No UV vision for trend-bucking blue-tongued skinks

Abstract

Squaring up to an adversary is always risky; you could break a limb, suffer damaged skin, or lose scales if you're a lizard. However, blue-tongued skinks may have a less confrontational strategy for diffusing conflict: they waggle their tongues at each other. But these are no ordinary tongues. As the lizard's name suggests, they are blue. But what exactly does a blue-tongued skink opponent see when a rival flashes its tongue? Well, that depends on the light-sensing photoreceptors at the back of the lizard's eye and which shades the receptors are tuned to, yet no one had taken a close look at blue tongued skinks’ eyes to find out how the reptiles see colour. So, Nicolas Nagloo and Jan Hemmi from the University of Western Australia and colleagues decided to take a close look at the back of one blue-tongued skink's eyes – the sleepy lizard, Tiliqua rugosa – to begin to understand what the animals see when a threatening rival waggles its tongue.It turns out that the reptile's eyes are more sensitive to shades of blue than other lizards and when the team checked for the essential light-sensitive proteins (photopigments) that specifically tune light-sensitive photoreceptors to certain shades of colour (wavelengths of light), they discovered that the sleepy lizard's eye has five types of photopigment: two tuned to shades of blue, two to shades of green and a fifth that picks up yellowy shades of green. Yet, when the team measured the electrical signals produced by the sleepy lizards’ retina, the blue-tongued skinks were barely able to see ultraviolet wavelengths, which are clearly sensed by other lizards that see ultraviolet light beyond the rainbow we know.However, the team is still a long way from understanding what the lizards truly see when another sleepy lizard shoots out its tongue. It is not clear which shades the blue-sensitive photopigments in the eye are precisely tuned to and how natural light filters formed by oil drops in the retina might modify the lizard's colour vision. But it is clear that the blue-tongued skinks see colours in a very different way from other lizards, which have a visual pallet that extends into the ultraviolet. Whether the sleepy lizard's lurid tongue or restricted colour vision range came first is also not apparent, but it is clear that various factors have moulded the blue-tongued skink's vision to be distinctly different from that of other lizards.