Monday, September 28, 2009

Why are animals colourful? Sex and violence, seeing and signals

Yesterday's problems of finding the venue, the missing proceedings, and the absence of Internet connectivity were made up for at the opening ceremony earlier today. The following invited papers were excellent. I cannot give a minute-to-minute account, so I will report on just one of the lectures: Justin Marshall from the University of Queensland in Brisbane lectured on Why are animals colourful? Sex and violence, seeing and signals.

The human visual system is much less colourful than that of many animals, so our world is much poorer. Justin Marshall studies how animals see the world, which comprises 3 main subject areas

  1. Visual ecology
  2. Camouflage
  3. Display — sex and violence

The function of colour is to add contrast. At least two channels are required to see colour. Why do mantis shrimp have 12 channels then?

Most cephalopods have only one channel and are therefore achromats. They use colour only for camouflage, not for display. However, some cephalopods use colour to display their toxicity.

Lungfish and birds have oil droplets in their retinas to tune their spectral sensitivities. They are tetrachromats. We do not understand the variability of the visual system in the various reef fish species.

The only way to enhance display is to use fluorescence.

To other fish, yellow fish are the same colour as the reef, because fish do not see in the red part of the spectrum (however, they see much farther in the UV domain than us humans), which is where the reef spectrum looks different from yellow fish. They do not need to match for camouflage in the reds. Stripes are disruptive and are meant for camouflage.

Stomatopods like mantis shrimp are the masters of colour vision. Four channels are in the UV portion of the spectrum and 6 channels have circular or linear polarisation vision through a kind of nanotube. To manage the architectural complexity of so many channels, the stomatopods are live-scanners.

Remote sensing is very interested in the eye structure of the mantis shrimp, because the current vision models are so much poorer compared to that of mantis shrimp.

The coral reefs are dying due to global warming and ocean acidification, as assessed among other with the Coral Health Chart. The current aspirational goals for CO2 are unrealistically high and will lead to the disappearance of all coral reefs in a few decades. Just in the Coral Reef Triangle alone live 500 million people who depend on the reef fish for survival and will have to mass-migrate to other parts of the world.

Justin Marshall and colleagues have given up trying to educate adults and are now focusing on educating children. Their new book for children will be available shortly though the web site

Now let us hope the UNSW IT department can figure out how to run a wireless LAN so I can upload this post. The DSL modem in my hotel room had been vandalized, so I am living in a pre-digital world. So much for Nathan's dream of attending the congress remotely through a blog. For now we still have to squeeze in a sardine box flung across the oceans to attend meetings.


  1. Six channels with circular polarization - nifty. Wonder if the shrimp have Hadinger's brush type entropic phenomena under the right viewing conditions? Well good to see you've been able to do a couple posts after the sardine can transport...

  2. Presumably a mantis shrimp would not need glasses to see the newer 3D theater movies, where the left and right images differ in their circular polarization. Perhaps 3 x 3D movies simultaneously? Filling the living room with water could be a bit of a pain though.