Some of the simplest eyes are just passive holes with a semi-clear membrane, such is the reptilian ‘pineal eye.’ This does little more than allow light to enter and thus allow the animal to determine whether it is light or dark outside. Even if their two main eyes are closed, they can still ‘see’ that it is light outside.

This pineal eye is sufficient to regulate their circadian rhythms. Knowing when it’s time to sleep and when it’s time to wake is about the main function of this simple unblinking eye, a simple clear dot which is about the size of a match head. It function  involves a multitude of health benefits to the animal however. Reptiles kept as pets have a particular need to not only have artificial light of a certain wavelength to simulate the sun for UV absorption, but they also require a sufficient period of darkness as well. Unable to close or ‘hide’ their unblinking pineal eye, they perceive perpetual daytime in an enclosure that maintains 24-hour continuous light or even the irregular human-dictated cycles of ‘day’ versus ‘night.’

This would be sheer torture for a wild mammal; it must also be unpleasant at best for a reptile too. While they will survive this unnatural circadian aberration they can become temperamental over time, moody and aggressive. Lethargy and weight loss can be the norm. Surviving but not thriving, breeding in captivity can become almost impossible. Often, returning to proper regulation of diurnal cycles is all it takes in an otherwise acceptable breeding program to induce reptiles to breed successfully in captivity.

Iguana Eye

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Cold and uncaring, the brightly-colored eye of a reptile while beautiful is also a bit scary for some. If they were glass marbles they would be a highly sought-after collectible. Prized like jewels for their colors, patterns and variations, they would be coveted and praised more than even the most brilliant gemstones.

Both animals and humans have phenotypic variations of eye color that are attributed to ratios of eumelanin, a substance produced by melanocytic cells in the iris. More, less, that is what makes all the difference.

Bright eyes of many bird species employ a different process determined by other pigments and possibly dietary preferences have lead to or contributed to this differentiation. If they are regularly consuming something that provides some element or micro-nutrient that favors a particular gene, that gene will be more effective at producing its specific pigment and thus it may manifest itself in very brightly colored eyes of that particular specie over time. Just as removing poison dart frogs from their environment causes them to eventually cease to be ‘toxic’ themselves, it is possible that wild birds removed from their exotic environment (and from their natural food selections) will in time cause their eyes to not be so brilliant and colorful.

Into Mysterious Eyes Like My Soul, Blue Smoke Falls into a Black Abyss

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We’ve probably all heard or read before that ‘blue eyes’ was a singular and random mutation having occurred perhaps 6000-10,000 years ago around the Black Sea region, or the Near East. Thus, all ‘blue-eyed’ people might all be related. Blue eyes with brown specks are not included in this hypothesis as this variation seems to be unrelated to this one notable, single random mutation.

The most common human eye color is brown and the least common is green. Some mutations can cause unusual eye colors including red, black/very dark browns and even shades and points of violet color have been recorded. The genes responsible for eye color are few, -perhaps as few as six and a study in early 2009 mentions a test in Rotterdam that has shown a greater than 90% accurate model for predicting brown and blue colored eyes using just the six suspected genes as predictor markers.

Did you realize that eyes can actually change color? A lightening or darkening of eye color can occur during a woman’s pregnancy, during childhood puberty and sometimes after a serious trauma. The melanocyte cells that produce the pigments which determine the color of the eyes is a continuous process so any hormonal change can result in eyes that change color.

Hormonal changes are cited for the reason why this happens. Some medicines (like glaucoma medications) can change the color of the eye as well. And this change is permanent.

Close-up of a Human Eye

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A refractive cornea, while useful out of the water, has no difference in refractive index between the vitreous fluid in the eye and the water outside of the eye when we return to the water. This is why when we go underwater and open our eyes, everything is blurry. The vitreous fluid relieves some of the refractive index of the lens as this gelatinous fluid is thicker/denser than the air, it helps refractive index of the light falling upon the retina. The lens need not work as hard and thus, is thinner as the focal length is aided by the denser-than-air vitreous fluid.

Useful out of the water, returning to the water which has a density about equal to the pressure of the vitreous fluid makes the vision blurry. No refractive difference. Therefore, creatures that evolved to partially return to the water (such as seals, walruses, penguins) have adapted to lose their refractive cornea and return to a lens-based vision which favors their need for underwater acuity.

Viewing a computer screen is tiring for the human eye and contributes greatly to eye strain. The colors black or any ‘non-white’ color is better for background-colors for web page as the color white strains eyes the most rigorously.

The color green, specifically the wavelength of around 540nm (which interprets to color-code #2f6533) is the most soothing, least stressful to the eye. Web pages using black backgrounds despite being easier on the eye consume more electricity per capita. It has been postulated that if some major web sites and portals (e.g., Google, YouTube, Facebook, MSN, etc) switched to an all-black background, there would be notable energy conservation realized.

Reptile Vision, a Snake or Turtle?

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Another reptile eye, up close.

Robber Fly Eyes

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