.Z.2015.90; male), domestic cat (Felis catus:
Royal Veterinary College, JRH uncatalogued personal collec-
tion; female), caracal (Caracal caracal: NMS.Z.2015.89.1;
male), ocelot (Leopardus pardalis: NMS.Z.2015.88; male),
cheetah (Acinonyx jubatus: data from Hudson et al. 2009a,
b), snow leopard (Panthera uncia: NMS.Z.2015.89.2; female),
jaguar (Panthera onca: NMS.Z.2014.67.2; female), Sumatran
tiger (Panthera tigris sondaica: NMS.Z.2015.91; female), and
Asian lion (Panthera leo persica: NMS.Z.2015.128; female)
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al.2

(Table 1). No specimens were euthanised for the purposes
of this research. The institutional abbreviation NMS refers
to the National Museums Scotland, Department of Natural
Sciences. All body mass and dissection data are included in
the Supporting Information (Table S1).
Dissection
All specimens were frozen shortly after death and then
defrosted (variably 24–48 h) prior to dissection except the
Asian lion, which was dissected 1 day postmortem without
any freezing or thawing. Initially, each specimen had the
limbs from one side removed and refrozen, allowing for
future dissection if the initial material was incomplete or
damaged. The muscles from the hindlimb and vertebral col-
umn were dissected individually and muscle architecture
was measured following standard procedures (e.g. Alexan-
der et al. 1981; Hudson et al. 2011a). For each muscle the
following architectural parameters were measured: muscle
belly length and mass, tendon length and mass, muscle fas-
cicle length and pennation angle (at least three for each
muscle, but up to 10 for some specimens, depending on
muscle size and variation of fascicle dimensions). These data
were used to calculate physiological cross-sectional area
(PCSA) for each muscle using Eq. 1:
Muscle volume ¼Muscle mass density;ð1Þ
where density is 1060 kg m
3
(typical vertebrate muscle,
Mendez & Keys, 1960), and then with Eq. 2:
PCSA ¼muscle volume cosðpennation angleÞ
fascicle length ð2Þ
In total 38 hindlimb muscles were measured for all nine
species, producing up to 228 metrics per species, and three
vertebral muscles, producing up to 18 metrics per species.
For most species, fewer than 12 metrics were missing in
total. The exception is the cheetah, as the data taken from
Hudson et al. (2011a) yielded only 50% completeness for
hindlimb measures (only muscle mass, fascicle length and
PCSA were usable; no tendon measurements were pro-
vided).
Scaling (regression) analysis
The data for muscle belly length and mass, tendon length
and mass, fascicle length, and PCSA were subjected to a ser-
ies of scaling analyses. Where tendon lengths and masses
could not be measured (because there were no tendons),
those data were removed before scaling analyses. Metrics
for which there were data from fewer than three species
were removed, but only metrics with at least six measures
will be discussed (although the results from metrics with
fewer measures, if significant, are displayed in Tables 1–6).
Thedatawerelog
10
-transformed, and then each logged
metric was regressed against log
10
body mass, using stan-
dardised reduced major axis (SMA) regression in the ‘smatr’
package (Warton et al. 2012) in R3.1.0 (R Core Team, 2014)
software. Significances of the regression line relative to
isometry and the correlation (r
2
) between each metric and
body mass were determined using bootstrapped 95% confi-
dence intervals (2000 replicates). Isometry is defined as scal-
ing patterns that match the slope expected for a given
increase in body size (i.e. maintaining geometric similarity),
and allometry represents increases or decreases from that
slope. For the logged metrics, isometry is defined as follows:
muscle or tendon masses scale against body mass with slope
equal to 1.00; muscle or tendon lengths scale against body
mass with a slope of 0.333 (i.e. length is proportional to
mass
1/3
); and muscle PCSA scales against body mass with a
slope of 0.667 (i.e. area is proportional to mass
2/3
).
As closely related species tend to have characteristics
more similar to each other, and as in felids large body
masses are only found in a few clades (Cuff et al. 2015), we
tested variables for phylogenetic signal. Each variable was
analysed using the phylosignal function in the ‘picante’
package (Kembel et al. 2010) in R, which measures phyloge-
netic signal using the K statistic. The phylogeny used for this
analysis was from Piras et al. (2013), which was pruned to
include only the taxa in this study. Metrics which were
found to have significant phylogenetic signal underwent
Table 1 Specimens dissected in this study. Sex: F =female, M =Male or Mix =both (unspecified).
Common name Species Sex Body mass (kg) General condition
Black-footed cat Felis nigripes F 1.1 Underweight
Domestic cat Felis catus F 2.66 Underweight
Caracal Caracal caracal M 6.6 Underweight
Ocelot Leopardus pardalis M 9.6 Overweight
Cheetah Acinonyx jubatus Mix 33.1 average Unknown
Snow leopard Panthera uncia F36OK
Jaguar Panthera onca F44OK
Sumatran tiger Panthera tigris sondaica F86OK
Asian lion Panthera leo persica F 133 Overweight
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al. 3

correction using independent contrasts in R, before the con-
trast data were subjected to SMA, as implemented in the
‘smatr’ package (Warton et al. 2012) in R. However, as phy-
logenetic SMA does not tolerate missing data, each metric
was analysed independently, dropping any taxa with miss-
ing data for that metric.
Principal components analysis and MANOVAs
Principal component (PC) analyses were also carried out on
the unlogged muscle data. As PC analyses require complete
datasets, any missing values were imputed based on
observed instances for each variable, using R3.1.2 software.
The imputed data were calculated iteratively until conver-
gence was achieved (German & Hill, 2006; Ilin & Raiko,
2010). The resulting ‘complete’ dataset was entered into
PAST 2.17c (Hammer et al. 2001) software. The ‘allometric vs.
standard’ option within the ‘removesizefromdistances’
tool was used to remove the effects of body size upon the
metrics. The felid species were assigned to groups first by
body size (i.e. small cat vs. big cat species, following Cuff
et al. 2015; although here defined as Panthera vs. non-Pan-
thera species), and in a second analysis by locomotor mode
(following Meachen-Samuels & Van Valkenburgh, 2009; ter-
restrial: F. nigripes,Acinonyx jubatus,P. tigris,Panthera leo;
scansorial: F. silvestris,C. caracal,L. pardalis,P. uncia,
Table 2 RMA results for log muscle belly lengths against log body mass, displaying only those that differ significantly from an isometric slope
value of 0.333. Results with significant r
2
are indicated in bold. No results were significant after phylogenetic correction. Upper and lower limits
represent 95% confidence intervals of the slope, ‘slope P’ represents statistical probability of the slope differing from isometry, the ‘r
2
P’ shows
the statistical significance of the correlation. All results including non-significant patterns are provided in Supporting Information.
Muscle Slope Lower limit Upper limit slope PIntercept r
2
r
2
Pn
Before correction
Piriformis 0.167 0.101 0.276 0.013 1.43 0.722 0.008 8
Peroneus brevis 0.192 0.112 0.330 0.047 1.14 0.677 0.012 8
Soleus 0.212 0.147 0.304 0.021 1.06 0.863 0.001 8
Gastrocnemius medialis 0.262 0.216 0.317 0.022 1.14 0.963 0.000 8
Semitendinosus 0.279 0.242 0.322 0.023 0.980 0.980 0.000 8
After correction
None
Table 3 Significant RMA (before and after phylogenetic correction) scaling results for log tendon lengths plotted against log body mass, displaying
only those that differ from an isometric slope value of 0.333. Results with significant r
2
shown in bold. Column headings as in Table 2.
Muscle Slope Lower limit Upper limit slope PIntercept r
2
r
2
Pn
Before correction
Superficial dig. flex. 0.887 0.369 2.134 0.031 2.48 0.007 0.846 8
After correction
None
Table 4 Significant RMA (before and after phylogenetic correction) scaling results for log muscle fascicle lengths plotted against log body mass,
displaying only those that differ from an isometric slope value of 0.333. Results with significant r
2
are shown in bold. Column headings as in
Table 2.
Muscle Slope Lower limit Upper limit slope PIntercept r
2
r
2
Pn
Before correction
Dig. ext. lateralis 0.185 0.300 0.114 0.022 1.26 0.684 0.006 9
Vastus intermedius 0.617 0.374 1.018 0.021 2.15 0.659 0.008 9
Peroneus brevis 0.716 0.349 1.469 0.038 2.60 0.234 0.187 9
Psoas major 0.936 0.417 2.101 0.019 2.11 0.580 0.078 6
Adductor magnus 1.20 0.567 2.523 0.002 2.02 0.162 0.282 9
After correction
None
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al.4

P. onca). Significant PC scores were then tested for body
size and locomotory signal using MANOVAs with and without
phylogenetic correction in the ‘geomorph’ package (Adams
& Otarola-Castillo, 2013) in R.
Results
Limb muscles
Prior to phylogenetic correction the belly lengths for M. pir-
iformis, M. peroneus brevis, M. soleus, M. gastrocnemius
medialis and M. semitendinosus all displayed significant
negative allometry (i.e. relative shortening as body mass
increases) (Table 2, Fig. 1). After phylogenetic correction,
only the M. soleus remained significantly negatively allo-
metric (Table 2, Fig. 2). None of the tendon lengths exhib-
ited significant allometry before or after phylogenetic
correction (Table 3). Prior to phylogenetic correction, the
fascicle lengths for M. extensor digitorum lateralis and M.
vastus intermedius showed significant allometry: the M. lat-
eral digital extensor fascicles scaled with negative allometry
(again, relative shortening), and M. vastus intermedius
scaled with positive allometry (Table 4). After phylogenetic
correction, no fascicle lengths scaled significantly differently
from isometry (slope of 0.333) (Table 4).
For the muscle belly masses, two muscles initially
showed significant allometry; the M. vastus intermedius
scaled with negative allometry (i.e. relatively less massive
with increasing body mass) and the M. gluteus medius
scaled with positive allometry (Table 5, Fig. 1). After phy-
logenetic correction, only the M. gluteus medius retained
significantly positive allometry (Table 5, Fig. 2). The ten-
don masses for the M. psoas major and M. extensor digi-
torum longus both showed significant positive allometry
prior to phylogenetic correction, but no tendon masses
scaled significantly differently from isometry after phylo-
genetic correction (Table 6). Before phylogenetic correc-
tion, seven muscles’ PCSAs scaled with positive allometry
(Table 7, Fig. 1) (i.e. relatively greater area with increasing
body mass): the M. gluteus medius, M. gemelli, M. biceps
femoris, M. tensor fascia latae, M. caudofemoralis, M. tib-
ialis caudalis, and the M. tibialis cranialis. After phyloge-
netic correction, only the PCSA of the M. tibialis cranialis
remained significantly positively allometric with body mass
(Table 7, Fig. 2).
Vertebral muscles
None of the vertebral muscle metrics showed significant dif-
ference from isometry either before or after phylogenetic
correction (Supporting Information Table S2).
Principal components analyses and phylogenetic
MANOVAs
PCA of all of the metrics for the hindlimb muscles alone
produced eight significant PC axes according to the Joliffe
cutoff, which is automatically generated in PAST.PC1repre-
sented 28.5% of the total variance, PC2 was 15.4%, with
PC3-8 representing between 12.8 and 4.5% (Fig. 3). There
was no significant separation between body size or locomo-
tory groups using either a MANOVA or phylogenetic MANOVA of
all PCs (P≫0.05 in all analyses). Adding data from lum-
bosacral vertebral muscles did not improve the ability to
Table 5 Significant RMA (before and after phylogenetic correction) scaling results for log muscle body mass plotted against log body mass, dis-
playing only those that differ from an isometric slope value of 1.00. Results with significant r
2
are shown in bold. Column headings as in Table 2.
Muscle Slope Lower limit Upper limit slope PIntercept r
2
r
2
Pn
Before correction
Vastus intermedius 0.796 0.650 0.976 0.033 2.619 0.947 0.000 9
Gluteus medius 1.22 1.12 1.33 0.001 2.800 0.991 0.000 9
After correction
Gluteus medius 1.25 1.08 1.45 0.010 0.010 0.978 0.000 9
Table 6 Significant RMA (before and after phylogenetic correction) scaling results for log tendon mass plotted against log body mass, displaying
only those that differ from an isometric slope value of 1.00. Results with significant r
2
are shown in bold. Column headings as in Table 2.
Muscle Slope Lower limit Upper limit slope PIntercept r
2
r
2
Pn
Before correction
Long dig. ext. 1.57 1.06 2.31 0.029 4.610 0.841 0.001 9
Superficial dig. flex. 1.71 1.15 2.54 0.014 4.47 0.836 0.001 8
Psoas major 1.72 1.08 2.76 0.042 5.129 0.999 0.024 7
After correction
None
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al. 5

distinguish among either body size or locomotor groupings
(P≫0.05).
Discussion
In quadrupeds, the hindlimbs are usually the main propul-
sive drivers (Alexander, 1977; Alexander et al. 1981; Hudson
et al. 2011a), and as such play more roles than just limb-
maintaining support against gravity. The muscles responsi-
ble for such roles are primarily the hip extensors (Alexander,
1977; Alexander et al. 1981; Usherwood & Wilson, 2005;
Williams et al. 2008, 2009; Hudson et al. 2011a). Therefore it
should be expected that these muscles will scale with at
least isometry, or possibly positive allometry, for the muscle
belly measurements and PCSA (a metric which is linked to
force production). Our results showed that most thigh mus-
cle metrics actually scaled isometrically, or at least with
allometry that is indistinguishable from isometry, in our
dataset. In the thigh only the M. gluteus medius, M. tensor
fascia latae, M. caudofemoralis and M. biceps femoris have
Fig. 1 Muscles displaying potential allometry (prior to phylogenetic analysis) in the studied felid species are shown in colour; others as white; for a
representative right hindlimb. (A) Lateral superficial muscles of hip and knee. (B) Lateral, deeper muscles of the hindlimb. (C) Medial muscles of
the thigh and shank. (D) Lateral muscles of the lower leg. (E) Medial muscles of the lower leg. Red =muscle belly length; orange =tendon length;
navy blue =muscle mass; light blue =tendon mass; green =PCSA. Stippling pattern is for negative allometry. Muscles not shown: M. psoas majo-
rum (Table 6); M. vastus intermedius (Tables 4 and 5); M. lateral digital extensor (Table 4); M. superficial digital flexor (Table 6); M. peroneus bre-
vis (Table 2).
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al.6

PCSAs that scale positively allometrically, with the M. biceps
femoris (weakly positively allometric), and the M. gluteus
medius being responsible for thigh extension (the rest are
used in adduction or rotation). Because the muscles’ cross-
sectional areas scaled isometrically proportional to mass
2/3
,
most muscles of the thigh appear to be relatively weaker in
larger species of felids.
In quadrupeds able to move rapidly, as taxa become lar-
ger, there tends to be a reduction in muscle mass towards
the distal ends of limbs, in which the distal muscles transmit
their forces down long tendons (Alexander & Jayes, 1983;
Payne et al. 2005; Smith et al. 2006, 2007). Cheetahs have
been noted to exhibit some similar degree of limb tapering
(Hudson et al. 2011a,b). This reduction of distal limb muscle
mass does not appear to be the case in felids in general,
with all distal muscles’ masses scaling isometrically, and only
the tendon mass of M. extensor digitorum longus scaled
with positive allometry. In felids, this would result in an
increase in inertial properties and therefore require more
work and power from the muscles to swing the hindlimbs
(Hudson et al. 2011b), and with no apparent increase in
elastic energy storage by the tendons (Alexander, 1984;
Fig. 2 Muscles displaying potential allometry (after phylogenetic analysis) in the studied felid species are shown in colour; others as white; for a
representative right hindlimb. (A) Lateral superficial muscles of hip and knee. (B) Lateral, deeper muscles of the hindlimb. (C) Medial muscles of
the thigh and shank. (D) Lateral muscles of the lower leg. (E) Medial muscles of the lower leg. Navy blue =muscle mass; green =PCSA. Stippling
pattern is for negative allometry.
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al. 7

Alexander & Maloiy, 1989), thereby reducing the overall
efficiency of the hindlimbs in larger taxa. This may be
because most felids have to retain limbs that are powerful
enough for climbing and capturing prey as well as being
‘light’ enough for fast locomotion. Perhaps owing to its fast
pursuit of prey, the cheetah is the only felid that shows
marked limb tapering and, as a consequence of its less pow-
erful limbs, tends to feed on relatively smaller prey. Interest-
ingly, a few muscle belly lengths actually scale with
negative allometry (Table 4), but this length is not compen-
sated for in any way with positively allometric tendons or
muscle fascicles that display unambiguous negative allome-
try. Previous work indicates that the bone lengths of felid
limbs scale isometrically (Anyonge, 1993; Christiansen &
Harris, 2005; Doube et al. 2009), but if there is a shortening
of some muscle bellies, and no corresponding increase in
tendon lengths, there may potentially be some subtle posi-
tional changes of these muscles between the taxa or an
increase in musculotendinous compliance (Roberts, 2002).
Alternatively, with the small sample size, there may just be
some outliers within our data, but this would require more
specimens to test.
The lack of general allometric increase in muscle PCSAs
suggests that felid limbs become relatively weaker at larger
body sizes, especially with no reduction in distal limb muscle
mass and no increase in tendon masses or lengths across
most of the limb, and no change in limb posture (Day &
Jayne, 2007; Zhang et al. 2012; Doube et al. 2009) as alter-
native compensatory mechanisms. As terrestrial mammals
get larger, maintaining a crouched posture becomes
increasingly energetically expensive due to the muscles of
the limbs having to balance the moments incurred by the
body weight, and the resulting vertical ground reaction
forces. The advantage of remaining crouched is that it max-
imises the horizontal component of the ground reaction
forces’ moment arms, potentially allowing for increased
locomotor performance in a horizontal direction (Biewener,
1989, 1990, 2005). However, as felid limb posture does not
seem to change with body mass and the muscle force-capa-
cities (linked to PCSA) appear to decrease, it might be pre-
dicted that larger felids become relatively slower and incur
greater metabolic costs during similar behaviours due to
lower mechanical efficiency. Indeed, Day & Jayne (2007)
found that the velocity of locomotion within felids (during
Table 7 Significant RMA (before and after phylogenetic correction) scaling results for log physiological cross-sectional area plotted against log
body mass, displaying only those that differ from an isometric slope value of 0.667. Results with significant r
2
are shown in bold. Column headings
as in Table 2.
Muscle Slope Lower limit Upper limit slope PIntercept r
2
r
2
Pn
Before correction
Biceps femoris 0.862 0.680 1.09 0.037 4.18 0.929 0.000 9
Caudal tibial 0.977 0.790 1.21 0.003 4.90 0.943 0.000 9
Gluteus medius 1.00 0.769 1.31 0.008 4.39 0.910 0.000 9
Tensor fascia latae 1.05 0.725 1.52 0.022 4.75 0.821 0.001 9
Gemelli 1.10 0.739 1.64 0.021 5.05 0.832 0.002 8
Tibialis cranialis 1.12 0.847 1.49 0.003 5.08 0.897 0.000 9
Caudofemoralis 1.17 0.781 1.74 0.012 5.40 0.788 0.001 9
After correction
Tibialis cranialis 1.14 0.698 1.85 0.036 0.017 0.743 0.006 9
Caudofemoralis 1.32 0.680 2.56 0.045 0.036 0.491 0.053 9
Fig. 3 Principal component analysis of hind
limb muscle architecture metrics. (A,B) Body
size groups, with blue for small felids and
orange for large felids (groupings follow Cuff
et al. 2015). (C,D) Locomotory mode groups
with red for terrestrial and pink for scansorial.
(A,C) PC1 (28.48% of total variance) vs. PC 2
(15.39% of total variance). (C,D) PC3
(12.83% of total variance) vs. PC 4 (11.24%
of total variance).
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al.8

walking) is broadly similar across all species, consistent with
the theory of dynamic similarity (Alexander & Jayes, 1983).
Furthermore, Garland (1983) found that larger cats (beyond
an optimal body mass of ~40 kg) move more slowly than
smaller ones. However, felids may partially compensate for
the near-isometric muscle scaling by the seemingly
increased mechanical advantage of the felid calcaneus
(G
alvez-Lop
ez & Casinos, 2012). Although evidence for
allometry of that mechanical advantage is not strong, if
present it may help counter the isometric scaling of the gas-
trocnemius, which is the largest (in terms of PCSA and thus
force potential) antigravity muscle in the hindlimb,
although further work is required on both the muscles and
bones.
Muscle fascicle lengths are linked to contractile speed
and range of motion, with longer fascicles able to contract
faster and over a longer range of motion than smaller ones
(Alexander, 1977; Alexander et al. 1981). Typically for most
Carnivora, the fascicle lengths scale indistinguishably from
isometry across the hindlimb (Alexander et al. 1981). Our
results broadly fit this pattern of near-isometric scaling,
with one exception. In our dataset, inverse allometry of
muscle fascicle length (where the slope is actually negative
rather than only less than the isometric slope) was detected
for the M. digitorum extensor lateralis. Thus, bigger cats
have shorter fascicle lengths (in an absolute and relative
sense) than smaller cats for the M. digitorum extensor later-
alis, which becomes increasingly multipennate in form,
resulting in a slower digital extension or more limited range
of motion in larger cat species. What role this may play in
their ecology and locomotion is, however, uncertain.
The limb muscles, nonetheless, do not work in isolation;
the vertebral muscles also play important roles in support
and locomotion. All vertebral muscles’ metrics from the
lumbosacral region scale isometrically in felids; therefore
the vertebral muscles also seem to become relatively
weaker with increasing body mass. However, this relative
weakening of the musculature of the vertebral muscles may
be compensated for by positive allometry of vertebrae and
the resulting moment arms in other vertebral regions
(Jones, 2015; Randau et al. in press). The combined results
for the vertebral muscles (here and Cuff et al. in press) show
that there is a relative reduction in force production capac-
ity in the spinal musculature of larger felids. This lack of
clear allometry of the intervertebral musculature may have
consequences for the maximum extension of the spine (a
vital component in maximising stride length and, therefore,
maximum speed: Hildebrand, 1959), although positive
allometry in the lever arms may compensate (Jones, 2015;
Jones & Pierce, 2016). However, how the complex interac-
tions of musculoskeletal anatomy, limb posture, range of
spinal motion and gait relate to tissue stresses or safety fac-
tors across the body size range of Felidae remains unclear
and deserves further study. We also accept there are limita-
tions to the current study as all the individuals were captive,
of varying degrees of health, and all of our measurements
were from a single individual from each species (or, in the
case of the lion and tiger, a single subspecies), and not all
ofthesamesex(withthelargest species all represented by
females), but we have no reason to expect this would
change our overall conclusions. For a more in-depth discus-
sion of these limitations see Cuff et al. (in press).
In the forelimbs of felids, only those metrics with the
strongest allometric signals remained significantly different
from isometry after phylogenetic correction (Cuff et al. in
press), and indeed broadly similar results were obtained
for the hindlimbs of felids, with only two metrics of 228
displaying allometry after correction. With so many mus-
cles scaling indistinguishably from isometry (or scaling only
weakly allometrically), there is no separation of the taxa
using PCAs or MANOVAs when assessing body mass group-
ings (Cuff et al. 2015) or locomotor mode either before or
after phylogenetic correction. This will remain an issue in
muscle scaling studies at least until larger sample sizes are
studied, particularly in felids, with many of the largest
felids being closely related members of the genus Pan-
thera (the exceptions being the cheetah and puma, which
convergently evolved larger body sizes: Cuff et al. 2015).
This close relationship of large-bodied felids (i.e. Panthera)
means that any potentially allometric patterns are more
difficult to tease apart from the null hypothesis of similar-
ity due to common ancestry, and it is thus more difficult
to distinguish modest allometry from true isometry in the
musculoskeletal system of Felidae. However, the dataset
provided here is an important step forward in understand-
ing how felid locomotor muscles scale with body mass,
and future efforts can test our findings by building on this
dataset.
Conclusions
Unlike the predominantly supportive, deceleratory and pre-
hensile roles of the forelimb muscles, the musculature of
the hindlimb is responsible for generating most of the
acceleratory forces during typical (e.g. steady-state) locomo-
tion in felids. However, the majority of propulsive (and
other) hindlimb muscles appear to scale isometrically across
Felidae, with only the strongest allometries remaining sig-
nificant after phylogenetic correction. As a consequence,
larger felids have relatively weaker hindlimb muscles than
those of their smaller relatives, consistent with the reduc-
tion in relative and even absolute locomotor speeds as
observed in other studies (Garland, 1983; Day & Jayne,
2007). The vertebral muscles emphasise these results fur-
ther, with all of the metrics scaling indistinguishably from
isometry. Furthermore, multivariate analysis (PCA) of muscle
metrics was unable to distinguish between locomotor
modes and body mass difference, which may be due in part
to the phylogenetic proximity of most large- and small-
bodied felids (Cuff et al. 2015).
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al. 9

Acknowledgements
This work was funded by Leverhulme Trust grant RPG 2013-124 to
A.G. and J.R.H. A.C.K. thanks the Aspinall Foundation (Port Lympne
Wild Animal Park), the Zoological Society of East Anglia (Banham
Zoo), the Cat Survival Trust, Thrigby Hall Wildlife Gardens, Cromer
Zoo and the Zoological Society of London (London Zoo) for dona-
tion of specimens used in this study. A.C.K. is grateful to the Negau-
nee Foundation for its support of the Curatorial Preparator at
National Museums Scotland. We thank two anonymous reviewers
for comments that improved the manuscript.
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Supporting Information
Additional Supporting Information may be found in the online
version of this article:
Table S1. Raw muscle architecture measures for hindlimb and
lumbosacral vertebrae.
Table S2. RMA results for all muscles before and after phyloge-
netic correction.
©2016 Anatomical Society
Hindlimb muscle scaling in cats, A. R. Cuff et al. 11