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I have a creature in mind that has almost no ability to turn its head left/right and has forward-facing eyes like primates and predators generally do. This would generally present a problem for a creature regarding anything sneaking up on it, but this animal species has developed a workaround and that is by having large ears that they can manipulate and has a reflective inner surface, allowing them to move the ears into their cone of vision and then angling them until they allow the creature to have an idea of what's going on behind them.

Problem is that I don't know how reflective such an organic structure will be able to realistically be with earth-like biology, whether the creature would be able to see things within the reflective inner surface of the ear with great detail like an actual mirror, or if it would only be able to make out that there is some sort of movement behind them but whatever that is they'll have to make the effort to use their legs to orient the whole body to be able to allow them to see what it was.

How reflective can a creature's body parts get?

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    $\begingroup$ Great question. @starfish prime and L. Dutch already gave examples, but there are many more, some with strong color or wavelength dependence from form birefringence in feathers or butterfly wings. But the second part of your question is also very interesting. Detecting motion for example is different than viewing and image. For detecting motion you don’t necessarily need to preserve the image, or have perfect mirror. $\endgroup$
    – UVphoton
    Mar 24, 2022 at 16:39
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    $\begingroup$ You can see your reflection in most vertebrates' eyeballs - does that count? If creature's entire skin were made out of cornea, they'd pretty mirror-like. $\endgroup$
    – Darrel Hoffman
    Mar 24, 2022 at 18:32

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I have no quantitative data, but one example from real life is the jewel scarab (in particular the Chrysina limbata from Costa Rica). They are not all shiny but some are highly reflective to the point where you can clearly see the photographers fingers:

Photograph of a jewel scarab

Had the shell not been curved I'm sure you would be able to get a near perfect mirror image.

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  • $\begingroup$ +1 for a nice clear reflection. There are a bunch of different kinds of shiny beetles, but the ones that live in my neighbourhood are singularly bad fliers and lose quite a lot of their glossiness by midsummer in exchange for scrapes and cracks. $\endgroup$
    – Starfish Prime
    Mar 25, 2022 at 13:06
  • $\begingroup$ That's rather reflective, though I don't know if chitin fits well with the creature and I may have to redesign the shape of the ears into being stalk-like with shell-like cupped ends, sort of like motorcycle mirrors come to think of it. I'll be accepting this answer based on pure reflectivity but I'll probably go with the silvering idea of the answer with fish scales for the creature's actual design. $\endgroup$
    – Lemming
    Mar 26, 2022 at 6:56
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Some fish scales have evolved to be highly reflective as part of a defense mechanism, with clever crystalline structures optimised for reflecting light in ways that confound the vision of would-be predators.

From the linked article:

As part of their study, Jordan and colleagues advanced nature’s design a step further. They calculated a scale structure that would make fish almost entirely invisible – due to the 100% reflection of light from the surrounding ocean.

Rohu fish scales

(image credit Rajesh Dangi)

Quote wikipedia,

Many teleost fish are covered with highly reflective scales which function as small mirrors and give the appearance of silvered glass. Reflection through silvering is widespread or dominant in fish of the open sea, especially those that live in the top 100 metres. A transparency effect can be achieved by silvering to make an animal's body highly reflective. At medium depths at sea, light comes from above, so a mirror oriented vertically makes animals such as fish invisible from the side.

They haven't evolved to form a nice even image-preserving mirror-like finish over a large area because that isn't so useful for their intended purpose, but the basic design could be adapted and work just fine. Even without perfect image formation, movement behind the mirror-user would be obvious even if the details of the moving object were not clearly visible. This would be a good cue to turn around, or dodge, perhaps.

Fish scales can be lost and regrown, by way of a bonus, so your critters don't have to worry about losing their all-around vision in their old age, or as a result of damage to their ears.

Additionally, the flexible, moveable mirrors might also be useful as a way to dazzle and confuse predators and prey alike.

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Tapetum lucidum is a biological structure with the very purpose of reflecting light

The tapetum lucidum (/təˈpiːtəm ˈluːsədəm/; from Latin tapetum lūcidum 'bright tapestry, coverlet'; pl. tapeta lucida) is a layer of tissue in the eye of many vertebrates. Laying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light available to the photoreceptors (although slightly blurring the image). The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores, while others are deep sea animals.

The only indication on its reflectivity I was able to find is indirectly given by this following statement

In the cat, the tapetum lucidum increases the sensitivity of vision by 44%, allowing the cat to see light that is imperceptible to human eyes.

Which I interpret as it reflects back 44% of the light reaching it.

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