Most lacertid lizards are content scurrying in and out of nooks and crannies in walls and between rocks. However, some have opted for an arboreal life style. Holaspis guentheri leap from branch to branch as they scamper through trees in the African forest. There are even anecdotes that the tiny African lizards can glide. But without any obvious adaptations to help them to upgrade a leap to a glide, it wasn't clear whether the reptiles really do take to the air and, if they do, how they remain aloft. Intrigued by all aspects of lacertid locomotion, Bieke Vanhooydonck from the University of Antwerp and her colleagues, Anthony Herrel and Peter Aerts, decided to find out whether H. guentheri really glide. Recruiting undergraduate Greet Meulepas to the team, they began filming dainty H. guentheri lizards,gliding geckos (Ptychozoon kuhli) and landlocked Podarcis muralis lacertid lizards as the animals leapt from a 2 m high platform to see if the African lizards really could glide(p. 2475).
Unfortunately, filming the tiny lizards was extremely difficult. Having startled the small animals into leaping off the platform, the team had little control over the animal's direction, and couldn't guarantee that it was parallel to their camera. It was also difficult to capture each trajectory with a single camera and tricky to get the lighting conditions right. But after weeks of persistence the team finally collected enough film, as the lizards leapt, to compare their performances.
At first, it didn't look as if the African lizard was gliding any better than the P. muralis. Both animals were able to cover horizontal distances of 0.5 m after leaping from the platform, while the gliding gecko covered distances greater than 1 m, aided by its webbed feet and skin flaps. But when the team compared the lizards' sizes, they noticed that there was a big difference between the two lacertid lizards. The tiny African reptile only weighed 1.5 g, almost 1/3 of the larger lacertid lizard's weight and 1/10 the gliding gecko's mass, so Aerts calculated how far each lizard would travel horizontally if they fell like a stone. This time it was clear that the tiny H. guentheri was travelling 0.2 m further than he would have expected if it were simply jumping off the platform. Holaspis guentheri was definitely delaying its descent and landing more slowly than P. muralis; it was gliding.
But how was the tiny lizard able to remain airborne for so long? Maybe the lizard was squashing itself flat while gliding to increase its surface area and generate more lift. But when the team analysed the lizards' trajectories, H. guentheri's shape did not change. And when Aerts calculated the amount of lift each lizard generated as they descended, it was clear that H. guentheri was unable to produce a lift force. The team realised that instead of increasing its surface area to generate lift, H. guentheri is able to glide because it is so light. The lizard's `wing loading' (mass:surface area ratio) was the same as that of the gliding gecko(assisted by skin flaps and webbed feet) so the lizard was able to glide like a feather because it was so light.
Curious to find out why H. guentheri is so light, Herrel contacted Renaud Boistel, Paul Tafforeau and Vincent Fernandez at the European Synchrotron Radiation Facility to scan all three lizards' bodies. Visualising the animals' skeletons with X-rays, it was clear that H. guentheri's bones were packed full of air spaces, making the lizard's skeleton feather light for gliding.