NICHE, MORPHOLOGY AND LOCOMOTION IN LACERTID LIZARDS 
Acanthodactylus, Meroles, Eremias, Holaspis and, in restricted 
form, in Pseuderemias. In Acanthodactylus a lateral scale row is 
present on the digits of manus and pes of all species, but an 
additional mesial row has developed on the manus perhaps three or 
more times in groups living mainly on soft sand (Arnold, 1983; 
Harris, Arnold & Thomas, submitted b). Meroles is similar in that all 
species have a lateral scale row on all digits, and a mesial row on 
those of the manus in a clade found on soft sand, consisting of the 
subgenus Saurites and Meroles anchietae. A mesial row occurs on 
the pes as well in Meroles anchietae which is found in the most 
extreme of such habitats (Arnold, 1991). Lateral and mesial scale 
rows have also evolved on all feet in the aeolian sand species of 
Eremias (Scapteira). Holaspis is distinctive in exhibiting additional 
scale rows only on some of the digits of the pes: digits 3 and 4 
possess lateral and mesial rows, while digit 5 has a lateral row which 
is continuous with similar scales on the trailing edge of the hind leg 
and the sides of the tail. 
In sand dwelling forms, the additional scale rows on the digits, 
which are often elongated and projecting, act rather like snow shoes 
during locomotion, reducing the tendency of the feet to sink into the 
yielding substratum (Carothers, 1986; Luke, 1986) and thus increas- 
ing effective thrust when running. However, it is notable that, 
although ground dwelling lizards obtain most locomotory thrust 
from the hind legs (p. 000), additional mesial rows of scales develop 
first on the manus. This may be because the forefeet especially are 
used in digging for food and to construct burrows and in this 
situation the fringes increase the efficacy of digging by broadening 
the toes so they shift more sand. Possibly, where sand is not 
especially soft, the functional advantage of an additional scale row is 
more critical in digging than running. 
Although lateral expansion of the digits appears to confer advan- 
tage when running and digging in soft sand situations, it is less clear 
why expansion should be achieved by separate additional scale rows 
in lacertids, since some sand-dwelling lizards in other families 
merely have the usual dorsal and ventral scale rows on the digits 
extended horizontally to form fringes (Luke, 1986). Indeed in sand 
lacertids without a mesial row, the dorsal scale row may project in 
this way. Possibly, separate rows of scales on the sides of the digits 
do not actually give better function, in impeding the toes when they 
are pressed into the substratum, than fringes produced from dorsal 
and ventral rows. They may however be advantageous in environ- 
ments where sand is very soft because fringes made up of independent 
scale rows can flex more easily ventrally, reducing impedence when 
digits are withdrawn from the sand. 
In contrast to their function in sand dwellers, the additional 
digital scale rows of Holaspis probably provide extra lift when this 
unique lacertid glides through the air (Arnold, 1989b). In some 
iguanians such fringes permit the lizards to run across the surface 
of water (Luke, 1986). Although fringes made up of additional 
scale rows on the digits thus occur in three superficially quite 
different situations, in all of them they slow or prevent passage of 
the feet through fluids. 
Not only have digital fringes in lacertids been elaborated by 
subsequent addition of separate lateral scale rows, but the length 
of the scales forming these also varies, often showing considerable 
correlation within a genus with the softness of the substratum 
usually occupied (Arnold, 1983). However, although some mem- 
bers of primarily sand dwelling clades appear to have reverted to 
firmer substrata, for instance Meroles suborbitalis, there are no 
certain cases where additional digital scale rows have been subse- 
quently lost even though their degree of projection may be 
reduced. 
81 
LOCOMOTION AND FUNCTION 
Some aspects of locomotion in habitual open ground lizards and in 
climbers are contrasted in Table 4 
Locomotion in ground dwellers of the Armatured 
clade (Figs 12—13) 
The following observations are based on Heliobolus lugubris, 
Meroles cuneirostris, M. reticulatus, M. anchietae, Eremias arguta, 
Acanthodactylus boskianus and A. pardalis. These were either 
videoed dorsally and laterally at 25 fields/sec and an exposure of 
1/1000 sec., or filmed at 16-48 frames/sec. Meroles cuneirostris was 
also videoed at 200 fields/sec. Most runs were conducted on a flat 
cork surface but animals were also allowed to sprint across soft sand 
and the footprints produced used to to check stride length and 
relative thrust of the fore and hind feet, as indicated by pressure 
waves in the sand produced at the trailing edge of the prints. 
Lacertid lizards use all four legs when running. The gait is 
sprawling, that is the humeri and femora project from the body 
roughly in the horizontal plane, and the steps of individual limbs can 
be divided into two phases: the power stroke when the limb is 
retracted and actually delivers thrust, and the recovery stroke when 
it is is brought rapidly forwards in preparation for the next step. 
Typically the fore and hind limbs work in diagonal pairs, for 
instance, the right foreleg and left hindleg are brought forwards in 
the recovery stroke at about the same time and are retracted more or 
less together in the power stroke; there may however be a slight lag, 
so that a hindlimb starts to move forwards after the contralateral 
forelimb. 
At extreme phases of the locomotory cycle, the forelimb on one 
side of the body is directed backwards and the hindlimb forwards, so 
they approach each other or overlap, while on the other side of the 
body the limbs are directed diametrically away from each other. In 
general, strictly ground-dwelling lizards of the Armatured clade 
carry the body well away from the substratum when running. At the 
end of the power stroke of a hind limb, the lizard may be balanced on 
the toes of a single foot and this is followed by a gliding phase when 
the animal ‘floats’ forwards with all limbs off the ground. 
Because of this floating phase, the total stride of each limb pair, 
that is distance between ground contact of left and right feet, may be 
substantially greater than the anatomical stride which is the distance 
between the feet of a limb pair when they are maximally spread 
forwards and backwards. As forelimb span is much less than hindlimb 
span in ground dwellers, the difference between total and anatomi- 
cal strides is much greater for the forelimbs and they are both off the 
ground for much longer periods than the hindlimbs. 
The posterior body flexes laterally to some extent during rapid 
locomotion towards the side on which the hindlimb is moving 
Table 4 Some characteristics of open ground and climbing locomotion in 
lacertids specialised to these activities. 
Fast ground Vertical 
Body close to substratum no yes 
Anatomical stride of forelimbs short long 
Hind leg delay some more 
Crus extended right forwards yes no 
Hind step length/snout-vent distance often >21 often < 1 
Floating phase yes no 
3 legs often in contact not usually yes 
Toe 5 makes positive grip no often 
Rise on toe tips at end of stride yes no 
