A tiny species of lizard is so light that it falls to the ground like a feather, scientists have discovered.
Outwardly, little of the animal's body seems adapted to flying, gliding or moving through the air in any way.
But a slow-motion camera has revealed that when the lizard jumps from a height, it can slow the rate of its descent and land gently on the ground.
The lizard's surprising aerial ability might help explain how some animals became true gliders.Details of the little lizard's talents are published in the Journal of Experimental Biology.
Controlled descent
Active flight, powered by the flapping of wings, has evolved in three living lineages of animals: birds, bats and insects.
But at least 30 different types of animal have evolved the ability to control their aerial descent, by parachuting or gliding to ground.
For example, gliding frogs use huge webbed feet, flying squirrels use long flaps of skin between their legs, and flying fish use their fins to glide.
Other animals have less obvious morphological adaptations.
Gliding snakes flatten and undulate their bodies, which helps to slow their fall while some species of ant are so tiny they can jump out of trees and freefall gently to lower on the trunk without hurting themselves.
So Bieke Vanhooydonck of the University of Antwerp became extremely interested when she read some old scientific papers reporting anecdotal evidence that a relatively ordinary species of lizard might also be able to glide from tree to tree.
Holaspis guentheri belongs to a group of lizards known as lacertids, which live in the Old World.
Though colourful, they do not stand out in terms of their behaviour, morphology or ecology.
"Also, compared to other gliding lizard species, it does not have any conspicuous morphological adaptations to an aerial lifestyle, ie no cutaneous flaps, webbed feet etc," says Vanhooydonck.
"It made me very curious about whether these animals were really able to 'glide' and if so, how they were accomplishing it."
Leaping platform
So Vanhooydonck and colleagues in Belgium and France filmed individual lizards leaping from a platform two metres above ground.
They compared the performance of H.guentheri with a rock-dwelling lizard ( Podarcis muralis ) that never takes to the air, and a highly specialised leaping gecko ( Ptychozoon kuhli ) that has a range of skin flaps that it uses to parachute to the ground.
For each, they examined the duration of each species' descent, the horizontal distance it covered and at what speed.
Both the rock-dwelling lizard and H.guentheri landed 50 centimetres from the base of the platform, while the gecko landed up to 1m away. But H.guentheri fell for longer, and more slowly than its rock-dwelling competitor.
"Much to our surprise, H. guentheri is able to slow down its descent and has low impact forces upon landing," says Vanhooydonck.
In fact, the lizard weighs just 1.5g, which is one third of the rock-dwelling lizard's weight and one-tenth of the gecko's.
Once weight was factored in, the researchers found that H.guentheri landed 20cm further away that it should have done had it fallen like a stone.
"Also its wing loading, the ratio of mass to surface area, is extremely low and in the same range as that of the gekko."
However, the two species achieve this aerial ability in different ways. As a result of its webbed feet and body flaps, the gecko achieves a low wing loading by having a large surface area.
H. guentheri has a low wing loading too, but by being so light.
X-ray scans of the lizard's body revealed its bones are packed full of air spaces.
Although the lizard's light weight and ability to fall gently are linked, it is still unclear whether its air-filled bones are an adaptation for parachuting, or whether they evolved for another reason.
It is also unclear whether H.guentheri glides from tree to tree to escape predators or move about more efficiently.
"Because of [the lizards'] secretive lifestyle, it is very hard to observe them in the wild, but it seems plausible they use it as an escape response," says Vanhooydonck.
And that could be just how other gliding animals took the first evolutionary steps towards an aerial lifestyle, she says.