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And we've also got filters, and slides, and other supplies even this very cool hydra t-shirt. All of that and more at microcosmos.store. One of the spectacular details of animals in our  world is just how varied their colors can be.

When you look at birds, for example, you’ll see  everything from mundane grays to iridescent blues. Birds some times craft those colors using melanosomes,  an organelle that holds the pigment melanin. And nothing about that seems  particularly unique to birds.

We have melanosomes, and we use  melanin to make our various skin colors. So then why don’t we ever shine with  the same iridescence as birds? Well, we could just blame it on light.

It’s always doing weird things,  playing tricks with what we see. So let’s just say that light is doing  something funny and call it a day. Except that’s very unsatisfying, isn’t it?

If light has so many tricks up its sleeves,   that only makes me want to know more about the  specific tricks it’s playing with bird feathers. And that means we’re going to have to start by  talking about color, specifically pigments. Pigments are the compounds that color  everything from our paints to our   cosmetics, and we see them all  the time in the microcosmos.

Perhaps most common of all of course would be the color green, found in cyanobacteria and euglenoids   and plenty of other photosynthetic organisms  thanks to the pigment molecule chlorophyll. When white light hits the chlorophyll, the  molecule absorbs the red and blue wavelengths. But it also reflects the green wavelengths back at us.

Which is how pigments work: they are molecules that absorb certain  wavelengths, and in turn they reflect   a particular wavelength of light back at us, which to our eyes, then looks like color. So when we look at these Haematococcus and see  a mix of green and red, what we’re actually   seeing is a mixture of pigment molecules  reflecting different wavelengths of light. There’s the chlorophyll reflecting green,   and the astaxanthin pigment  molecule reflecting red light back at us.

And so the lesson we take from pigments  is that light doesn’t just shine. It reflects and absorbs. But it also does more than that.

That became very clear to Robert Hooke,   who in 1665 published what is probably one of  the most important texts in microscopy history: Micrographia, or Some Physiological Descriptions  of Minute Bodies Made by Magnifying Glasses: With Observations and Inquiries Thereupon. This work contained detailed descriptions and  illustrations of objects Hooke examined under   the microscope, including the feathers Hooke  found from peacocks, ducks, and other birds. Hooke compared the structures he saw on   the iridescent peacock feathers  to pearls, which, as he wrote quote: do not only reflect a very brisk light, but  tinge that light in a most curious manner; and by means of various positions, in respect  of the light, they reflect back now one colour,   and then another, and those most vividly.

As scientists began to better understand  what light is and how it interacts with   the world around it, they were able  to understand what was happening. The light was not getting absorbed  and reflected by a pigment. It was interacting with the physical  structures within the feather,   refracting and diffracting and scattering  and whatever else the light can do.

The result, to our eyes, is structural color: a color whose wavelength is the result of  the structures the light was moving through. And for some structural colors, the  angle of the viewer relative to what   they’re looking at will change the  color itself, and this produces iridescence. You can think of the difference  between pigments and structural color   as kind of like the difference between  looking at a crayon and a rainbow.

When we see a rainbow, we’re  not looking at colored water. We’re seeing light refract and reflect  and disperse in those water droplets. Except at the end of the day, a  rainbow is a function of the rain   and sunshine and your eyes, making it an  illusory object that you can of course never touch or find the end of.

Feathers, on the other hand, are very tangible. And the structures they use to craft their  colors are very present and observable. But before we dive too deep into  how feathers use structural color,   we should note that birds can and do use  pigments as well for their coloring,   mixing carotenoids, melanins, and  porphyrins for their various hues.

But birds can also expand on that range,  using structural colors. The blues you see in these  feathers here are structural color. Blue pigment is unusually  difficult to find in nature.

So when the color does appear, like in these  feathers or in the wings of a butterfly,   it is often from structural color. Now the structures that make that happen  are located in the barbules, those tiny,   interlocking threads that make  a feather feel, well you know like, feathery. Inside the barbules are keratin and melanosomes,   those melanin-containing organelles we  mentioned in the beginning of the episode.

And it's the arrangement of these  melanosomes within the keratin   that create the structures that will in  turn determine the color of the feather. Now a lot of animals, including again us, have melanosomes. But birds are unique because  their melanosomes can be hollow.

And the more hollow the melanosome, scientists  have found, the more iridescent the feathers. And if they produce thinner melanosomes that  are shaped more like a flat plate than a rod,   the iridescence will only increase. These features of the melanosomes are essentially   changing the way that light moves through  the feather, crafting a different color.

Scientists are still learning about how these  structural colors have evolved in feathers,   and why there’s so much variety in color and iridescence across different birds species. These are structures that once contained pigments,   but evolved to contain nothing, because being  empty somehow made even more brilliant colors. It is a wild leap to make, but of course,  evolution doesn’t think, if color is produced,   and it confers an advantage, evolution  doesn’t care or know how this happened,   the photons that would have existed whether or not life ever evolved in our universe  doing a marvelous dance in the empty  pigment organelles of a birds feather,   it evolved without anyone ever knowing  how it worked, until us, until now.

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