Square eyes

Another entry on the list of Things I Should Have Known: Tetrachromacy. Mentioned in a podcast about birds' eggs; never heard of it. Now I've done 10 minutes research, so I know everything. 

Many birds, reptiles and amphibians are tetrachromats; most mammals are not. Tetrachromats have 4 different kinds of colour-sensitive cells in their eyes, each type sensitive to a different range of wavelengths - of course they also have the neural mechanisms to receive and process the signals from those cells. This means they can distinguish between colours that look identical to mere mammals; they can also see light that is outside the visual spectrum of mammals, from ultra-violet to red

A hard-to-prove hypothesis is that mammals were reduced from tetrachromacy to dichromacy during the 150 million years that Tyrannosaurus rex and his cousins made it too dangerous to go out in daylight. Nocturnal mammals needed high light sensitivity but not colour vision, so this sense atrophied. Somehow, higher primates have regained one of the cell-types, so they are trichromats, and can see red (but not UV, of course), but most other mammals remain dichromatic - hence the oft-quoted pedantic point that the bull doesn't really care what colour the rag is, because he can't see red

Interestingly, bees are trichromats, but their 'third' cell-type is sensitive to ultra-violet light, rather than red - I wonder how the extra looks to them. Recent research also suggests some marsupials may be trichromatic. There is also a suspicion that some people, principally women, may be tetrachromats. There seems to be sold evidence that a minority of women have four cone cell-types, but no firm evidence yet that they have the neutral mechanisms to process the signals in a way that gives a finer colour sensitivity

It's also the case that we all get a bit of tetrachromacy at low light intensities, when the rod cells in our retinas supplement the colour processing of our three types of cone cells. Is that why we love morning, evening, spring and autumn light so much?

I read that yesterday set a record for the overall global temperature. Somewhere must have been very hot to compensate for our day. This morning dawned a little showery still, but warm enough to fit in a little emergency beekeeping before we set off on a mini-trip. I've just had a shower in our holiday cottage and noticed that I still smell smoky. I don't usually notice at home. Perhaps our entire house is smoky. People are very polite

A community of photographers may be an incendiary place to write about colour vision. If I'm just catching up with what everyone already knows, I'm sorry, please indulge me