Very simple. Most monkeys live in forests and woodlands, where blue and purple colors really standout. A few species, such as baboons, do live in savanna type habitats, where blue color is less conspicuous. However, blue or purple, even in such situations, is not better than brown for camouflage. Besides, it is not easy to produce blue or purple color in mammals, since blue is a structural color, meaning it is produced by scattering blue light. The eyes of some people are blue because the molecules inside the eye scatter blue light, but these people also need to have no other pigment for the blue color to be produced. If there is other pigments present, then it may change the eye to green, red, brown, or gray.
A few mammals, including the mandrill, do have blue colors. The mandrill male has a blue face and rump, produced by structural colors. However, these areas are vulnerable to damage by the sun, because they can only produce blue color if there is no melanin. Without melanin the skin is not protected from cancer causing UV rays (one reason blue eyed people are more susceptible to eye cancer). the mandrill happens to live in forests, so they can have blue face and rump coloration and not have a higher cancer risk. But even in the mandrill, the females lack blue coloration, because the blue color makes the animal more conspicuous. It is better for the female to not look conspicuous. The blue color is used for display to females and to intimidate immature male monkeys, who lack the blue color. Even though mandrills are at a higher risk of being seen by predators, they have evolved the blue color because the advantages in breeding success and territorial defense outweigh the cancer risk. For most other species, they seem to get by just fine without any blue color.
All the monkeys that stood out were picked off long ago by predators . The purple ones all stood out .
I guess because once long ago there was a boy hairless monkey thing who was asked by God what color he wanted to be for all eternity. He said not pink or purple because those are girl colors. He said brown. Years later: "Aw sh*t! I look like sh*t!"
for the same reason you asked the question
they're just not , ok ?
because i need 2 know
