Bull. not. Hist. Mus. Land. (Zool.) 68(2): 155-163 



Issued 28 November 2002 



Holaspis, a lizard that glided by accident: 

 mosaics of cooption and adaptation in a 

 tropical forest lacertid (Reptilia, Lacertidae) 



E.N. ARNOLD 



Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD. 



SYNOPSIS. Holaspis is the most morphologically apomorphic lacertid taxon with 42 or more derived morphological features 

 arising on its exclusive lineage. Nearly all of these confer advantages in three specialised activities, or ameliorate problems 

 resulting from them. The activities are: climbing on the often vertical open surfaces on tree boles and branches, utilising very 

 narrow crevices in wood and beneath bark, and the ability, unique among lacertids. to glide from tree to tree. Although many of 

 the features related to these activities are likely to result from direct adaptation to the situations concerned, exaptation has been 

 critical in the development of gliding. Two behaviours present in the earliest lacertids have been coopted to this activity: rib 

 spreading associated with basking contributes to an effective aerofoil, and balance control associated with running helps maintain 

 appropriate posture in the air. Features originally developed in the context of crevice use also contribute to the aerofoil and a high 

 surface: weight ratio. So, while natural selection has moulded Holaspis for its present activities, multiple accidents of history have 

 also been important, as they also have in the evolution of bird flight. 



INTRODUCTION 



Sometimes there has been a flurry of adaptation on a lineage after a 

 long period of little or no obvious change. A plethora of apomorphies 

 may have been produced, often in association with shift into a new 

 and demanding niche or a succession of these. For instance, this 

 occurred in the lacertid lizard genus Meroles where apomorphies 

 accumulated in a series of increasingly extreme soft-sand environ- 

 ments (Arnold 1990, 1991 ). In other cases, not all the features that 

 confer performance advantage in such a selective regime necessar- 

 ily arose by natural selection in its context. In some instances, 

 features developed by natural selection in a different situation or by 

 some other means, and were only later coopted to a new function. 

 Darwin ( 1 872) was aware of this process which was named exaptation 

 by Gould & Vrba (1982). Cases of exaptation are very widespread 

 (Arnold, 1994; Gould, 2002) and contribute to the ability of lineages 

 to enter new selective regimes. Usually, optimum survival in these 

 involves combining exaptations with new features that are built by 

 the new selective regime. Exaptations are typically a small propor- 

 tion of the necessary features, but there are examples where a 

 number of characters really critical to invading the new regime are 

 exaptations. A case in point is the aberrant lacertid genus, Holaspis, 

 the only member of the approximately 1 700 species of Scincomorpha 

 known to glide regularly and effectively. 



TAXONOMY AND RELATIONSHIPS 



Until recently Holaspis was regarded as a single species with two 

 well-defined subspecies, but these are now each given species 

 status (Broadley, 2000) as Holaspis guentheri and H. laevis. H. 

 guentheri occurs in Sierra Leone, Ghana, Nigeria, Cameroon, 

 Congo, Uganda, Gabon and Angola, and H. laevis in Tanzania, 

 southeast Congo, Malawi and Mozambique. H. laevis has six dark 

 longitudinal stripes on the body instead of eight and has on aver- 

 age fewer, larger scales comprising the semitransparent window 



present in the lower eyelid which is generally rather better devel- 

 oped than in H. guentheri. 



Within the Lacertidae, morphology indicates that Holaspis is a 

 member of the subfamily Eremiainae (Harris, Arnold & Thomas, 

 1998) and within this of the Equatorial African clade (Arnold, 

 1989a, b.), which is relatively basal and has a generally primitive 

 morphology, most of its members not differing much from members 

 of the generally primitive subfamilies, Lacertinae and Gallotiinae. 

 The Equatorial African clade is characterised by the following 

 combination of derived features: pineal foramen lacking; medial 

 area of the clavicle not markedly expanded; only one postnasal 

 scale; parietal scale extending laterally to the edge of parietal table 

 of the skull, and the tympanic scale small. All except Holaspis also 

 have the postorbital and postfrontal bones fused, the absence of this 

 condition in Holaspis being secondary (Arnold, 1989a). Among the 

 Equatorial clade. morphology suggests Holaspis is the sister group 

 of two species of Adolf us, A. africanus and A. vauereselli (Arnold 

 1989a). Studies of mitochondrial DNA sequence (Harris & Arnold, 

 pers. obs) corroborate this relationship, although with only low 

 bootstrap support. 



MORPHOLOGY OF HOLASPIS 



The following account concentrates on those characters that are 

 peculiar to Holaspis and derived within the Equatorial African 

 group, and usually within the Lacertidae as a whole. These 

 autapomorphies and are listed in Appendix 1. For illustrations of 

 living Holaspis see Schmidt, 1919 (reproduced in Arnold 1989a), 

 Schiotz (1960) and Branch (1998). 



Holaspis are small lizards growing to a maximum of only about 

 53mm from snout to vent and a total length of 130mm. The whole 

 animal is extremely depressed, and more so than any other lacertid. 

 The index, head depth/ head width, measured on alcohol-preserved 

 specimens somewhat exceeds that found in most other flattened 

 lacertids (see for instance Arnold 1998a, p. 344), averaging about 

 0.54 when measured in adults (n = 10). This however does not give 



© The Natural History Museum. 2002 



