Could an animal evolve iridescent horns that appear metallic like chrysina resplendens' chitinous exoskeletons?

Question

Sort of two questions here, but very related. One about whether a mammal-like animal could evolve to have a chitinous endoskeleton, I guess. Then a question of whether, if they grow horns, that skeleton/bone could be iridescent, highly reflective and apppearing a single hue. This second question is more important to me though, and I see the mammal thing as more of a secondary priority since I am not sure if my animal even has mammary glands yet. It is furred, big and a biped though. I am hoping for some biologists' takes on this. I am wondering if it is even biochemically possible.

Would it be possible for an animal, for instance something like a mammal, if they evolved horns, to have said horns be iridescent and possibly even highly reflective like a metal, even appearing pure solid gold? For instance, say we redid evolution and mammals ended up with endoskeletons made of chitin rather than keratin? Or what if we just have keratin?

On chrysina resplendens' golden iridescence

For instance, the chrysina resplendens of the jewel scarab/Ruteline genus or Rutelinae subfamily of Scarabaeidae, have intricate nano-structures in their chitin exoskeleton, whose spacing and repeating layers are found to vary over a specific range through the exoskeleton, which causes the simultaneous reflection of a range of visible colours of light. The nano-structured exoskeleton is composed of natural materials including chitin and various proteins, and in addition to their strong reflectiveness, the structures are remarkable in how they manipulate the way that polarised light is reflected. They produce circularly-polarised light, where the orientation of the light's oscillations rotate as the light travels. The two possible directions of rotation of said light are referred to as "left-handed" and "right-handed" in the paper. This explains why they appear to be completely made of gold.

The solid golden colouring of chrysina resplendens jewel scarabs is explained in a research paper, Optically ambidextrous circularly polarized reflection from the chiral cuticle of the scarab beetle Chrysina resplendens, by Ewan D. Finlayson, Luke T. McDonald and Pete Vukusic, published June 25, 2017 in the University of Exeter.[1].

On iridescent keratin or similar material that can form skeletons, as well as on toughness of material

Now, polarimetry in insects is nothing new, but I was just thinking if it would be possible for a mammal to have it, and if so, if it ever could appear a solid metal coloring like in the C. resplendens? And still be as tough as a keratin horn or antler?

[1]: Finlayson, Ewan D. and McDonald, Luke T. and Vukusic, Pete. Optically ambidextrous circularly polarized reflection from the chiral cuticle of the scarab beetle Chrysina resplendens June 25, 2017. University of Exeter. https://royalsocietypublishing.org/doi/10.1098/rsif.2017.0129

Answer 1

Absolutely! I don't know of any mammals that display Iridescence, but I think that you could have evolutionary paths where that could be reasonable. This video shows a variety of animals with iridescent properties.

https://www.labroots.com/trending/videos/11889/science-iridescence-animals

But basically if you have structures that start to approach the dimensions of the wavelength of light and are organized in patterns then you can have interference of the reflected light. This is most famous in butterflies with the scales on their wings, and other insects as you mention. However feathers like on the peacock or other birds can also display the phenomena. From the video, apparently snakes with their scales.

You don't have to have pigments, in fact for diffractive optical components it is easier if the material is optically transparent. However typically you do want to have similar sized structures. Nature can often do that pretty well. For example opals are composed of small silica spheres.

The reflected light is often polarized, and depending on the structure of the small objects and how they are organized can also be circularly polarized.

To detect polarized light could be done several ways, for example another polarizing transparent membrane that goes in front of the eye. But the structure of the rods and cones, and how you choose to make those could also be sensitive to polarization. Humans can actually see polarized light. If you look at the clear blue sky with the sun to your back, Haidinger's brush is a light yellow dog bone like structure that will change its direction according to the polarization. I think there was a recent story that the Vikings may have used it to help navigate. In our modern society with laptops if you are looking at a white screen and see a yellow tinge it can be because of the polarization caused by the liquid crystals.

I suppose with fur, part of the purpose is to trap air for insulation and I don't know if that is stronger evolutionary advantage.

So to answer your question. I think any transparent structure (or colored if not two absorbing) that the body can make could have potential for form birefringence and iridescence. Why something would evolve that way is a little harder to justify, looking pretty and attracting more mates might be one.

Answer 2

Treating the horns as a different question as to how for the horns could be shiny and iridescent, you can look mother of pearl on the inside of an oyster shell as a model. This is called Nacre. It is little flat plates of a calcium carbonate (aragonite) that is organized as little flat plates or tablets. This also turns out to be very strong.

When you break the shell and look at it under the electron microscope it looks like the diagram.

That may be a stretch but apparently people actually study differnt types of horns like in this paper. https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2018.0093

From the micrograph you can see that the keratin is at least a little bit organized, and perhaps in your world, with a high calcium diet or other mineralization and the right selection pressures the creature evolved with something like the mother pearl horns.

Of course it might be simpler to have the creature decorate his/her horns somehow. If you look at powder horns from the days on muskets that were used to store gun powder some of those are pretty shiney, just because they are polished.

Answer 3

You have two questions that really aren't related at all. First, on the question of alien creatures evolving a skeleton based on chitin, it seems plausible, but I don't know enough about the properties of chitin to give a definitive answer.

The structural parts of bones are composed of mineral crystals, mainly calcium phosphate (hydroxyapatite), embedded in a matrix of collagen fibers: https://en.wikipedia.org/wiki/Bone There doesn't seem to be any fundamental reason why a different evolutionary path couldn't use chitin in place of collagen, just as for instance sea shells are formed of calcium carbonate plates embedded in a protein matrix.

But using chitin in place of keratin would imply an entirely different evolutionary path, as the genes for that are AFAIK buried way back before the divergence of primitive chordates. You couldn't have some random mutation suddenly producing a chitin-using mammal.

Now for gold color and iridescence, that's generally produced by structural coloration, as with your beetle example. The underlying material doesn't have to be chitin. It could be almost anything.

For instance, it is certainly possible to have iridescent structures made of keratin. This is pretty common among birds, see for instance the structural coloration of peafowl feathers: https://magazine.scienceconnected.org/2015/06/structural-coloration-in-bird-feathers/ (And for an example from today's news, the cassowary: ps://www.cnn.com/2020/05/13/world/cassowary-bird-feathers-scn/index.html ) There are some birds with a more or less golden color, for instance the Golden Pheasant https://en.wikipedia.org/wiki/Golden_pheasant though I don't know if the color is structural or pigment.

The reason you don't see more of this sort of coloration (or indeed, any sort of color other than usually drab variations on brown, black, and reddish-orange) in mammals is (IMHO) down to the fact that most mammals other than primates don't have good color vision. It's no use evolving a gold-colored horn to attract mates if potential mates can't distinguish your shiny golden horn from your rivals' drab brown one.

Indeed, mammals in general seem to have gone in for camoflage coloration, rather than using bright colors as a sexual adornment, as many birds do. The only exception I know of is the mandrill https://en.wikipedia.org/wiki/Mandrill and here again, the colors are structural (using keratin) rather than pigments.