Is it true that  all mammal species  are colour vision deficient including humans? Have mammals really lost
much of their colour vision during the age when dinosaurs and all their close relatives roamed
and dominated the earth and our mammalian ancestors had to live in the shadows to survive? Could this explain
that birds,  insects,  etc. have superiour colour vision to humans?

Hi Christina,

Interesting question... Let's start by saying that *most* of the abilities that animals display now have been brought about through time by natural selection, which favoured individuals that had these abilities.

For example, Cheetahs run so amazingly fast because, in all likelihood, faster cheetahs got more prey and survived better, and their offspring inherited this ability which in turn they passed on to their offspring, and so on... (a simple explanation for a very complicated process!).  The ability to climb trees, for example, was probably not very important to the survival of early cheetahs so there was never any strong pressure favouring those individuals that could climb trees. As a result, today's cheetahs are pretty pathetic tree climbers.

About colour vision: Most early mammals were likely to have been nocturnal (as most small mammals today still are). At night, colours are not very important, so there was probably no great pressure favouring the development of sharp colour detection. It's a different matter for monkeys and apes, which are diurnal. Those that could see in colour could, for example, tell a ripe banana from an unripe one, thus feeding healthier and having healthier babies that could pass on this colour detection ability. Humans, having come from this stock, have pretty good colour vision.

Hope this helped,

Carlos.

Last edited by Carlos Grau (25th Mar 2010 14:33:03)

"At night, colours are not very important."

Why should that be true?  Surely an orange fruit is still orange at night, and a nocturnal mammal that can find orange fruit is at an advantage over one that can't?

If I'm honest I don't know too much about vision, but the mammalian retina has two types of photoreceptors - rods which are more sensitive to light in general but do not distinguish colour, and cones that do discriminate colour but are less sensitive. It has been shown (I think... haven't checked the literature myself!)  that nocturnal mammals have proportionately more rods than diurnal ones. So as an evolutionary hypothesis I'd say that it not so much that seeing colour in the dark is not a good thing, but rather the advantage of having more rods (better movement detection, pattern discrimination, etc etc) outweighs any advantage of seeing colours. In other words for increasing fitness it's probably more important to see a predator moving in the dark than to know what colour the predator is.

Alistair, that makes sense, but leaves me with essentially the same question: why, if it's better at night to be able to detect a predator than to see what colour it is, would the same not be true during the day?

The dual retina of humans and most vertebrates consists of multiple  types of cone for colour vision in bright light and one single type of rod, leaving these animals colour-blind at night. Instead of comparing the signals from different spectral types of photoreceptors, they use one highly sensitive receptor, thus improving the signal-to-noise ratio. However, nocturnal moths and geckos can discriminate colours at extremely dim light intensities when humans are colour-blind, by sacrificing spatial and temporal rather than spectral resolution. The advantages of colour vision are just as obvious at night as they are during the day. Colour vision is much more reliable than achromatic contrast, not only under changing light intensities, but also under the colour changes occurring during dusk and dawn. It can be expected that nocturnal animals other than moths and geckos make use of the highly reliable colour signals in dim light.

Kelber, A and Roth, L.S.V. (2006) Nocturnal colour vision – not as rare as we might think. Journal of Experimental Biology 209, 781-788.

...and working off the premise that "Colour vision is commonly defined as the ability to discriminate between two visual stimuli that only differ in their spectral composition, independent of their relative intensities."

I would also argue that an Orange fruit is not necessarily orange at night. Oranges are only orange because they don't absorb light at 590 nm, but if the only available light at night is coming from reflected or scattered ambient, there is no guarantee that there is much 590 nm light still kicking around. To us at least, Oranges in dim light appear more brown than orange, which may reflect a perceptual change with the reduced intensity.

Last edited by Jim Caryl (26th Mar 2010 13:32:14)

"An Orange fruit is not necessarily orange at night. Oranges are only orange because they don't absorb light at 590 nm, but if the only available light at night is coming from reflected or scattered ambient, there is no guarantee that there is much 590 nm light still kicking around."

Of course, that makes perfect sense.  Thanks for clearing that up.

I was waiting for our very own colour vision expert Innes Cuthill to answer this one, but I'll jump in to the melee. The reason why mammal colour vision is "deficient" compared to birds and insects has been discussed, although I would add that mammal hearing and olfaction (sense of smell) is far better developed than it is in most birds. Colour cues provide information about ripeness of fruits, toxicity, availability for mating and so forth, but scent cues are also available, which is what mammals tend to pick up on with their excellent sense of smell. It is almost certainly no coincidence that the most diurnal mammals (the primates) tend to rely less on their sense of smell than do the nocturnal mammals. In low light conditions smell is far more useful than colour detection, which is probably part of the reason why mammals never had sufficient evolutionary pressure to develop good nocturnal colour vision. The kiwi is a great example of a bird that has lost much of its visual ability as its olfactory senses became more important due to its nocturnal habits.

As to the mammal "deficiency" in colour vision - this has been discussed in other posts http://www.askabiologist.org.uk/punbb/v … hp?id=1454 but it's worth noting that animals are all adapted to their environments pretty well - birds may see into the ultraviolet, but most mammals do perfectly well without that ability - so it's not really a deficiency, it's just a different set of adaptations.