86 
Often the digits are spread radially with all the claws inserted in 
minor irregularities in the substratum and the well developed toe 5 
contributing positively to the grip of the hind foot. Toes 1—3 are 
often directed more or less anteriorly, 4 laterally or somewhat 
posteriorly and 5 posteriorly. Sometimes, instead, toes 3 and 4 may 
both be directed obliquely backwards, or toes 1-4 are all directed 
forwards. 
As the crus flexes on the femur and the body of the lizard moves 
forward, it becomes directed posterolaterally, changing its orienta- 
tion to the foot. This results in the metatarsal segment being 
directed more laterally and its posterior edge rising; because the 
claws are firmly inserted, digits 1-4 flex mesially to accommodate 
this change in orientation of the metatarsal segment (right hind leg, 
Fig. 14b; Fig. 15). There is also a tendency for the crus to thrust 
diagonally backwards at this stage which accentuates the bending 
of the toes. At the same time, the proximal parts of toes 14 flex 
upwards in the vertical plane mainly at the following phalangeal 
articulations toe 1 —0/1, toe 2 — 1/2, toe 3 — 2/3, toe 4 — 2/3 and 3/4. 
The femur is then retracted and the crus is extended posteriorly 
relative to it, thrusting the body of the lizard upwards (right hind 
leg, Fig. 14b). The metatarsal segment does not rise much as a 
whole but its hind edge continues to do so and, as this happens, the 
claw of toe 5 becomes detached, followed by that of toe 4 (if this 
digit is not directed forwards), and then those of the remaining toes 
as the foot moves rapidly forwards to gain a new grip further up the 
rock face. This recovery stroke takes place with the foot close to the 
substratum. 
In contrast to ground locomotion, the femur of specialised 
climbers seems to be rotated forwards around its long axis for most 
of the step cycle, allowing the limb to work largely in a plane more 
or less parallel to that of the substratum. 
Movements of the fore limb 
After its recovery stroke, the forelimb is extended forwards with the 
humerus directed roughly anterolaterally, the lower limb forwards 
and the digits broadly spread (right limb, Fig. 14b,d) As the 
humerus 
retracts and the lower limb flexes on it, the latter rotates in a 
parasagittal plane, becoming orientated first normal to the substra- 
tum and then directed posteroventrally as the limb thrusts backwards 
(right limb, Fig. 14a, c). After this the digits flex dorsally and the 
claws are then released from their contact with the rock face, as the 
next recovery stroke begins. 
Other patterns of locomotion in specialised climbing lacertids 
On less steep surfaces a climbing lizard like Lacerta oxycephala 
shifts to a locomotory pattern essentially similar to that which 
specialist climbers use on the ground (p. 85). When running down 
a very steep slope, upward motion is presumably powered substan- 
tially by gravity, but descent is controlled by the lizard taking short 
steps in which the hindlimbs are turned back with toes 4 and 5 and 
often 3 directed posteriorly (Fig. 16). At the end of a step, in which 
the femur is not moved much, the ventral tendons of these digits are 
relaxed, loosening the grip of the claws. The foot is then brought 
forwards, still directed posteriorly, and the claws flexed and in- 
serted again; after this the leg extends backwards and the cycle is 
repeated. 
Problems of upward vertical locomotion 
The problems encountered by a lizard climbing a vertical face are 
quite different from those of an animal running on relatively level 
ground. 1. There is a need to keep upward thrust parallel with the 
surface being climbed. Although the oblique thrust delivered to the 
E.N. ARNOLD 
Fig. 16 Position of toes of right hind foot in Lacerta oxycephala 
descending a rock face; 3,4 and 5 are turned posteriorly. 
substratum by the hind limbs of arunning lizard tends to push it a way 
from the ground into a floating phase, gravity returns it rapidly. There 
is no such automatic restoration of contact on a vertical face and 
oblique thrust would push the lizard right off the substratum. Thrust 
must consequently be applied in a direction parallel to the face. 2. 
There is a constant danger of falling from the face being climbed. In 
particular, were there no foreleg contact, a lizard would tend to fall 
outwards because it is then in a position of unstable equilibrium with 
its centre of gravity above the remaining hindleg contact. The con- 
verse condition, with both hind legs free, is less precarious as the 
posterior part of the body tends to rotate towards the rock. 3. As 
gravity acts in a direction diametrically opposite to that of locomo- 
tion, momentum will be lost very quickly once upward thrust ceases; 
this must therefore be regular and continuous. 
Many characteristics of locomotion, in lacertids that climb vertical 
faces regularly, appear to ameliorate these problems. Keeping the 
body and limbs close and parallel to the surface being climbed 
ensures that backward thrust delivered through the claws is also more 
or less parallel to it. The danger of falling off the face is minimised by 
the way the number of feet in contact with it is maximised including 
those of the particularly important forelegs. This positive engage- 
ment of all feet in upward locomotion maximises thrust and makes it 
available throughout the cycle. Thrust is also maximised by the way 
flexion of the toes enables the claws to be kept in place as long as 
possible. Bringing the crus forwards until it is not much more than 
normal to the body axis is equivalent to moving in a relatively low 
gear, compared with the anterolateral extension found in ground 
runners travelling at speed, something that is also appropriate when 
moving against gravity. Keeping the body and limbs close to the 
substratum also maximises stride and restricts the downward lever- 
age that the body would exert if it was held away from the substratum. 
The tail also plays a part in ensuring the foreparts of the lizard do not 
fall away from the face. It is held very close to the substratum and, if 
the front legs cannot get a grip (for instance if a piece of smooth card 
is interposed), the lizard can hold its upright position by stiffening its 
body and tail and pressing the latter against the surface. 
Functional aspects of the limbs and feet of specialised climbing 
lacertids 
The greater equality of fore and hind limb pairs in habitual climbers, 
when compared with open ground dwellers, is important in allowing 
the stride lengths of the two pairs to be matched and for the fore feet 
