84 
place in three dimensions and are not always easy to envisage from 
a written description and diagrams. However a clearer idea of some 
of the main aspects can be obtained by making a simple model out of 
a strip of card with folds inserted to represent articulations between 
the main elements (Fig. 13). The model can be be used to demon- 
strate the pattern of flexion between the femur and crus, the 
subsequent reorientation of the latter element in the parasagittal 
plane and associated lifting of the metatarsal segment brought about 
by partial retraction and rotation of the femur, the benefits of femoral 
rotation in allowing the limb to be partially retracted and extended in 
the horizontal plane as it is brought forwards in the recovery stroke, 
and the restricted nature of the problem of rotation in the crural 
region. It should however be born in mind that there is more play in 
the actual joints than the model indicates. Such a model is also useful 
in appreciating the rather different motions of the hind leg in 
climbing species. 
Other hind limb gaits in ground-dwelling lizards — continuous 
gearing 
Although lizards are often stated to have only a single gait, in 
contrast to many mammals, the hind limbs are used in a range of 
ways that are largely correlated with speed. Stationary lacertids may 
commence movement by thrusting with both hind legs, especially if 
startled, so accelerating before a step pattern is established. In slow 
walking, the excursion of the femur may be restricted and, instead of 
being brought forwards, the crus may be kept largely flexed, so that 
it is never directed forwards and the soles of the feet may be 
orientated rather laterally, a result of forward rotation of the femur. 
At increasing speeds, femoral excursion is greater and the crus 
may be brought forwards until it is roughly perpendicular to the 
body with the foot directed anteroposteriorly. Finally, the crus is 
extended fully forwards and the femur rotated backwards at the 
beginning of the power stroke, as described above. These substantial 
changes in the way the hindlegs are used act like continuously 
variable gears. As might be expected, the body is held closer to the 
ground in the slower gaits as forward rotation of the femur during 
these permits a more lateral use of the whole limb. 
Movements of the foreleg in ground-dwellers 
At the beginning of the power stroke, the humerus is directed antero- 
laterally and the lower limb and digits point forwards. During 
retraction the forelimb turns over until its underside is uppermost. At 
first the manus is placed palm-down, but the lizard rises on the distal 
toes as the lower limb becomes more or less vertical. However, the toes 
usually dorsiflex at the end of the stride. As with the hind leg, the fore- 
leg is raised high when it is brought forwards in the recovery stroke. 
Functional aspects of the limbs and feet of ground-dwelling 
lacertids 
It is now possible to assess the functional importance of limb 
morphology in ground dwelling lacertids. The long legs, in which 
the more distal elements — crus, metatarsal segment and digits — are 
differentially elongated, are responsible for the extended stride of 
these species, and the way the metatarsal bones are bound closely 
together in some forms increases the rigidity of the metatarsal 
E.N. ARNOLD 
segment. The way the main adductor muscles, especially the 
caudifemoralis, are attached proximally to the femur confers high 
mechanical advantage on the locomotory system, which in this 
respect and the elongation of its distal elements parallels those of 
other fast amniote runners such as horses. 
The regular downward curve of the toes, maintained by joint 
capsules and tension in the dorsal and especially ventral tendons at 
the end of the stride, and the restriction on medial flexion, ensure 
that thrust is delivered to the ground efficiently. The robust phalanges 
with joints of restricted flexibility are clearly suitable for resisting 
the compressive and shearing forces produced at this time, when the 
lizard may sometimes be balanced on the tips of very few toes. 
Steady increase in length from the first toe and its metatarsal to the 
fourth means that the claws of these digits can be well-spaced when 
inserted in the ground, ensuring a wide area of contact with the 
substratum so a good grip is more likely, even on shifting surfaces; 
the generally large size of the foot also contributes to this spread and 
the long lightly curved claws are more likely to gain effective 
purchase in earth or sand than short recurved ones. Reduction of the 
fifth toe is comprehensible in as much as it is virtually unused in fast 
locomotion. 
The very robust phalanges of the manus may not be specifically 
associated with locomotion but could be important in digging, 
something advanced ground lacertids accomplish largely (or en- 
tirely in the case of Heliobolus lugubris, personal observations) with 
their forelegs. Possibly the relatively large manus of soft-sand 
dwellers is also functionally associated with digging. 
Ground locomotion in climbing species 
Lizards that habitually climb, like Lacerta oxycephala, L. perspi- 
cillata and to a lesser extent, L. nairensis, run quite efficiently on the 
ground and often extend the crus fully forwards. However, they tend 
to carry the body less high than specialised ground-dwellers, partly 
because their limbs are generally shorter and the crus especially so, 
and these features also limit stride length. Habitual climbers do not 
rise on to the tips of their toes at the end of the stride and, instead of 
the digits flexing downwards, they flex dorsally, toes 24 bending at 
the penultimate articulation between the phalanges, so the pes 
rotates over the inserted claws (Fig. 17b). This shortens effective 
stride length still further. Climbing species also tend to keep the hind 
limb closer to the substratum during the recovery stroke. 
The distinctive features of ground locomotion in habitually climb- 
ing forms all have functional advantages during climbing (p. 000).A 
similar but more extensive carry over of features advantageous in 
climbing to horizontal locomotion occurs in the gecko, Gekko gecko 
(Zaaf, Aerts et al., 1997). 
Locomotion in climbers on steep open surfaces 
(Figs 13-17) 
Most detailed observations were made of Lacerta oxycephala, 
which was filmed dorsally and laterally when climbing on a near 
vertical rock slab. L. perspicillata, Algyroides nigropunctatus and A. 
marchi were also examined by film or video; in most cases, speeds 
and exposures were the same as for many ground dwelling lizards 
but Algyroides nigropunctatus was also videoed at 200 fields/sec. 
\ 
digits 
1 
1 \ 
metatarsal 
; 
1 
segment \ 
\ 

Fig. 13 Simple model of right hind limb of lacertid. A strip of card cut and folded as indicated by broken lines can be used to demonstrate the main 
movements of the hind leg elements in a running lizard. 
