A low-ranking monkey, or just blue balls?

By Sophia Giarrusso, class of 2020 (@GiarrussoSophia on Twitter)

Vervet monkeys have small cute faces and bright blue testicles.

Over the male’s lifetime, the blue colour will change according to social status. In other words, a low-ranking monkey will have much lighter nuts than an alpha.

The male vervet monkey.
Image credit Malingering on Flickr (CC BY-NC-ND 2.0)


Vibrant shades of blue are everywhere in nature. Blue hydrangeas and forget-me-not flowers are stunning examples of blue plants. The eastern bluebird, peacocks and blue jays all have vibrant shades of blue in their feathers. Even reptiles like the electric blue gecko and the blue iguana, are blue.

Image credit: Ian Glover from Flickr (CC BY-NC-ND 2.0)


All the species I just mentioned, even vervet monkey testicles, aren’t actually blue. It’s just an illusion.

*Drops microphone.

Science has revealed that almost all the blue we see in nature isn’t actually blue. There are two exceptions to this rule, but I’ll talk about them later.


Forget-me-not flowers
Image credit: allispossible.org.uk from flickr (CC BY 2.0)

So, why aren’t the monkeys testicles blue?

Looks can be deceiving.

Like that date you went on with a guy who looked tall in his Tinder pictures. You meet him at a bar where he is already sitting down waiting for you. You chat for a few hours, flirt a little, and then mutually decide it’s time go watch Netflix.

But when you both stand up, he disappears…

Only for you to look down and realise he’s half your height.

Anyway, back to blue!

Male eastern bluebird
Image credit: dpbirds from flickr (CC BY NC-ND 2.0)

Round two: So, why aren’t the monkey’s testicles blue?

I will try to explain this in a simple way. Please bear with me…

The blue colour is created by physical structures, not pigments, that interfere with visible light. The physical structures are teeny tiny, often at the nanoscale, and exist on the surface of an organism.

When light hits these small structures, different structural angles reflect different wavelengths of light, and different wavelengths of light have different colours.

At a certain angle, red is reflected. At another, shorter angle the colour green will be reflected. And finally, at an even shorter angle the colour blue is reflected.

Pigments, on the other hand, appear the colour of the light they don’t absorb, but instead, reflect.


So, if you looked at the hair covering the monkey’s scrotum under a transmission electron microscope you wouldn’t see a blue pigment. Instead, you would see different angled ridges, with thousands of microscopic branches arranged in a way that makes you see the colour blue.


Electric blue gecko (Lygodactylus williamsi)
Image credit: Corinne Benavides on flickr (CC BY-NC 2.0)

The two blue exceptions that will ‘blue’ your mind

Only two terrestrial animals are known to have true blue pigments.

They are the olive-wing butterfly (Nessaea aglaura) and the blue poison dart frog (Dendrobates tinctorius “azureus”) – two absolutely spectacular looking animals. Take a look…

The olivewing butterfly (Nessaea aglaura) 
Image adapted from Insects, Insekten, Insectes, Insetti! on flickr (CC BY-SA 2.0)


Blue poison dart frog (Dendrobates tinctorius “azureus”)
Image credit: MantellaMan on flickr (CC BY-NC-ND 2.0)



To conclude, blue is one of the rarest colours in nature, even though it’s practically everywhere – just look at the sky and the ocean!

The structure of microscopic structures and the way they reflect light make us see the colour blue in the majority of plant and animal species.

…and finally, if you want to watch a video that sums up what I’ve just said, click on this link.


Some journal links