NICHE, MORPHOLOGY AND LOCOMOTION IN LACERTID LIZARDS 
to play a positive role in upward locomotion, presumably contribut- 
ing thrust as well as attaching the foreparts. This contrasts with 
ground runners where the forelimbs have at most a minor role in 
delivering thrust. The fact that the hind limbs of habitual climbers 
are relatively short overall is partly responsible for the low gear 
nature of upward locomotion, as is the shortness of the crus com- 
pared with the femur; when the crus is flexed towards the substratum 
at some phases of the step cycle, its shortness permits the upper 
limbs and body to remain close to the substratum. 
The short sharp recurved claws on the feet of climbing forms 
allow a firm grip on substrata like rock that do not permit much 
penetration. The insertion of the ventral tendon on the distal phalanx 
of each digit well away from the actual articulation (Fig. 17a, b) 
means that it has high mechanical advantage and can flex the claw 
effectively against the weight of the body, ensuring its grip is 
maintained. 
At the end of the recovery stroke, when the hind foot is reattached 
to the substratum, the long third metatarsal allows the third toe to be 
deployed easily forwards, laterally or backwards, depending on 
where its claw can be inserted. The mobility of this toe and of 
numbers 4 and 5 means that some or all of them can be opposed to 
the remaining toes to give a positive grip on the substrate. The fact 
that toe 3 can be turned backwards is also important in allowing its 
claw to join those of digits 4 and 5 in acting as an intermittent brake 
when the lizard runs rapidly down steep slopes. When the digits of 
the hind foot are spread with their claws flexed and in the process of 
insertion in the rock face, the dorsal and ventral digital tendons 
contract emphasising the kinking of the phalanges in toes 3—S and so 
shortening these digits. This shortening ensures a positive grip by 
the opposed claws. 
Shortness of the hind toes in specialist climbers helps to reduce 
lateral foot displacement produced by outward thrust of the crus. In 
the later stages of the power stroke, mesial flexibility of toes 2-4 
permits the claws to remain in place. As the metatarsal segment turns 
more laterally, these toes often become quite sharply bent in a plane 
parallel to the substratum. This permits the claws to remain in place 
and upward thrust to be generated for as long as possible. As the back 
of the metatarsal segment lifts, downward flexion of the second 
phalanges of toes 3 and especially 4 (Fig. 15a) enable the claws of 
these often backwardly or outwardly directed digits to remain in 
place longer, prolonging a positive grip. 
Not only do forwardly directed toes flex mesially but, as the 
metatarsal segment lifts and turns over, hind toes 3 and 4 bend 
dorsally in the parasagittal plane if they are directed forwards (Fig. 
17b). This flexion is concentrated at particular joints which enables 
it to be more acute than if it were distributed throughout most of the 
articulations of the toe; the shortness of some intermediate phalanges 
also contributes to this. Such acute flexion means that the metatarsal 
segment can stop closer to the rock face instead of being displaced 
outwards. 
Concentration of dorsal flexion is combined with the simultane- 
ous ventral flexion of the claw, necessary to maintain its grip and, in 
toes 3 and 4 and when backwardly directed, additional ventral 
flexion of phalanx 2 on phalanx |. The areas of ventral flexion are 
produced by tension in the main ventral tendon. Although tension is 
likely to be more or less the same throughout the length of the 
tendon, ventriflexion is combined with the intervening area of the 
toe flexing dorsally. This differential action is an additional result of 
toe kinking, coupled with the varied positioning of the tendon 
relative to different articulations in the toe (Fig. 17a, b). Essentially 
under the joints where the more distal phalanges flex downwards, 
for instance in toe 4 at the articulation of phalanges | and 2 and 4 and 
5, the tendon is displaced away from the joint. This differential 
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Fig. 17 Effects of digit kinking and tendon position. a. Fourth hind toe of 
Lacerta oxycephala with claw newly inserted in rock face. b. Same toe 
towards end of stride when metatarsal segment is lifting. Because the 
ventral tendon (black) is displaced well away from from joints A and D 
and consequently has greater mechanical advantage at them, the 
articulations can be kept ventriflexed while joints B and C, where the 
tendon is closer and mechanical advantage less, can simultaneously 
dorsiflect in response to the movement of the metatarsal segment. Claw 
grip can consequently be maintained right to the end of the stride. c. 
Fourth hind toe of Lacerta agilis; because there is no inbuilt kinking or 
marked differential tendon displacement, the toe simply bows upwards 
when the ventral tendon is under tension 
positioning means that the mechanical advantage of the tendon 
varies with the particular articulation to which it is applying a 
turning moment; thus advantage is great at the two articulations 
where it is displaced downwards but weaker in between where, in toe 
4, phalanx 2 articulates with phalanx 3 and 3 with 4. Consequently 
the latter area can flex dorsally in response to lifting and forward 
movement of the metatarsal segment, while those bordering it retain 
their ventral flexion, maintaining the lowering of the toe below the 
level of the metatarsal segment and the grip of the claw. The way the 
toes of habitual climbers can flex simultaneously in two directions in 
a plane perpendicular to the substratum and also bend mesially 
contrasts with the situation in specialised ground dwellers. In these, 
because joints are double headed and because there is no kinking and 
the main ventral tendons do not show variation in degree of separa- 
tion from particular joints, the digits simply curve upwards into a 
regular arc (Fig. 17c); this places substantial restrictions on the 
possibility of vertical climbing in these forms (see below). 
