illuminating science

Last Friday, I went to a fascinating talk by Justin Marshall, an professor here at UQ. I’ll give his abstract, then briefly discuss what he talked about:

My principle aim and that of the team I work with is to understand how other animals perceive their environment. As arrogant humans we tend to assume we are the pinnacle of evolution, however, certainly in sensory terms this is far from true. By taking an approach to sensory systems which is based around ecology and includes physiology, anatomy, behaviour, neural integration and machine vision, we hope to decode signals and their intension in the animal kingdom. As part of the desire to look at the world
through the eyes of crustaceans, fish, and even birds, I spend much of my time looking at the colours of The Great Barrier Reef and other underwater environments (see National Geographic May 2005). This led to the discovery of a prawn with a visual system almost identical to push-broom cameras and we are now part way through a project known as “Prawns in Space”, an effort to help re-design man-made imaging systems using biological systems.

As you can probably guess, he was a very entertaining speaker. The main focus of his talk was about vision, and the role of colour in various environments, particularly the Great Barrier Reef. He had some brilliant pictures of the different camouflage techniques used by fish, from simply blending in to using stripes to disrupt their lines. (He had an amazing picture where he Photoshopped stripy fish onto a picture of coral (since they wouldn’t all co-operate together) - I didn’t notice two of them until he actually pointed them out - and they were so obvious!) It was also interesting that even though some fish appeared really visible to us, we see differently to fish, so to other sea creatures they probably just blend right into the water.

But the most interesting topic for me was the Stomatopod, or mantis shrimp, which is a cute little guy that lives in the shallows of tropical and sub-tropical waters. Although they max out at about 25cm long, these stomatopods have “raptorial appendages” that are capable of delivering a strike equivalent to a 22 calibre bullet! A 9-10cm guy can smash an aquarium (glass) tank if he wants to, or cut straight to the bone of your finger… Apparently, it’s the second fastest action in the animal kingdom (I’m not sure what the first is - any ideas?) and might make a cool first year physics problem (5kg of force in 3ms - calculate the impulse, and perhaps compare the resulting pressure to the breaking stress of skin…)

But their super powers doesn’t stop there. They have the most sophisticated visual system in the world - they have 16 different receptors in their eye, capable of detecting 12 different colours, compared to human’s 3 (red, green, blue). They can see in the UV (4 colours) and they can detect the polarisation of light! (We can’t see this property directly - but Polaroid sunglasses can easily show that it’s there. Just rotate the glasses (or your head) while looking at the ocean, say.) From the talk, I don’t think anyone really knows why it’s necessary, but it might be to do with very sensitive communication methods that involve colour.

So why don’t we get this fancy vision setup? Well, partly it’s overkill - three colour sensors give fine colour vision. Even just six apparently does all you could ever want. But there’s also another question - if you look at what colours of light our sensors absorb (on this picture, or this one) you see that we detect blue, green and red light as expected, but the red and green sensors overlap big time, pretty much on the yellow part of the spectrum. Surely it would make more sense to put the green sensor in the middle?

Apparenty, it all comes down to bananas! Since we evolved from primates, we needed to be very good at seeing fruit, leaves and berries. What colours are these? Red, green and yellow! It would be a big advantage for us to be able to tell exactly when fruit (say, a banana) is ripe, so being able to detect shades of yellow well is an evolutionary advantage - and lo and behold, the eyes have it! (There’s a joke there, in case you missed it…)

So this is just a smattering summary - if you ever get a chance to hear Justin Marshall talk, definitely take advantage of it - he’s on my top speakers list for certain!