If you’re anything like me, you may have wondered why some monkeys have bright blue skin. Even if you don’t pay careful attention to monkey butts, you may still have wondered what makes some people’s eyes blue.

To answer these questions, it is important to first understand how light interacts with living stuff, which we call biological tissue. Light is a collection of little packets of energy, called photons, that whiz through the air. Photons come in all different colors, and when these colors are all together, we see white light (like sunlight or light from a light bulb). But photons of different colors act differently when they enter biological tissue.

You can think of a photon as a drunk person walking through a forest. The drunk person enters the forest and walks into a tree, D’oh!, changes direction (scatters) and walks another short distance and into another tree, D’oh! D’oh! D’oh! D’oh! …. and before you know it the drunk person exits the forest in a random place going a random direction.

When photons enter the skin, it is just like the drunk person walking in the forest. Photons bounce off collagen fibers (instead of trees) and emerge from the skin at a random place. In optics, we call this diffuse reflection. A visual representation of diffuse reflection is when a LASER beam hits the surface of milk, giving off a “glow ball” around the LASER spot. (You can also catch my personal demonstration HERE).

Glowball via

Before it gets detected by your eye, a single photon that penetrates a biological tissue (like skin) will randomly scatter within the tissue many times. But it is important to consider that skin is more complex than the forest analogy. Skin is composed of many layers and contains pigments such as melanin, and this affects how photons scatter. This added complexity is the secret to the blue monkey butt.

To understand how photons move through a complex tissue like skin, lets compare a red photon with a blue photon as it hits two tissue layers. Tissue “A” is on top, and Tissue “B” is underneath. If layers A and B were both white (like milk), they would reflect red and blue photons equally and you would see both colors.

But in real skin, the top layer (Tissue A) has only a little melanin, while the next layer of skin (Tissue B) can contain a lot of melanin, which is important because melanin absorbs light, “killing” any photons that encounter it!

Seeing Blue
Seeing Blue via

In the picture above, both the red and blue photons bounce (scatter) 4 times while they are penetrating the skin. But because the red photons travel a longer distance in between scattering events, they manage to penetrate both layers of skin. The melanin in Tissue B absorbs the red photon so it can’t leave the skin and reach your eye.

In contrast, the blue photons take very short steps in between their scattering events, and they don’t penetrate very deep. They do not make it past Tissue A before they emerge and hit your eye.

A baboon’s butt tissue is arranged somewhat like the illustration above so that blue photons are reflected and all of the other photons (like the red ones) are absorbed. Only the blue light makes it out and gets into our eyes. This is why we see we see blue monkey butts!

So now you will never look at a monkey’s butt the same way: you will know what makes it blue! You will even be able to hold your own if this topic ever comes up on a first date…

More about photons: Another demonstration of optical diffusion can be found HERE. If you want to go way off the mathematical deep end of diffusion theory, equation 15 of THIS PAPER is a great way to start. Alternately, if you are sick and tired of diffuse light and just want to see me rock out using ballistic laser light, check out this RAP MUSIC VIDEO.

Or if you’d prefer to invoke the Goldilocks Postulate, and want to try something that is just right in the middle: head over to this online biophotonics course.

P.S. Stay tuned for next time’s discussion about imaging cats through a layers of milk, another wonder of light diffusion.

This article was republished with permission from its author. See the original post over at

Lesula Butt

Lesula Butt

The lesula, a newly discovered monkey that, from the front, looks like a man in a monkey suit, also has some weird stuff going on behind. Unique for its family, Cercopithecus, the lesula’s entire back end, including its perineum, buttocks, and scrotum, are all a bright blue-white. It’s theorized that this bright coloration will help female lesulas find the mostly gold-and-brown male lesula when it’s time for mating.
Hamadryas Baboon Butt

Hamadryas Baboon Butt

Many members of the baboon family, like this hamadryas baboon, have prominent buttocks. Unlike many other primate species, the baboons are largely terrestrial, living on the ground in fields and deserts rather than up in the trees. They, like us humans, have the need for some kind of padding on the rear end so they can sit down comfortably for long periods of time. These baboons have what can best be described as calloused, puffy skin on the buttocks; not super attractive, at least to us, but very effective for sitting, which is the baboon’s favorite position.
Gelada Butt

Gelada Butt

The gelada is an old world monkey that lives, like the hamadryas baboon, around the southern coasts of the Red Sea in Africa. It’s not actually a baboon, though it is closely related. Unlike some monkeys, like the baboons, the gelada doesn’t brighten and puff up its buttocks when the female is in estrus. Instead, it has a hairless patch on its chest that gets inflamed, like a pus-filled necklace. But it does have a similar adaptation in the buttock area–these are called “ischial callosities,” very thick calloused pads on the buttocks that make it comfortable to sit and nap or eat, which is mostly what the gelada does.
Celebes Crested Macaque Butt

Celebes Crested Macaque Butt

The Celebes crested macaque, also known as the crested black macaque, is perhaps best known for its fantastic selfie skills. But it’s also an interesting creature, butt-wise. Both the males and females of the species are highly promiscuous, and the female macaque uses its outrageously swollen rear end to indicate that it’s receptive.
Mandrill Butt

Mandrill Butt

And that brings us to the primate with the most colorful buttocks of all: the mandrill. The mandrill is the largest non-ape primate, and also arguably the most colorful, with a trademark bright red and blue nose and a frankly gorgeous rainbow-colored butt. These are secondary sexual characteristics, present in both sexes but much more vibrant in males. Just look at that butt!