
Tarantulas evolved blue colour 'at least eight times' - jackgavigan
http://www.bbc.co.uk/news/science-environment-34944735
======
kibwen
Is it possible that the blue color exploits a kind of colorblindness in the
spider's predators that makes it effective camouflage? In the same way, you
could imagine that red would be an effective camouflage against a green
background if your predators were red/green colorblind humans.

~~~
jjoonathan
Yep! Actually, this is _always_ how it works. Every living creature that can
see is "colorblind"[1] in the sense that it projects the infinite dimensional
space of visible light intensity (a histogram of intensity vs wavelength[2])
onto a small finite-dimensional subspace of perceived "color". For us, n=3.
For dogs, n=2. For the mantis shrimp, n=16. There's nothing special about 2 or
3 or 16 other than that's how many different types of photoreceptors we have.
The fact that we can represent "color" as an RGB triple is purely an artifact
of the ways our eyes are built.

Anyway, matching color _really_ involves making sure that the projection of
its spectrum on the perception space of its predators is the same as that of
its background (there are other camouflage techniques, let's ignore them for
the time being.)

    
    
        Pa=Pb     P=projection, a=animal, b=background
    

P is, as you note, strongly dependent on the predator! What fools humans might
not fool birds and dogs (or vice versa) because we have different P due to
having different spectral responses in our photoreceptors. Each "row" of P is
a photoreceptor sensitivity spectrum.

Why should a programmer care? Funny story, about a year ago there was someone
in here who was going on about how his company made bill validators that
couldn't be fooled by photocopied money. He was convinced the secret sauce had
to do with machine learning. Of course the more likely explanation is that the
photocopier (which is acting like the "prey" in our example and matching
colors) was designed against the P of a human eye, not the P of whatever
photosensor+light combo was used to inspect the incoming currency. The copier
was tailor made to fool humans but not to fool silicon photodetectors, which
will naturally have quite different spectral response functions (rows of P).
If that's all there was to the story (I don't know enough to say definitively
one way or another), someone familiar with the way spectra work would know
that the printer was quite possibly one matrix multiplication away from
working for this alternative purpose. Oops! Or "tee-hee-hee" depending on
which side you're on, I suppose.

For a more benign example, imagine creating a physically based effect for a
video game or movie. The physics equations all take "inputs" and give
"outputs" parametrized on wavelength. But all your art assets and screen
colors are in RGB. When translating into the language of physics you're forced
to invent information that you don't have (spectral details outside our
perception space) and when you're translating out of the language of physics
you're forced to discard this information. The latter is fine, but the former?
If you're not aware that you are making an arbitrary decision (i.e. if you
just plug in our sensitivity spectra or ink spectra or delegate to a library
that does so), you might find yourself unable to match nature, and it might
take you a long time to find the knob you need to tweak in order to get
correct behavior.

[1] By "see" I mean "resolve an image". I'd be willing to bet that there's a
creature out there that uses chromatic aberration or continuous variability in
the activation spectra of its light sensitive neurons to its advantage. But
I'd also be willing to bet that it doesn't independently resolve a full
spectrum for each angle of incoming light.

[2] and direction of travel and location in space and polarization and
probably a few more I'm forgetting.

~~~
vive-la-liberte
>The fact that we can represent "color" as an RGB triple is purely an artifact
of the ways our eyes are built.

I've thought a bit about this before and came to think, what if humanity
disappeared and only our creations were left and they were discovered by an
alien species but to them it would just look like the way we see the colonies
of ants, the shell of shell fish, etc., that is to say just a pile of matter
organized for housing, and the aliens were unable to percieve the fact that we
have encoded vast amounts of knowledge in our artifacts. That would've been
quite a cosmic tragedy.

------
namenotrequired
Not a biologist: is it possible that they kind of imitated each other?

If one species develops the color as a warning to predators, then once those
predators learn to ignore this species, I can imagine it'd be advantageous for
similar species to look like this as well.

~~~
detaro
Possible. Other example: Hoverflies can neither sting nor bite, but many have
yellow-white striping and therefore "look dangerous" like e.g. wasps.

