
NICHE, MORPHOLOGY AND LOCOMOTION IN LACERTID LIZARDS 
forwards. This increases the length of the hind limb step which in 
the more long-legged species may be substantially longer than the 
body. Wild Meroles anchietae about 60mm from snout to vent had 
step lengths of 80-150mm (measured from tracks at Gobabeb, 
Central Namibia in April, 1994). As might be expected from the 
greater relative lengths of time they are in contact with the ground, 
hind limbs are far more important in fast ground locomotion than 
forelimbs. That they deliver more thrust can be seen from tracks in 
sand where hind limbs produce footprints with a strong posterior 
pressure wave caused by their powerful backward extension, 
whereas forelimbs tend to produce simple shallow pocks, indicat- 
ing that their main role is to provide intermittent support to the 
foreparts. 
Movements of the hind limb (Fig. 12) 
When animals are running fast, the hind leg is brought forwards so 
that it is extended in generally anterolateral direction with the main 
axis of the metatarsal segment often lying approximately para- 
sagittally or somewhat anterolaterally and digits 1-4 directed 
forwards and spread (Fig. 12a). The femur lies roughly in the 
horizontal plane, while the crus is directed obliquely downwards and 
the foot is placed flat on the ground with the claws of toes 14 flexed 
downwards and inserted into the substratum. Toe 5 often projects 
more laterally. 
In the first phase of the power stroke, the crus flexes on the femur 
(Fig. 12b). This results in the femur moving forwards but, as the line 
of flexion of the knee is offset mesially, its distal extremity passes 
over the crus which changes orientation so that, from being directed 
anterolaterally, the crus swings until it is directed ventroposteriorly 
in a parasagittal plane. 
At this stage, the femur begins to be retracted, its distal end 
descends somewhat and it also rotates forwards (when viewed from 
above) about its long axis (Fig. 12c). The crus also again becomes 
less flexed relative to the femur and these various movements 
change its orientation, so that it becomes more or less horizontal but 
still lies in a parasagittal plane. As this occurs, the metatarsal 
segment rises proximally, beginning with its lateral edge, so that it is 
now directed downwards and outwards. In firm substrata, the claws 
maintain their position so that this reorientation of the metatarsus 
then results in some mesial bending of the toes in the horizontal 
plane to accommodate it; however flexing is limited by the stiffness 
of the toes in this direction. 
The femur continues to be retracted until it is directed 
anteroposteriorly (12d).At the same time the crus unflexes further so 
that it maintains its parasagittal orientation. By now, the metatarsal 
segment is completely lifted from the ground and this raises the base 
of the toes which, as well as being bent mesially, become flexed 
downwards and the lizard rises on to the tips of toes 14 and then just 
2-4 so that, at this stage, it is hyperdigitigrade. Final thrust in the step 
is thus delivered entirely through the claws which act like the spikes 
on an athlete’s running shoes. During this phase the whole leg 
extends and the upper surface of the metatarsal segment may even be 
directed anteroventally. 
During a step, the lizard thus uses extension of all parts of the 
hindleg to provide thrust: femur, crus, metatarsals and digits. After 
this the muscles controlling the ventral tendons of toes 24 may 
relax so these digits dorsiflex and the claws are pulled free. Toe 5 
plays very little part in fast locomotion in specialised ground dwell- 
ers and leaves the ground at an early stage. 
In the rapid recovery stroke, where the hind limb is brought 
forwards before the next step, it is raised high, partly flexed and then 
extended forwards. During this process, the femur is protracted and 
its forward rotation is maintained, so that forward flexion and 
83 
extension of the leg takes place more or less in the horizontal plane 
and the foot is oriented with its mesial edge downwards. This allows 
the distal portions of the limb to be kept well clear of the ground, so 
that it is less likely to be impeded by any irregularities in the 
substratum or by projecting plants. It also means that when the foot 
does make contact with the substratum at the beginning of the power 
stroke, it may still be orientated with its mesial edge downwards, 
although it is then immediately placed flat on the ground as a result 
of backward rotation of the femur. If the toes do encounter an object 
that hinders their forward motion during the recovery stroke, the fact 
that the upper surface of the foot is directed forwards means that they 
can simply be passively ventriflected and brushed aside, so the leg 
can still progress anteriorly. The toes are also capable of passive 
lateral movement around their joints with the metatarsals, especially 
when the foot is in the process of being placed sole-downwards on 
the ground. 
There is some variation in fast hind leg motion in armatured 
ground-dwellers, which may partly result from the nature of the 
substratum and its irregularities. Thus the foot may be clearly 
directed anterolaterally at the beginning of the power stroke and the 
claws may slip in loose soils so that the foot tends to rotate outwards 
more at the end of a step. Some species also have characteristic 
features during fast ground locomotion; for instance, in Acantho- 
dactylus boskianus the foreparts are carried particularly high. 
Rotation of the femur and supposed restrictions on its movement 
Rotation of the femur about its long axis is a very significant feature 
of hind leg movement during locomotion (Rewcastle, 1983). It 
enables the path of extension of the crus during the power stroke to 
be different from that of its flexion, allows the leg to be brought 
forwards orientated more or less in the horizontal plane well above 
the ground, and explains why the foot may be initially put down 
mesial edge first. 
It has sometimes been assumed that the femur in lizards cannot be 
adducted far posteriorly because its trochanter was believed to jam 
against the ventral rim of the acetabulum (Rewcastle, 1983). How- 
ever, in all the lacertids studied, substantial posterior adduction is 
regularly observed and no restriction of the kind envisaged is 
observable in skeletal material. 
The supposed problem of crural rotation 
There has been considerable discussion of a supposed problem of 
rotation within the distal hind limb (see for instance Rewcastle, 
1983). If the foot is assumed to maintain its position during the 
power stroke, while the angle of the femur in the horizontal plane 
changes relative to it during adduction, there would have to be a 
rotational twist within the intervening crural area, to accommodate 
the change in relative position of these elements. The screw-like 
nature of the mesotarsal joint between the crus and foot actually 
permits some twisting (Rewcastle, 1980) and various other factors 
reduce the amount that is actually required: 1) The angle of the knee 
joint allows the crus to swing, from being in line with the femur at 
the beginning of the power stroke to being directed backwards, 
without disturbing the foot; 2) forward rotation of the femur and 
descent of its distal extremity also helps minimise twisting of the 
lower limb; this is also true of 3) reorientation of the metatarsal 
segment, 4) bending of the toes, and 5) the general mobility of the 
tarsal area. These factors, involving changes in orientation of the 
distal femur and of the proximal foot preclude any substantial 
problem of crural rotation. 
A partial model of hind limb movement 
The movements of the hind leg of lizards during locomotion take 
