A low-ranking monkey, or just blue balls?

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




18 Responses to “A low-ranking monkey, or just blue balls?”

  1. sgiarrusso says:

    Thank you, Andrew! Trying to explain the science in a simple way isn’t as easy as you may think! I’m glad I got the message across 🙂

  2. sgiarrusso says:

    Thank you Russell! Glad you enjoyed it 🙂

  3. Russell says:

    Found this on twitter! Crazy how only two terrestrials are BLUE. Thanks for the great read sgiarrusso

  4. Andrew says:

    Such a great post! I love how you made something so complicated so simple.

  5. sgiarrusso says:

    Thanks Wayne 🙂 your comment is really encouraging!

  6. sgiarrusso says:

    Thanks isherburn. I know, it’s incredible! I love seeing blue butterflies and birds even more now!

  7. sgiarrusso says:

    Thank you, reader Billyboy123 😀

  8. sgiarrusso says:

    Thanks tcdavies! and YES, it’s not just the case for the colour blue.

    Multi-layered structures produce iridescence. If you’ve ever looked at a peacock feather you would notice the colours change when you look at it from different angles. These colours can range from blue, green, orange and yellow.

    The colour reflected will also depend on the nanostructures refractive index and the surrounding medium.

    Maybe the grass isn’t actually green! What do you think?

  9. Wayne Du says:

    Amazing work! Love the topic and love the way you introduced it. It’s also great to see you incorporate personal events that other people can relate to, it really makes it feel like a blog.

  10. isherburn says:

    The title hooked me right in! Absolutely hilarious. Also had no idea only two animals were actually blue-coloured. The poison dart frog is so stunning!!!

  11. sgiarrusso says:

    Thanks for the feedback Josie!

  12. sgiarrusso says:

    Thank you Angela for your ‘beautiful’ comment, haha! Happy Sunday!

  13. sgiarrusso says:

    Thank you Jaslyn! Glad you enjoyed it 🙂

  14. Billyboy123 says:

    Interesting. Thank you for explaining this in simple terms, author. The blue on the butterfly and frog is electric ⚡️

  15. Tom Davies says:

    Absolutely fantastic and fascinating! Do we think this may be the case for other colourful parts of nature?

  16. Josie says:


  17. Angela Rutherford says:

    So informative but funny at the same time. You are a beautiful writer Sophia.

  18. Jaslyn Goette says:

    This is so well written and hilariously informative!