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Small vertebrates running on 
uneven terrain: a biomechanical 
study of two differently specialised 
lacertid lizards
françois Druelle  1*, Jana Goyens
1, Menelia Vasilopoulou-Kampitsi1 & peter Aerts1,2
While running, small animals frequently encounter large terrain variations relative to their body size, 
therefore, terrain variations impose important functional demands on small animals. Nonetheless, we 
have previously observed in lizards that running specialists can maintain a surprisingly good running 
performance on very uneven terrains. The relatively large terrain variations are offset by their capacity 
for leg adjustability that ensures a ‘smooth ride’ of the centre of mass (CoM). The question as to how 
the effect of an uneven terrain on running performance and locomotor costs differs between species 
exhibiting diverse body build and locomotor specializations remains. We hypothesise that specialized 
runners with long hind limbs can cross uneven terrain more efficiently than specialized climbers with a 
dorso-ventrally flattened body and equally short fore and hind limbs. This study reports 3D kinematics 
using high-speed videos (325 Hz) to investigate leg adjustability and CoM movements in two lacertid 
lizards (Acanthodactylus boskianus, running specialist; Podarcis muralis, climbing specialist). We 
investigated these parameters while the animals were running on a level surface and over a custom-
made uneven terrain. We analysed the CoM dynamics, we evaluated the fluctuations of the positive and 
negative mechanical energy, and we estimated the overall cost of transport. Firstly, the results reveal 
that the climbers ran at lower speeds on flat level terrain but had the same cost of transport as the 
runners. Secondly, contrary to the running specialists, the speed was lower and the energy expenditure 
higher in the climbing specialists while running on uneven terrain. While leg movements adjust to the 
substrates’ variations and enhance the stability of the CoM in the running specialist, this is not the case 
in the climbing specialist. Although their legs are kept more extended, the amplitude of movement does 
not change, resulting in an increase of the movement of the CoM and a decrease in locomotor efficiency. 
These results are discussed in light of the respective (micro-)habitat of these species and suggest that 
energy economy can also be an important factor for small vertebrates.
Locomotion requires mechanical work to counter inertia (and gravity when moving upwards) and to overcome 
resistive forces from the environment. Issues relating to substrate structure and organisation alter the locomotion 
of animals, and adaptations for ecologically-relevant ways of moving can be found in various aspects of the animal 
biological system1–3. The design of the limbs4,5, the type of gait6 and the posture7 can, therefore, influence loco-
motor performance and efficiency. Relative to their body size, small animals are more prone to encounter large 
terrain variations than larger animals do. Apart from the fact that their locomotor cost (J/kg/m) is already high 
compared to large animals8–10, terrain structure and organisation at the scale relevant to the animal may impose 
important additional energetic challenges in small animals. For instance, uneven terrain requires manoeuvring 
and intermittent running to bypass obstacles or, can require moving up and down along the running trajectory 
to cross obstacles. Therefore, running over uneven terrains will unavoidably result in both perturbations of the 
overall goal directed movement as well as higher costs.
Encountering large terrain variation relative to body size is a very common scenario for small lizards. 
Investigating the impact of an uneven substrate on the kinematics of running lizards is, therefore, essential to 
gain insight into the relationship between fitness and performance in an appropriate ecological context. Although 
1Laboratory for Functional Morphology, University of Antwerp, Antwerp, Belgium. 2Department of Sport Sciences, 
University of Ghent, Ghent, Belgium. *email: francois.druelle@uantwerpen.be
open
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previous studies have explored obstacle negotiation in lizards11–14, hardly anything is known about crossing exten-
sive uneven terrains. In our previous study15, we made the observation that Acanthodactylus boskianus, a running 
specialist, is not only specifically adapted for high-speed running on an even, level surface, but is also able to 
maintain its high performance on structurally uneven terrain. The relatively large terrain variations were offset 
by important capacities in leg adjustability that ensured a ‘smooth ride’ for the centre of mass. Despite being a 
desert species, A. boskianus is also adapted to deal with large terrain variations at a scale relevant to its size. The 
question as to how the effect of an uneven terrain on running performance and locomotor costs differs between 
species exhibiting different locomotor specializations remains. In this context, and assuming that the functional 
demands imposed by natural environments (in terms of structure and organisation) are reflected in the locomo-
tor system2,3, lizards with different morphologies should exhibit different locomotor performance. According to 
the physiological and biomechanical theory, the sprinters would benefit from a laterally compressed body, long 
hind limbs (with primarily long and slender zeugopods and autopods) and a more parasagittal running limb 
posture. This enables these lizards to take large strides and to reach high speeds. In contrast, the climbers would 
benefit from dorso-ventrally flattened bodies and strong, short equal fore and hind limbs with a sprawling posture 
to enable them to keep a close and firm contact with the substrate1,16–21.
Here, we compare locomotor performance and kinematics in a running specialist belonging to the Lacertidae 
family, A. boskianus, to a climbing specialist, the lacertid Podarcis muralis, when negotiating an uneven terrain. 
Both species are described as active foragers, i.e. species for which the locomotion accounts for a large portion 
of the energy budget (mean number of moves per minute: 2.01 and 3.05, respectively, and percentage of time 
moving: 28.80% and 20.54%22), but they are representative of different locomotor specializations. P. muralis is 
described as a specialized climber primarily seen as a rock-dwelling lizard, thus commonly encountering both 
highly uneven and vertical structures as well as flat terrains23–25. A. boskianus is considered a specialist in fast 
running and acceleration in an open desert environment26–29.
According to our previous results15, we presently hypothesise that the running costs (J/kg/m) should hardly 
be affected by the imposed terrain variations in A. boskianus. The anticipated good performance of the running 
specialist on the uneven terrain may be related to naturally occurring sand ridges in its (micro-)habitat. In the 
present study we also compare the centre of mass dynamics, limb behaviour and locomotor costs of A. boskianus 
with results for P. muralis when tested on the same terrain. Although the climbing specialist commonly lives in 
rocky and uneven environments, our experimental terrain should strongly perturbate the running performance 
of this species because its habitat commonly offers many hiding places that do not require running any great dis-
tance. Furthermore, their anatomy, i.e. a flattened body with short limbs (see above), allows it to maintain close 
and firm contact with the substrate, thus P. muralis are expected to follow the uneven substrate topology closely, 
leading to perturbations in their running mechanics. In this context, we hypothesise that, on the flat terrain, A. 
boskianus will show better locomotor performance and lower costs to those of P. muralis17,30,31. Furthermore, 
the latter should be more perturbated by the uneven terrain than the running specialist. The respective limb 
and CoM dynamics should result from the differences in limb length and design4,32 and from the respective 
ecologically-relevant escaping strategies of these species, i.e. running a great distance in A. boskianus and hiding 
as fast as possible in P. muralis.
Methods
Subject details. 
Seven adult male A. boskianus were obtained from a commercial dealer (Amfibia, 
Antwerpen, Belgium) and seven adult male P. muralis were collected using hand foraging techniques in the wild 
(Mechelen, Belgium; the P. muralis individuals were released in their natural environment after the experiments). 
All animal care and experimental procedures were carried out in accordance with the regulations and guidelines 
of the University of Antwerp. The present protocol was approved by the ethical committee of the University of 
Antwerp (ECD-dossier 2013-76).
Experimental protocol and acquisition of data. 
We constructed an adjustable racetrack including a 
central part that could remain flat (control) or be covered with hemi-spheres (uneven terrain). The hemi-spheres 
were 25 mm high, i.e. equal to ≈0.4 times snout vent length of our animal sample (63.95 ± 3.18 mm in A. boski-
anus and 61.26 ± 3.19 mm in P. muralis). We painted the flat and uneven terrains with adhesive paint and sand 
was additionally spread and glued to the surface. This significantly increased the roughness of the substrates to 
enable the animals to run at top speed.
The experiments took place in the morning in November 2017 for A. boskianus and in April 2018 for P. mura-
lis. All the animals were first kept in an incubator set at 37 °C for A. boskianus and 30 °C for P. muralis to optimise 
their respective locomotor performance26,28. For each individual, 15 anatomical parts were marked with white 
using water based paint: top of the snout, back of the head, side of the head, shoulder, mid-trunk, hip, mid-tail, 
knees, proximal part of the feet, elbows and proximal part of the hands. During a 3-week period, we tested each 
lizard randomly on each substrate every day. The lizards were encouraged to run along the racetrack by means of 
hand chasing and one or two consecutive trials were performed per substrate [a minimum of 30 minutes rest time 
(in the incubator) between the different per-substrate trials was ensured]. We recorded the running animals with 
four synchronized high-speed digital video cameras operating at 325 frames.s−1 and 1/800 shutter speed (© 2018 
NorPix Inc., system 10 GigE Vision, 1920 × 1080). The cameras were positioned perpendicular to the runway, 
at the top and in diagonal for increasing the accuracy of the 3D reconstruction (see Figure A in Supplementary 
Material). Calibration was performed using a custom-made calibrated construction (477 × 143 × 96 mm) on 
which 40 dots were digitized. After the recording, the digitization of the body markers was performed manu-
ally frame-by-frame using Matlab (R2019a) and the DLTdv5 application developed by the T. Hedrick lab33. A 
strong selection criterion was applied on the selection of the sequences to be digitized. Sequences were considered 
appropriate when the running individuals were crossing the substrate in a straight line and at a constant speed. 
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This resulted in 61 strides analyzed for A. boskianus and 51 for P. muralis. Further information about the present 
experimental protocol can be found in our previous paper15.
Locomotion analysis. 
On the raw data (digitized markers), we first applied a fourth order low-pass 
Butterworth filter with a cut-off frequency of 60 Hz. This is well above the mean stride frequency in our study 
(mean frequency = 11.74 Hz in A. boskianus and 11.69 Hz in P. muralis). Second, a general filter using a piecewise 
cubic spline interpolation method was applied for missing data. For instance, the very fast movement of the limbs 
during the swing phase sometimes made few dots impossible to digitize correctly. In these occasional cases, we 
kept the running sequence with the few missing dots (usually 1–5 frames). If more than one third of the frames 
was missing, we removed the complete stride from the dataset. We then estimated the position of the body cen-
tre of mass (CoM) based on the dissections of three A. boskianus cadavers15 and one P. muralis cadaver. After 
freezing and segmenting the body, the body parts were subsequently weighed on a micro balance (MT5 Mettler 
Toledo, Greifensee, Switzerland; precision: 0.01 mg), and each marker was provided with a percentage of the total 
body mass (the limb CoMs are estimated at the knees and elbows). The weighted arithmetic mean of all markers 
enabled us to calculate the instantaneous position of the CoM in all digitised frames. We corrected the height of 
the CoM for substrate height by substracting 25 mm from CoM height on the uneven terrain (i.e. the radius of 
the hemi-spheres). In our sample, the average position of the CoM was estimated to be 23.9 ± 1.9% of the trunk 
from the hip joint for A. boskianus and 23.7 ± 4.9% for P. muralis. The estimated trajectory of the CoM from the 
slope of the regression line in the XY-plane allowed us to recalculate the global frame of reference using a rotation 
matrix, with an X-axis aligned with the direction of motion, and the Y-axis perpendicular to the X-axis in lateral 
direction; the Z-axis is aligned with the gravity vector.
Morphometrics and body movements were used to determine the instantaneous mechanical energy of each 
body segment (head, proximal trunk, mid-trunk, distal trunk and tail) over a stride period:
E
mgZ
m Z
X
Y
I ya
pi
(
)
2
(
)
2
si
2
2
2
2
2
=
+
+
+
+
+





Where m is the mass of the segment si, g is the gravitational acceleration (9.81 m.s−2), Z is the instantaneous 
height of the CoM of the segment considered (the segment CoM is estimated from the different markers), Z, X 
and Y are the linear velocity of the segment CoM, ya and pi  are the angular velocity of the segment si in the fron-
tal and sagittal plane, respectively; note that the roll rotation is not included in the calculations as it is expected to 
be minimal comparing to the yaw and pitch. I is the inertia of the segment si and is estimated using the moment 
of inertia calculation for a uniform rod, as follows:
I
mL
12
si
2
=
Where L is the length of the segment si. Each limb was considered as a point mass at the level of the elbow or knee 
and the instantaneous mechanical energy was calculated as follows:



E
mgZ
m Z
X
Y
(
)
2
pi
2
2
2
=
+
+
+
The total instantaneous minimal energy is calculated as the sum of all Esi and Epi and the time differential of 
the total energy yields the instantaneous power during the stride. The integral of the positive power allows us to 
calculate the average positive work and the integral of the negative power allows us to calculate the average neg-
ative work over a stride.
The overall efficiency of the muscles depends on their contractile properties as well as their elastic compo-
nents. Although the elastic components stretched during the preceding phase of negative work may increase the 
efficiency of the muscles, the maximal efficiency of the conversion of chemical energy into the positive mechani-
cal work is approximately 25% in animals34–36. It has also been shown that large animals should benefit more from 
elastic energy savings than smaller animals5,35. Therefore, in the present study, we are assuming that muscles can 
perform positive work with a maximal efficiency of 25%. We therefore estimated the energy cost of transport from 
the sum of the positive work times 4 and the negative work times 1.
Statistical analysis. 
Assessing morphological differences between species. Comparisons in morphometrics 
(body mass and segment lengths) were conducted between both species using exact Permutation tests for inde-
pendent samples. In this context, the statistical unit is the individual and permutations are an appropriate test for 
the small sample size (n = 7).
Assessing kinematic differences among and within species. In the present protocol, a strong selection criterion had 
been previously performed on the running sequences (see previous). Each selected stride comes from a different 
running sequence, thus ensuring stride independence. In addition, using dimensionless quantities is a way to 
control for potential random effects related to individuals because we expect individual differences in running 
kinematics to be related to size. Hence, we have considered the strides as our experimental units and the strides 
are compared, on the one hand, across speed and species on the control substrate (a), and on the other hand, 
across speed and substrates within species (b). All kinematic data were log10-transformed before analysis in order 
to ensure normality and homoscedascity assumptions.
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 a) Between species on the control substrate Using analyses of variance (ANOVA), we first tested for differences 
between species in mean speed and dimensionless speed [assessed using the Froude Number 
×
v
l
g
2 ; where v 
is the stride average speed, l is the length of the tibia of the individual considered15,37, and g is the gravita-
tional acceleration (9.81 m.s−2)]. Second, a set of covariance analyses (ANCOVAs) were performed on 
different response variables including species as factor and dimensionless speed as a covariate. The 
response variables tested are: the dimensionless spatio-temporal parameters [dimensionless stride length 
(stride length divided by tibia length), dimensionless stride frequency (squared frequency divided by tibia 
length times g) and duty factor (proportion of stance phase relative to stride duration)], the amplitude of 
CoM and foot displacements on the Y-axis (lateral) and Z-axis (vertical), the relative average position of 
the foot in the 3 planes, and the relative height at which the CoM is maintained.
 b) Within species between substrates The same statistical tests were performed but the substrate was included 
as a factor instead of the species in the ANCOVAs.
In addition, we compared the average cost of transport between species and substrates using ANOVAs. 
We also compared the slopes of the linear models between the cost of transport and absolute speed using the 
“lsmeans” package in R. All the statistical analyses were performed using R (version 3.3.2), but the permutation 
tests were performed using StatXact (version 3.1). The significance level was set at P < 0.05.
Results and Discussion
Morphological features associated to running and climbing skills in Lacertidae. 
Figure 1 shows 
the morphological differences between A. boskianus and P. muralis. These differences can be related to their 
respective running and climbing skills. While the snout-vent length is not different between both species, A. boski-
anus has longer hind limbs (femur + tibia) than P. muralis (independent Permutation test = 3.062; P = 0.0006; 
Table A in Supplementary material). The mass of the hind limbs is more than 2 times larger than the mass of the 
forelimbs in A. boskianus (5.5 g vs 2.2 g), while fore- and hind limbs masses are almost equal in P. muralis (1.8 g 
vs 2.3 g). Both species exhibit longer hind limbs than forelimbs (A. boskianus: paired Permutation test = 2.551; 
P = 0.0156; P. muralis: paired Permutation test = 2.514; P = 0.0156), but the difference between fore- and hind 
limb lengths is significantly larger in A. boskianus than in P. muralis (independent Permutation test = 2.806; 
P = 0.0012). The long hind limbs of A. boskianus relative to the forelimbs may enhance their running capacities, 
while the small difference in fore- and hind limb lengths in P. muralis certainly enhances their climbing skills21.
Kinematic differences between runners and climbers when running on level surface. 
According 
to the trade-off hypothesis, being a specialist in one locomotor mode should impair performance in other 
modes17,21,30,31. Table 1 shows the average spatio-temporal parameters for a running specialist (A. boskianus) and 
Figure 1. Comparisons of the measured morphological features between the running and climbing specialists. 
A. boskianus are in orange and P. muralis are in green. Symbol significance: *P < 0.05, **P < 0.01, ***P < 0.001. 
Lizard drawings are from Menelia Vasilopoulou-Kampitsi.
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a climbing specialist (P. muralis) when running on a flat/even substrate. A. boskianus run significantly faster than 
P. muralis (ANOVA F = 12.89, P = 0.0008). After correcting for size effects, there is no significant difference in the 
average speed between A. boskianus and P. muralis (ANOVA F = 2.395, P = 0.128; Fig. 2A). Correcting for size and 
speed effects, A. boskianus exhibits a higher stride frequency (ANCOVA F = 4.548, P = 0.0382; Fig. 2B) and a lower 
duty factor (ANCOVA F = 68.84, P < 0.0001; Fig. 2D) than P. muralis. The amplitude of the upward displacements 
of the foot (i.e. the foot clearance) is larger in A. boskianus (ANCOVA F = 15.53, P = 0.0003), while the lateral dis-
placements of the foot are larger in P. muralis (ANCOVA F = 16.44, P = 0.0002; Fig. 3). On average, P. muralis places 
its feet further, laterally, from the hip (ANCOVA F = 12.65, P = 0.0009), i.e. the posture is relatively more sprawled, 
and the same happens in the fore-aft direction (ANCOVA F = 13.01, P = 0.0007), i.e. the foot is more retracted in P. 
muralis (Fig. 4). There is no difference in the amplitude of CoM translation in the lateral and upward directions on 
the flat terrain. However, the CoM is maintained at a significantly lower height in P. muralis compared to A. boski-
anus (9.21 ± 3.03 mm and 17.15 ± 3.66 mm, respectively; ANCOVA F = 43.74, P < 0.0001). To sum up, A. boskianus 
use more parasagittal hind limb postures with a larger foot clearance, exhibit lower duty factor, higher stride fre-
quency and keep the CoM relatively higher than P. muralis. These specificities thus emerge in A. boskianus which is 
a fast runner in general and a better sprinter than P. muralis on level surface. P. muralis run with a CoM very close to 
the surface, which is advantageous for balance in lizards that climb vertical surfaces21, while A. boskianus keep the 
CoM higher, avoiding touching the substrate and providing space for parasagittal limb displacements. In this way, A. 
boskianus run much faster, as observed in lizards living in open habitat38.
A. boskianus
P. muralis
Flat (control)
Uneven
Flat (control)
Uneven
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Speed (m.s−1)
1.72
0.48
1.56
0.45
1.16
0.27
0.92
0.15
Stride frequency (Hz)
11.74
2.52
12.24
2.89
11.69
2.29
11.19
2.46
Duty factor (%)
26.67
5.49
33.43
7.28
53.25
11.66
55.97
6.22
Stride length (mm)
141.18
20.6
122.47
23.21
77.09
16.13
67.34
11.97
Table 1. Mean ± SD for spatio-temporal parameters.
Figure 2. Average dimensionless speed (A) and spatio-temporal parameters calculated for each species and 
for each substrate: Dimensionless stride frequency (B), dimensionless stride length (C), duty factor (D). The 
brown colour is for A. boskianus, the green colour is for P. muralis. Within each species, darker bars represent 
the control (flat surface), lighter bars represent the uneven terrain (i.e. hemi-spheres). Error bars show standard 
deviations. Symbol significance: *P < 0.05, **P < 0.01, ***P < 0.001.
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Impact of running over an uneven terrain for a running specialist. 
According to previous research15, 
the small fast running specialist A. boskianus, is able to cope with complex substrates without there being any 
impact on sprint speed (Fig. 2). Although the stride length decreases (141 mm vs 122 mm; ANCOVA F = 4.075, 
P = 0.0482) and the duty factor increases (27% vs 33%; ANCOVA F = 9.134, P = 0.004), the average speed is 
not significantly impaired in this context. The orbit characterizing the movement of the foot relative to the 
hip also remains similar across the substrates in the sagittal plane (the XZ-plane) and in the frontal plane (the 
XY-plane; Fig. 5). We observed a significant increase in the amplitude of the foot clearance (ANCOVA F = 10.655, 
P = 0.0018; see Fig. 3) and the height at which the CoM is maintained decreases significantly (17.15 ± 3.66 mm 
vs 12.44 ± 3.28 mm; ANCOVA F = 16.565, P = 0.0001). In general, the complex terrain impacts few kinematic 
aspects of A. boskianus (Figs 3 and 4). The changes occur mainly at the level of the legs that adjust instantaneously 
to the substrate variations through larger amplitudes of the foot clearance. This enables A. boskianus to keep the 
trajectory, as well as the movement amplitude, of the CoM stable.
Impact of running over an uneven terrain for a climbing specialist. 
Contrary to A. boskianus, 
the average speed when negotiating the uneven terrain decreases significantly in P. muralis (1.16 ± 0.27 m.s−1 
vs 0.92 ± 0.15 m.s−1; ANOVA F = 5.346, P = 0.025; Fig. 2). The stride frequency also decreases significantly 
(11.69 ± 2 Hz vs 11.19 ± 2 Hz; ANCOVA F = 9.148, P = 0.004; Fig. 2). Although the amplitude of the foot move-
ments does not change on the uneven terrain, the centre of the orbit of the foot movement shifts downwards 
on the sagittal plane (ANCOVA F = 7.67, P = 0.008; Figs 4 and 5); it does not change in the frontal and trans-
versal planes. The CoM translation in the Z-direction increases (ANCOVA F = 5.09, P = 0.029) and the relative 
height at which the CoM is maintained decreases significantly (9.21 ± 3 mm vs 6.22 ± 2 mm; ANCOVA F = 7.732, 
P = 0.0078).
Costs of transport in running and climbing specialists. 
The cost of transport does not differ between 
the two species when running on a flat substrate (ANOVA F = 0.029, P = 0.87), however A. boskianus still run 
on average 50% faster than P. muralis. The substrate type (flat or uneven) does not impact the cost of transport 
in A. boskianus which supports the hypothesis that the morphology of A. boskianus is strongly adapted for fast 
running27,29. When these animals encounter large terrain variations relative to leg length, they can continue to 
minimise the energy costs related to running. In A. boskianus, leg movements adjust to the substrates’ variations, 
enhancing the stability of their CoM15. On the contrary, the complex terrain provokes a significant increase in 
the cost of transport in P. muralis (ANOVA F = 4.445, P = 0.041; Fig. 6) but the relationship between the cost of 
transport and speed is not impacted, i.e. there is no significant difference between the slopes of the regression 
lines among and within species. The general increase in the cost of transport in the climbing specialist P. muralis 
Figure 3. Average amplitudes of the CoM and foot displacements in the Y- and Z-directions. See Fig. 1 for 
symbol significance.
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is mainly related to an increase of the positive external energy (Fig. 7). Although it can keep the energy costs 
associated with running at a low level on an even terrain, the costs increase significantly when the demands of the 
terrain become too high. In the context of the present uneven terrain, the running performance of P. muralis is 
strongly affected and the locomotor economy is lost.
Overall, our results show that runners and climbers have the same cost of transport on level terrain, although 
climbers run at a slower speed. Contrary to our hypothesis, the cost of transport is not higher in the climbers 
Figure 4. Average position of the foot relative to the hip per stride and corrected for size in the three planes of 
movement. See Fig. 1 for colour and symbol significance.
Figure 5. Mean trajectory of the foot movement relative to the hip on the sagittal (Z) and frontal (Y) planes 
(note that the orbits are not corrected for size). The hip is represented by a white circle. The orange colour is for 
A. boskianus (adapted from15), the green colour is for P. muralis. Within each species, darker orbits represent the 
control (flat surface), lighter orbits represent the uneven terrain (i.e. hemi-spheres).
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running on a level surface. Nevertheless, it is possible that the lower running speed of climbers allows them to 
maintain low energy costs (the slope between speed and cost of transport does not differ between the two spe-
cies, but if P. muralis was able to run faster, Fig. 6 suggests even higher costs relative to A. boskianus). Thus, the 
morphological features associated with climbing impair sprint speed, as there is no difference in dimensionless 
speed. As anticipated, on the one hand, the uneven terrain has no influence on the average speed and the cost of 
transport in the running specialist, A. boskianus. On the other hand, the climber, P. muralis, encounters many 
more difficulties when negotiating uneven terrain. Indeed, speed and energy expenditure are impaired in P. mura-
lis running on uneven terrain. In general, their legs are kept more extended, but the amplitude of movement 
does not change. Hence, leg movements do not adjust to the terrain as observed in A. boskianus, resulting in an 
increase of the movement of the CoM and a decrease in locomotor efficiency.
Movement is obviously related to muscle effort, and it can be expected that the maximal locomotor power 
output will be limited by the force that can be generated by the muscles. Some authors have argued that small 
animals (mammals and reptiles) do not rely on elastic energy mechanisms for locomotion, thus they exhibit 
important metabolic costs27,35. Although sprawled leg postures should increase the required muscle forces32,39,40, 
Figure 6. Relationship between speed and cost of transport (estimated from the fluctuations of the minimal 
mechanical energy) in P. muralis (in green) and A. boskianus (in orange). Squares indicate the strides performed 
on the flat (control) substrate and the solid lines represent the respective linear models, circles indicate the 
strides performed on the uneven substrates and the dashed lines represent the linear models.
Figure 7. Positive and negative minimal mechanical energy in A. boskianus and P. muralis on the control 
and complex terrain. The solid colour represents the positive energy (+) and the diagonal lines represent the 
negative energy (−).
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our results suggest that, overall, the costs of transport associated with running are similar in two lizard species 
adopting either more parasagittal leg postures (A. boskianus) or more sprawled postures (P. muralis). However, 
we have observed that the costs of transport are larger in P. muralis than in A. boskianus when crossing uneven 
terrain. The necessity to limit metabolic costs may be less important in fast climbers than in fast runners because 
climbers from rocky environments primarily rely on explosive power generation in order to find shelter rapidly 
within close proximity. This is indeed a typical behavioural strategy of P. muralis25. On the other hand, small run-
ning specialists such as A. boskianus definitely need endurance too, in order to escape to the nearest hiding place, 
which can be located at a distance, certainly for a desert species such as A. boskianus. As a result, they need to be 
able to keep the energy costs associated with fast locomotion at a low level; when targeted by a predator, making 
a stop in the middle of the pathway is not an option. A. boskianus can minimize the energetic challenges imposed 
by uneven terrain by limiting the movement amplitude of the CoM. Our study, therefore, supports the assump-
tion that locomotor economy is optimized in accordance to the ecological relevance35.
conclusion
The capacity to negotiate uneven terrain at the scale of the animal size is not a common capacity shared by liz-
ards in general. The climbing specialist tested in this study displays the lowest performance on uneven terrain. 
Saxicolous habitats are the primary niche of P. muralis, and it certainly poses many opportunities for hiding and 
escaping. In this way the obstacles and vertical substrates that have to be dealt with are commonly much larger 
than the size of these lizards. Our finding of the lower velocity and a higher energy cost on the uneven terrain 
for P. muralis compared to A. boskianus, support the theory that the former uses a behavioural strategy of swiftly 
escaping to a close hiding place when confronted with danger. For them, short running burst can be very effective. 
The running specialist A. boskianus, on the other hand, presumably runs away rapidly over long(er) distances 
under similar circumstances. This can, again, be linked to its specific structural microhabitat. A. boskianus lizards 
live in open environments such as deserts, where hiding spots can be located a long distance away. The specific 
structural microhabitat found in the desert may resemble most closely the uneven terrain in our experiments 
because on sandy substrates, sand ridges are often present, as a result of complex interactions between flowing 
sand masses and wind. This microstructure of a substrate that is very flat on a larger scale, may challenge small 
lizards such as A. boskianus in a very similar way as the uneven terrain in our experiments. This could explain why 
they perform so well on this substrate, both in terms of velocity and energy expenditure. Our study, therefore, 
supports the hypothesis that microhabitats impose functional demands that species are adapted for, rather than 
large ecological niches41. Furthermore, locomotor costs can also be important factors in small vertebrates. Given 
their ecological niche, locomotor economy may represent a significant constraint for the evolution of lizards.
Data availability
All the data used in the statistical tests can be found in Supplementary Material (Dataset 2).
Received: 31 May 2019; Accepted: 29 October 2019;
Published: xx xx xxxx
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Acknowledgements
We are very grateful to Gregory Desor and Jan Scholliers for their valuable help for designing and building the 
complex terrain. We thank Lise Eerdekens, Nick Van Hul and Ilse Goyens for their valuable help in digitising a 
large number of running sequences. We thank the referees for their constructive and detailed comments on the 
first version of the manuscript. We are very grateful to Josie Meaney-Ward who revised and improved the English 
of the manuscript. This work was supported by Fonds Wetenschappelijk Onderzoek (FWO project G0E02.14N). 
J.G. was funded by an FWO postdoctoral fellowship (12R5118N). M.V.-K. was funded by the Department of 
Biology, University of Antwerp.
Author contributions
Conceptualization and methodology: F.D., J.G., P.A.; Data collection and investigation: F.D., M.V.-K.; Analyses: 
F.D., J.G., P.A.; Data curation: F.D.; Writing - original draft: F.D.; Writing - review & editing: F.D., J.G., M.V.-K., 
P.A.; Project administration: P.A.; Funding acquisition: P.A., J.G.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s41598-019-53329-5.
Correspondence and requests for materials should be addressed to F.D.
Reprints and permissions information is available at www.nature.com/reprints.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and 
institutional affiliations.
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© The Author(s) 2019

Small vertebrates running on uneven terrain: a biomechanical study of two differently specialised lacertid lizards.
Druelle, François,Goyens, Jana,Vasilopoulou-Kampitsi, Menelia,Aerts, Peter