EVOLUTION OF HOLASPIS 



159 



BEHAVIOUR 



Holaspis guentheri occurs especially in rain-forest situations while 

 H. laevis also extends into savannah. These lizards are nearly always 

 observed at some height on the trunks and branches of standing 

 trees, occurring at least up to 30m, and do not usually come down to 

 the ground (H. guentheri: H. Lang in Schmidt. 1919; Perret & 

 Mertens. 1957; Schi0tz & Vols0e, 1959; Laurent, 1964; Dunger, 

 1967; P. Agland, pers. comm. A. P. Mead, pers. comm. H. laevis: 

 Barbour & Loveridge, 1928; Loveridge, 1951, 1953; De Witte, 

 1953; Branch, 1998; D. G. Broadley pers. comm.), although they 

 can occur on fallen timber (H. Lang in Schmidt, 1919). Holaspis 

 spp. are active hunters, constantly moving and searching and often 

 investigating crevices (P. Agland, pers. comm.) in which they also 

 frequently hide when disturbed and at night (H. Lang in Schmidt, 

 1919; Loveridge, 1951; Laurent, 1964; pers. obs. on captive ani- 

 mals). They are extremely agile, moving with ease on vertical and 

 overhanging surfaces (H. Lang in Schmidt, 1919). Holaspis appear 

 to thermoregulate and at times basks in patches of sunlight for at 

 least up to ten minutes (Dunger, pers comm.; P. Agland, pers 

 comm.). As in many other basking lacertid lizards, the body is 

 spread and flattened by the dorsal ribs being rotated forwards and in 

 Holaspis the body becomes as flat and round as a coin (Dunger pers. 

 comm.. P. Agland pers comm.). 



Holaspis is unique among lacertid lizards in being able to glide 

 between trees. This behaviour was first formally reported in H. 

 guentheri in Ghana by Schi0tz & Volsoe (1959) and subsequently 

 confirmed by P. Agland in Cameroun and A. P. Mead in Nigeria 

 (pers. comms). Earlier reports also provide some collaboration. C. J. 

 P. Ionides (quoted by Loveridge, 1955) noted that in Tanzania H. 

 laevis covers long distances in leaps between trees and Laurent 

 (1964) reported that local people in northern Angola said that H. 

 guentheri can fly. According to Schi0tz & Vols0e (1959). this lizard 

 starts from a head-downwards position, high on a tree trunk from 

 which it leaps outwards and glides steeply downwards. The trajec- 

 tory later becomes shallower, and just before the lizard alights, it 

 turns slightly upwards. For most of the glide, the lizard is orientated 

 with its sagittal axis along the direction of motion, but towards the 

 end this becomes perpendicular to it, the lizard stalling and reducing 

 speed by this means. In one measured leap a lizard travelled 10.5m 

 horizontally while dropping 9m, an overall angle of about 42° from 

 the horizontal. Holaspis appears capable of selecting a target before 

 launching itself, and of changing direction in mid-flight. 



Among the H. guentheri observed by P. Agland (pers. comm.) one 

 glided 30m at an angle of 10-20°, another travelled 25m and a third 

 6m. Motion was fast and straight and again appeared to be directed. 

 In some cases there was an initial drop before the trajectory levelled 

 out but in one instance a lizard running horizontally on a branch 

 launched itself into the air without much fall before stabilising its 

 flight path. At the end of a dive animals again alighted head 

 upwards, landing very fast and sometimes immediately running 

 upwards. Holaspis clearly has the ability to maintain its belly- 

 downwards posture in the air with limbs spread and to change 

 orientation as appropriate. 



FUNCTIONAL ANATOMY 



In this section an assessment is made as to whether particular 

 morphological apomorphies of Holaspis could have evolved through 

 direct adaptation by natural selection in connection with one or more 

 of its special behaviours: frequent locomotion on very steep often 



vertical open surfaces, use of very narrow crevices, and gliding. 

 Assessment is made on two criteria: 1 . perceived functional benefit 

 of the apomorphies in the activities concerned; and 2. whether 

 similar apomorphies have appeared independently in other lizards 

 that have evolved similar behaviours. The second criterion is most 

 convincing if there are multiple independent origins of the apomorphy 

 and if these origins are correlated with appearance of the relevant 

 behaviour on the lineages of the taxa concerned. Even if there is a 

 prima facie case for functional advantage of an apomorphy in 

 connection with a particular behaviour, its absence in forms that 

 have evolved the behaviour independently raises the possibility that 

 it is not connected with the activity concerned. Alternatively, it may 

 represent one of several strategies with other taxa gaining similar 

 advantages in different ways. 



LOCOMOTION IN tree BOLES. The functional advantages of near- 

 equality in fore and hind limb spans, and of characteristic foot 

 architecture, in climbing on steep open surfaces has been discussed 

 elsewhere (Arnold 1998b). These features are particularly well 

 developed in Holaspis and presumably related to the abundance of 

 such surfaces in its environment. The unique manus features of 

 Holaspis suggest the forelimbs are sometimes used in parasagittal 

 planes (Arnold. 1998b). This may be when the lizard launches itself 

 from a head-downwards position on a steep surface. Extending the 

 forelimbs at this time would push the foreparts of the body out into 

 a more horizontal position, putting it closer to its orientation when 

 gliding and making an outward leap easier. 



USE OF CREVICES. Features that confer advantages in crevice use 

 and the functional basis for this has already been surveyed (Arnold 

 1998a). Many derived features of Holaspis occur in other lacertids 

 that use rock crevices, having developed independently at least once 

 in archaeolacertas (Lacerta spp.), and in Oma.nosa.ura cyanura and 

 some populations of Podarcis hispanica. These forms show many 

 apomorphies similar to those of Holaspis although the features are 

 less developed than in this form, especially the degree of flattening 

 of the head, body and limbs. These low vertical dimensions enable 

 lizards to enter narrow crevices and a variety of cranial features 

 (Appendix 1. numbers 4, 6, 7, 8, 10 and 12) results in a deformable 

 skull that can be inserted into irregular spaces. Increased cranial 

 kinesis enables the skull to be flattened further by protraction on 

 entering a crevice and locked into place by subsequent retraction. As 

 a result of flattening of the skull, the eyes, which are large, project 

 well above it during normal activities and this potentially impedes 

 entry into crevices. However, in lacertids including Holaspis each 

 eye is pushed downwards as the lizard enters a crevice by contact 

 with the crevice roof so that its lower surface deflects the flexible 

 membrane crossing the greatly enlarged inferior orbital foramen. 

 This enables the lower part of the eye to project into the buccal 

 cavity, so that it can be housed within the depth of the head. 

 Reduction of the supraocular osteoderms increases the flexibility of 

 the skin over the eyes so that its geometry can alter during their 

 depression. Reduced overlap of collar and belly scales increase 

 smoothness enabling lizards to move easily both forwards and 

 backwards in crevices. Some or all these features are paralleled in 

 many non-lacertid crevice users including skinks (such as Mabuya 

 laevis and M. sulcata), xantusiids (Xantusia henshawi), geckos 

 (Afroedura) and iguanids (Sauromalus, Opiums). 



Holaspis has other features not found in other crevice-using 

 lacertids but present in the most extremely flattened exploiters of 

 rock crevices in other families, such as Platysaurus (Cordylidae) 

 and Tropidurus semitaeniatus (Iguaninidae, Tropidurinae) and prob- 

 ably functionally associated with such strong depression. Among 

 these is modification of the scleral ossicles, so that there is one or 



