88 
Kinking of the hind toes of climbing lizards then is a very simple 
feature that has profound effects on foot function: toes 3-5 can be 
shortened to provide a positive grip; when directed backwards or 
outwards, they can be displaced downwards so that they maintain 
their claw contact with the substratum, even though the posterior 
part of the metatarsal segment to which they are attached is rising; 
simultaneous flexing in different directions in the parasagittal plane 
is possible. Not surprisingly, such a simple but elegant and produc- 
tive mechanism has arisen many times in climbing lizards (see 
p. 77). As noted, it seems probable that the numerous variants in the 
exact pattern of kinking within the foot that are found in lizards as a 
whole (p. 77) are to a large extent functional alternatives rather than 
adaptations to different situations. 
The forefoot shows some functional similarities to the hind one. 
The digits are spread very widely when the claws are first inserted 
and possibly contraction within the palm draws the metacarpals 
closer, tensioning the fingers. As in the hind limb, the shortness of 
intermediate phalanges in digits 3 and 4 probably concentrate dorsal 
flexion allowing it to be sharper and letting the forelimb be turned 
over without being displaced much outwards. The peculiarities in 
Holaspis have not been investigated in a living animal but they may 
allow the limb to act even more effectively in a parasagittal plane. 
In general the digits of climbing lacertids act differently from 
those of habitual ground dwellers. Instead of the weight of the 
animal being balanced on columns of phalanges at times, it is 
supported by tension in the ventral tendons. The phalanges are 
subjected to a compressive force by this but, because the tendons are 
firmly attached by ligamentous sheaths at each joint, such force is 
along the length of the phalanx and consequently exerts little shear. 
Also, as the tendon insertion on the claw is offset from the pivot for 
this on the penultimate phalanx, thus increasing its mechanical 
advantage, compressive forces along the axes of the toes will be 
reduced. The largely tensile role of the toes in climbers is reflected 
in their slender phalanges and robust ventral tendons and the net 
lateromesial compression of the toe this produces compared with the 
toes of ground dwellers (Figs. 9b, 10). 
Climbing in specialised ground dwelling lacertids 
Members of the ground dwelling clade consisting of Latastia and its 
sister group are incompetent climbers. In trials using single lizards 
of each species, Meroles reticulatus could not climb a concrete slab 
that was at a much steeper than 60°from the horizontal; the max1- 
mum angle for Acanthodactylus erythrurus and A. scutellatus was 
70°, and for A. boskianus 80°. In these species and other ground 
dwellers such as Lacerta agilis, the hind toes cannot flex mesially or 
dorsiflect as they do in specialised climbers; as already noted they 
simply bow upwards instead. In contrast, specialised climbers like 
Lacerta oxycephala and L. perspicillata could climb the slab with 
ease when it was vertical or even overhanging by 10° or 20°. 
CONCLUDING REMARKS 
Limb proportions and foot morphology of lacertid lizards are obvi- 
ously evolutionarily plastic and numerous changes in these features 
have taken place within the family, often in different directions. 
However, although extreme variants are quite different, virtually no 
anatomical changes are obviously likely to be irreversible, in the 
way that loss of phalanges or claws that occur in many gekkotans 
seem to be. (Development of extra rows of scales on the sides of the 
toes may be a possible exception). 
Across the family, changes in limb proportions and foot structure 
correlate quite closely with shifts in structural niche and the different 
E.N. ARNOLD 
locomotory problems that these entail. It is possible to interpret the 
different morphologies in functional terms as conferring perform- 
ance advantage in these situations. Clearly, locomotion in different 
habitats requires different morphological features, in particular, 
running on open ground, climbing on open surfaces and traversing 
vegetation matrixes. Adaptation to any one of these reduces locomo- 
tory effectiveness in the others. For instance, the robust, stiff digits 
that allow ground dwellers to run partly on their toe tips restrict 
climbing ability, while the flexible toes advantageous to climbers are 
inappropriate for the most effective kind of ground locomotion. 
Species which occur in a range of structural habitats consequently 
must compromise in locomotory terms and are probably not 
maximally effective in any one situation. Whether they always 
converge on a functionally intermediate morphology or whether it is 
sometimes more effective to be efficient in one area but accept 
penalties in another is not yet clear. However, Podarcis pelopon- 
nesiaca at Stymphalea, S. Greece, runs effectively on the ground and 
also climbs readily on rock outcrops but it is very clumsy in the latter 
situation compared with rock specialists. (Arnold, 1987). 
The conflicting mechanical demands of locomotion in different 
environmental situations and the fact that they are largely unresolvable 
is one of the main reasons why mechanical aspects of habitat 
comprise such an important parameter in the structure of lizard 
communities (Arnold, 1984, 1987). Actually, it is not habitat per se 
that causes the conflict but the fact that really efficient physical 
compromises seem impossible. 
Overall there is great homoplasy among lacertids not only in 
structural niche but also in the locomotory mechanisms associated 
with these. 
ACKNOWLEDGEMENTS. N. P. B. Arnold helped with the video work, and 
P. Crabb and G. Summons (Ministry of Defence, Woolwich Arsenal) pro- 
vided some high-speed video facilities. J. Vindum, W. R. Branch, R. Arnold 
and C. J. P. Arnold were active in field collection and observation. H. in den 
Bosch donated essential specimens and, with W. R. Branch, M. Largen and J. 
Vindum, provided information about habitat and behaviour. L. Hartley 
collected data on the caudifemoralis muscle. C. J. McCarthy helped in a 
variety of ways. N. Tinbergen and A. J. Cain supervised some of the earlier 
parts of this study. I am grateful to all of them. 
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