Australasian Science Magazine

Two mantis shrimp wrestling. These animals have the most comprehensive set of colour receptors known to humans. Credit: Roy Caldwell

By Noor Gillani

Colour perception is more advanced in goldfish than humans, yet researchers have tended to focus on vision in animals similar to us. Justin Marshall says this is “fundamentally stupid” and is setting his sights on a marine creature with 12 different colour receptors.

Humans may have come a long way in their use of colour since the days of finger-painting, but we will never see it the same way creatures of the deep do. Prof Justin Marshall of the Queensland Brain Institute says that most animals live in a world more colourful than the human one. “We think that we’re quite good with colours,” he says. “Some humans will tell you we’ve got the best colour vision in the world.

“We don’t,” he adds. “Most of the other animals out there have better colour vision than we do. Even things like goldfish.”

Marshall’s specific focus is on the mantis shrimp, which possesses 12 colour receptors in the retina. This compares with just three in humans: red, blue and green. Hence Marshall says that mantis shrimp have “ the world’s best colour vision system. They’re four times better [than humans] – and they’re a shrimp.”

Yet Marshall says that there has not been sufficient research on the role that colour perception plays in the lives of animals. “I’d say we know 1%.”

Marshall says that scientists often neglect in-depth research on animals that seem dissimilar to humans. “There’s a great deal that still needs to be learnt about colour vision in other animals. In particular, other animals other than primates. We tend to be a bit fixated on things that are close to us because we want something that’s going to do us good. This is a fundamentally stupid way to do research.”

Marshall said the majority of our knowledge about our own biological systems comes from observing weird and wonderful creatures. “For instance, almost everything we know about your nervous system – so how your ear is transmitting the sound to your brain, or how you can pick up your phone and hold it, what the nerves are doing – that was discovered by looking at squid,” he said.

Marshall’s earlier research has examined how mantis shrimp vision could help medical scientists design cameras to detect cancer and visualise brain activity. “The importance of looking at other animal systems and other animal visual systems in this context can’t be overemphasised,” he said.

Marshall recently co-authored an international study in Science (http://tinyurl.com/ydd4yhl2) examining the importance of colour vision in animals as it evolved for purposes of social signalling, reproduction and survival. The study also explored the interaction certain species have with forms of colour unknown to the human eye, such as ultraviolet light.

This field of visual ecology is interdisciplinary and rapidly growing, fuelled by technological advances such as spectrophotometry, digital imaging and large-scale comparative analyses. As old questions are answered, new ones take their place. For instance, researchers can now question the evolution of camouflage based on what visual capability a prey’s predator has.

Marshall said his overarching conceptual question is: “Why are animals colourful? I’m also interested by animals which don’t use colour, which is the cephalopods – so the cuttlefish, octopus and squid – which are completely colourblind yet they manage to camouflage themselves very well.”

Cephalopods use colour-changing cells called chromatophores that lie below the skin. The cells contain an elastic sac of pigment that, when squeezed, pushes pigment to the surface.

“Changing colour is a good defence, so good camouflage, but it’s also a way of communicating both between themselves and also with other animals,” Marshall said.

Besides manipulating colour as a defence mechanism, in many ways animals associate with colour in the same way as humans. “Charles Darwin and [Alfred] Wallace studied animals and animal colour a lot, and couldn’t really see the point of colour,” Marshall said. “And in Wallace’s case he sort of believed that there’s actually an aesthetic sense in animals – that they just like it because it might evoke an emotion or it might make them happy in some way. So he actually believed that it’s possible that animals just liked colours.

“Why does a parrot have to have 14 different colours on its body? What’s wrong with five? The birds of paradise are ridiculous in the way they use and manipulate colour.”

Humans are animals too, although not very colourful ones, so we “tend to borrow colour strategies. We use colours for camouflage; we use colours for appetising; a lot of our colour world is to do with sex, so a lot of appetising for mates. We do that, whether we think we do or not. The clothes that we wear, the way in which we accentuate our secondary sexual characteristics.

“We change colour. If someone really embarrasses you, you might go red. If someone really pisses you off, you might go red.”

Marshall says that humans rely on colour-related physiological mechanisms to communicate things like pleasure, pain and anger, “either to do with the predatory relationships, defence or sex… one of the three things in life: don’t get eaten, eat things, and make more of yourself.

“And that’s what colour’s all about.”