Wednesday, 6 June 2007

Primate colour vision: was fruit or sex the evolutionary driver?

A long time ago, I used to do research in animal behaviour. And one of the questions that stayed with me was why did promates evolve colour vision. 3D is easy, if you climb about in trees a lot having 3D helps you avoid killing yourself, but colour? Yes you need to know when fruit are ripe but an augmented monochrome vsion would do that as long as there was contrast with the surrounding vegetation.

And then there's birds. A long time ago I got interested in this topic, but really through looking at the evolution of berry colour and foraging strategy in birds.

Basically my question then was 'why are so many berries red' which given that a lot of birds perceive different colours to us is slightly
wierd. And of course 'why are those which are not red blue-black?' Which is kind of interesting as while read are easy for us ape derived beings to see, blue black is markedly less so (and why do some go through a red phase of ripening?)

Certainly birds rapidly learn to associate all sorts of colours with either acceptable or distatstful food objects so there's some
flexibility there, it's not as if the red preference is hard wired. I've done a little proof of concept experiment on this and Ian Soane did a much better on one on 'Why are distasteful prey not cryptic?'.

Fruit bats are also kind of interesting. Like primates they have binocular colour vision and eat fruit and live in a tropical area with
no real seasonality meaning that brightly coloured (ripe) fruit ares something they should be looking for if they want to live well.

So, my question is, which came first? Fruit colour to denote ripeness or colour vision to detect ripeness. And if the former why is our
colour vision so good at detecting variations when really all that's needed is a detction system that says 'fit to eat (or not)'.

And some people at Ohio University may have the answer, or part of it at least. (There's also a fuller report here.)

A preference for red made us evolve a better response for red coloration. Which would also mean that red coloured fruits would stand out more and be selected. (Which is good if you're a fruit, as you get eaten and your seed shat out somewhere else with a nice pat of fertiliser to start some more trees).

But did the red preference come first as a response to the availability of red fruit because birds (and fruit bats) had a preference for them (our augmented redness detector does the enhanced monochrome thing)? And why the hell do we havd difficulty with blue-black. Was red just simply good enough?

No comments: