162 



E.N. ARNOLD 



coopted from an earlier activity. All lacertids. including Holaspis, 

 appeal - to spread their ribs when basking in relatively cool conditi- 

 ons, increasing surface area and rate of heat intake. 



Permanent depression of the head, body and limbs of course also 

 contributes to the aerofoil and we can ask whether this is a special 

 feature of gliding or whether it is coopted from crevice use. As 

 already noted there is some phylogenetic evidence of its earlier 

 origin for crevice use, something that has occurred in many inde- 

 pendent lineages. 



Modifications for gliding in Holaspis are quite extensive, several 

 organ systems being involved. It is therefore rather surprising that 

 Holaspis has not developed a more efficient aerofoil such as occurs 

 in Draco. However to do this would probably involve the develop- 

 ment of a delicate patagium or extensive lateral skin flaps. It is likely 

 that such structures would interfere with the lizard's ability to enter 

 and move in the narrow crevices it regularly utilises. Consequently 

 Holaspis is restricted to using means of broadening the body that do 

 not project exteriorly. 



As a stable glider, Holaspis is very dependent on its long tail, but 

 this can break easily, even close to its base where loss might make it 

 unstable in the air and reduce its ability to control its glide path. 

 Nonetheless the tail is lost and often regenerated in many individuals 

 (Arnold, 1984). This emphasises the importance of tail loss as an 

 antipredator device and suggests the cost: benefit ratio still favours 

 frequent tail loss in Holaspis even though locomotory costs may 

 well be high. 



It might be thought that cases like Holaspis, where entrance into 

 a new life mode has been dependent on multiple exaptations, are 

 rare. But this phenomenon occurs in another instance where aerial 

 locomotion has been attained, that of birds where feathers and the 

 complex mechanism of wing folding arose long before gliding or 

 active flight (Gauthier & Gall, 2001). 



ACKNOWLEDGEMENTS. I am grateful to P. Agland, D. Broadley, G. Dunger, 

 the late A. Loveridge, and A. P. Mead for information about the behaviour of 

 Holaspis in the field, and to Garth Underwood for helpful discussion. An 

 earlier version of this paper formed part of a D. Phil thesis submitted to the 

 University of Oxford. In connection with this 1 thank my supervisors, the late 

 A. J. Cain and the late N. Tinbergen, and the Scientific Research Council of 

 the United Kingdom for providing funding. 1963-1966. 



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