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Monday, May 23, 2005

Moloch horridus

David Attenborough has narrated a documentary on lizards. What I saw of the documentary focussed on the lizards known as "dragons", Agamidae, which are mostly found in Australia. In particular there was a sequence on Moloch horridus, the Thorny Devil.

Both this website and Attenborough refer to the curious forwards-backwards shaking movement the lizard makes as it moves slowly along. That is, it rocks backwards and forwards as it walks. Both the website and Attenborough imply that this is a kind of camouflage or cryptic movement, to help the lizard resemble swaying vegetation. The Thorny Devil is a prey item as well as a predator and it is conjectured that the rocking movements help its camouflage.

Other animals that make similar rocking movements as they walk include some chameleon lizards, if I recall correctly, as well as many stick insects and praying mantises. In the case of these insects, a cryptic resemblance to swaying vegetation has also been proposed.

On the other hand, as I proposed in a paper some years ago in an entomological journal, the rocking movements made by these large, typically arboreal insects may actually be to aid in achieving vision. By moving their body (and therefore their eyes) backwards and forwards the animal can detect objects and gauge their distance from the objects' parallax movement against the background. This would be particularly valuable for animals with comparatively simple vision that do not move quickly and freely enough to generate relative motion - and therefore parallax movement in the visual field - in any other way. That is, rocking movements among these insects are a way of obtaining visual information when the whole animal is not able to move freely and quickly.

What applies in the case of these large, slow-moving arboreal insects may also apply to the slow-moving Thorny Devil. That is, the rocking movements of the lizard may be to help the Thorny Devil see objects, not to help it resemble swaying vegetation.

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