Blue-spotted tree monitor’s mysterious colour

By Bec Crew | April 30, 2015

This PNG lizard boasts a colour that’s highly unusual in nature

Contributor

Bec Crew

Becky Crew is a Sydney-based science communicator with a love for weird and wonderful animals. From strange behaviours and special adaptations to newly discovered species and the researchers who find them, her topics celebrate how alien yet relatable so many of the creatures that live amongst us can be.

OF ALL THE COLOURS in the world, blue is one of the most unlikely pigments to appear on the feathers, fur, scales, skin, and exoskeletons of the creatures around us. In fact, unlike the browns and greys that are easy to produce, and even the bright yellows, pinks, and reds that birds in particular can bring out through the foods they eat, blue is an impossible option for almost everybody.

“Blue is fascinating because the vast majority of animals are incapable of making it with pigments,” Yale University ornithologist Rick Prum told Rae Ellen Bichell at NPR late last year.

Even many of the creatures that you do see adorned in blue, such as the gorgeous Australian Ulysses butterfly (Papilio ulysses), the bright blue neon cuckoo bee (Thyreus nitidulus), and the common peacock with his almost electric take on the hue, don’t actually produce the pigment either. They manage to produce it through trickery known as ‘structural colour‘.

“They have evolved a new kind of optical technology, if you will, to create this colour,” Prum told NPR. So instead of lining their external structures with blue-coloured cells, these butterflies, birds, and bees produce sheets of colourless cells that are structured in such a way to reflect light that appears blue to onlookers.

How does monitor produce its blue colour?

While it hasn’t been studied directly, the brilliant blue colouring of the blue-spotted tree monitor (Varanus macraei) perhaps comes from the same structural trickery.

Endemic to the tiny island of Batanta, off the western coast of New Guinea’s Vogelkop Peninsula, this gorgeous reptile is covered in black scales patterned in blue ‘ocelli’ – eye-shaped formations.

Due to its tiny size of just 450 sq.km, the island of Batanta holds the smallest known distribution of any tree monitor in the world.

Blue-spotted tree monitors can grow to just over a metre for adult males – the females are slightly smaller – with a tail that extends almost twice the length of its body. As you can see in the video below, their tails are fully prehensile, which allows them to grip to tree parts to stay aloft in the forest canopy.

While the species if becoming increasingly popular in the pet trade, very little is known about their habits in the wild. What I’d like to know is how and why they ended up so blue, and if there’s an equally flamboyant courting routine to match.