Colour, sex and violence: the bizarre world of vision

According to Justin Marshall, the animal with the most sophisticated visual system is the stomatopod. “At least at the retinal level,” Marshall, a biologist at the University of Queensland’s Brain Institute, says. “It has 20 channels of information – 12 in colour, six polarised and two in black and white.”

Also known as the mantis shrimp, the stomatopod is a small reef-dwelling crustacean and one of the many examples cited in Marshall’s recent review paper, published today in the journal Science with 26 other co-authors, from biologists to neuroscientists.

The paper cites many of the roles that the animal’s appearance — in academic parlance, animal coloration — plays, from social interaction to microbial protection, from UV filter to defence mechanism. Consider, for instance, the broad-tailed hummingbird and its iridescent throat, which changes between black to magenta depending on the angle of sight. Or, of course, the mantis shrimp, which not only displays a variegated colour pattern (they even fluoresce during mating rituals) but possesses extraordinary vision across the visible and the UV range. “We are interested in brains and the extensions of those brains out into their surrounding world,” Marshall says. “I am mostly interested in the visual information. Animal behaviour and why they do things when having seen a visual stimulus.”

We talk to Marshall about the biology of colour, stomatopods, and how coloration influences sex and violence.

WIRED: What first motivated you to study the biology of colour?  
Marshall: My mother was an artist and my father a marine biologist working on fish. They wrote a book on tropical marine biology and helping to collect specimens and watch my mother paint got me interested. I have been a marine scientist from around the age I learned to walk. Like anyone who has been to a reef, I am continually blown away by its colours. Colour is cool, sex and violence, camouflage and conspicuousness. So many answers to this question [of colour]. I see colours in everything and learning more about how animals use colour language is an endless source of wonder. ‘One fish two fish red fish blue fish.’ Thank you Dr. Seuss!

What’s the function of colour from an evolutionary point of view?  
Many, but sex and violence just about covers it. Colour is used to hide from predators in camouflage. It is used to lure unsuspecting animals to their doom (the red of carnivorous plant traps, anglerfish lures). It is used in violence in other ways – in territoriality and the display of ownership by strutting males – and it is used to attract and impress mates: ‘Look what I can make, mate with me and your offspring will be able to do that too or be as successful a forager as I am.’
Part of your research focuses on the mantis shrimp. Can you describe their colour vision?  
No. Nobody can yet as the smart little shrimps have come up with an entirely new way of encoding colour. In fact we struggle to describe any colour vision other than our own as in the end there is no such thing as colour. Colours are a set of subjective labels we use to describe subsections of the spectrum, or spectral combinations of it, that we can see using our three colour sensitivities: cones for red, green and blue. Many animals see ultraviolet and we do not. We have no labels for that part of the spectrum. Mantis shrimps see more of the UV than any other animal and have 12 receptors for colour, four times as many as humans. All that chromatic information flowing into a tiny little shrimp-brain and we still do not understand what they use it for, other than behaviourally – there is a lot of sex and violence in their world. They pack a nasty punch or stab, hence the name mantis shrimp. They cavitate water. Nasty! Actually, the best guess I have at the moment is that their eyes encode colour as a frequency pattern, more akin to the way the ear decodes the frequency space of sound.
Can you give me a few examples of animals that use fluorescence and enhanced colours and for what purpose?  
If we look for fluorescence on the internet, a whole array of nightclub tattoos appear, which show up under the excitation of UV lights. Secret sexual communication. Birds, their breasts and other areas such as crowns and cheeks got there first. Several parrot species fluoresce, including the budgerigar, and they use it in mate choice just like us. Then there are flowers and pitcher plants and other things that glow under black-light (UV) or blue-light. The problem is many pigments in nature fluoresce but not all of them do so for a visually meaningful reason. We tend to like to give things that glow in the dark a function as they look cool but we need to be sure to ask the animals. Back to behaviour. For a fluorescence to have a function, we need to show a change in behaviour associated with that fluorescence alone, not just the colour that it may overly enhance.
How does your research apply to humans and add to our understanding of how we use colour to interact with the environment? 
Two answers. Firstly, not at all. I am not interested in applying my research to humans but in discovering more about the fabulous secrets of nature. I am a pure research biologist immersed in the deep blue of blue-sky research. Animals and animal colours and their uses of them are a constant source of wonder and excitement. Ask Sir David Attenborough to describe a bird of paradise.

Secondly, there are spinoffs. Any sort of comparison with animals that do colour vision differently helps us understand our own colour world and the ways in which our brains interpret it. I am lucky in some ways as my work is helping us to redesign satellites, enhance data storage on computers and potentially detect cancer early. All this in fact from the eye of the mantis shrimp or stomatopod.

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