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I have seen species of birds, insects and fishes with splendid colours. But when it comes to mammals (including us humans), they almost always appear in shades of brown, grey, orange, or in black and white. I don't recall seeing any pink, green and purple. Why is that so?

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Oreotrephes
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    That mandrill face in the picture you uploaded is pretty brightly colored. – Oreotrephes Sep 01 '13 at 09:07
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    Great question. At least according to this BioScience article, it seems like most of the answers are still unknown. No time to fully review it now, but this paper in PTRSB by the same author looks very good, and basically takes as its starting point that black and white stripes / spots is as colorful as mammals get, and is already surprising in mammals, "a class of vertebrate generally thought to be drab and cryptic". – Oreotrephes Sep 01 '13 at 09:31
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    Most mammals have poor colour perception, which might be the reason for the lack of bright colours. Plenty of mammal species are very contrasty with deep blacks and bright whites on the same animal, which might be all that's required. Primates have much better colour perception than most mammals, and there are a few monkey species that have bright blues and oranges. Of course, I have no idea whether mammals were more colourful before colour vision was lost; this is pure speculation on my part. – Chinmay Kanchi Sep 01 '13 at 09:32

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I don't think, there is a precise answer about the evolutionary mechanisms, but "mechanically":

  • mammals have principally just two types of pigments: eumelanin and pheomelanin, both of which have their color variants, but within a known range. Bird pigments, besides melanins, include carotenoids and porphyrins. Arthropods generally have carotenoids, melanins and ommochromes [and other pigments?]. E.g. carotenoids and ommochromes alone can create rather "exotic" coloration from a mammal point of view (green, pink, violet).
  • both birds and insects actively utilize iridescence. With fur it seems to be technically much more difficult to achieve than with feather or scales.
  • many (most?) mammals do not differentiate colors. Birds have much better vision abilities in this respect. From a selectionist point of view this cuts out a considerable part of selection acting upon coloration, which could otherwise produce broader spectrum of phenotypes.
alephreish
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In addition to the arguments presented in the accepted answer, I would like to add that most species of mammals are red/green colour-blind. This has to do with the fact that for much of their evolutionary history, mammals were nocturnal creatures and so the ability to see different colours wasn't necessary.

This dates back to the mesozoic period when mammals had nocturnal lifestyles in order to avoid predation from dinosaurs.

Here's an article that summarizes recent research on this topic: http://www.utexas.edu/news/2012/10/29/effects-prehistoric-nocturnal-life-mammalian-vision/

Aidan Rocke
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The Answer is simple..EVOLUTION.. The colour of an living organisms depends on 2 things 1) attracting other sex. 2) escape from the predators.

In birds and insects: They doesn't much depend on other sex for living(except for the process of reproduction)and there is no need t exhibit physical work for attracting other sex... For example if you take a peacock the male needs to be colourful to attract female. but there is no physical work....Let The Other example be some Insect,then if it is colourful.. a predator may not eat for its bright colour.. this made insects and birds evolute into colourful..

But in mammals. the attracting and escape both depend on physical work... a lion being handsome cannot attract lioness.. :P It need to be physically healthy to escape from predators and give an enough competition to other lions in his territory... this made evolution for mammals much depend on their capability of body.. rather than physical attractiveness.. for example in a forest a tigress will obviously pair with a Tiger which has more territory than a more attractive handsome tiger without territory(this is not a case with insects)..

Alex markston
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  • This isn't really a good argument. Mate choice in mammals (as in other creatures) is complex. For example, I'm pretty sure the character of the mane (size?, colour?, fluffiness?) plays a role in mate choice, since it's the most obvious reason for the strong sexual dimorphism. – Chinmay Kanchi Sep 01 '13 at 09:12
  • I think that the basic concepts here are right on – there's a balance between attracting the opposite sex and hiding from predators (crypsis); among other strategies, including putting off predators with bright coloration (aposematism). However, the answer is a bit unclear and it would be very nice to see some references. – Oreotrephes Sep 01 '13 at 09:14
  • sorry for the ambiguity.. i have just edited my answer adding some more examples.. I have a video on y insects and birds have more colourful than mammals.. Its a documentary by discovery "PlanetEarth".. – Alex markston Sep 01 '13 at 12:18
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    Unfortunately, your edits don't address the main problem. Your entire premise that mate choice in mammals doesn't depend on "physical attractiveness" isn't supported by evidence. Lionesses choose their mate partly by the attractiveness of the mane, several primates have rather conspicuous sexual displays (the inflated cheek pouches of orangutans, the silver fur on the back of the silverback gorilla, the bright facial patterns on mandrills, etc.). – Chinmay Kanchi Sep 02 '13 at 00:26