Theoretical Morphology and Physiology of the Sasquatch Eye: Karl Sup

NOTE:  This is a special posting by respected Sasquatch field, technical and theoretical researcher, Karl Sup.

Theoretical Morphology and Physiology of the Sasquatch Eye
Author Karl Sup 04/30/2014

One of the reported traits of numerous encounters with Sasquatch peoples includes an observation by the witness of eyeshine and/or eyeglow. I myself have witnessed both events firsthand more than once, and remain innately puzzled by the latter.

Each instance of observed eyeglow was observed in total darkness, with no moonlight. While the starlight may have played a factor in the observation, the same observations occurred on cloudy, dark nights as well. The muted but glowing, iridescent color appeared to fluctuate between a red-orange-yellow in a very organic manner. In contrast, each instance of observed eyeshine was in correlation to a direct application of infrared (IR) light to the subject(s).


We’ve all experienced eyeshine. Flip through your family photo album and look for the ‘red eyes’ of aunt Martha and little cousin Susie. Many cameras have red eye reduction capabilities that pre-flash to dilate our pupils to deter demonic-looking photos of our loved ones. Because the light of the flash occurs too fast for the pupil to close, much of the very bright light from the flash passes into the eye through the pupil, reflects off the fundus at the back of the eyeball and out through the pupil. The camera records this reflected light. The main cause of the red color is the ample amount of blood in the choroid which nourishes the back of the eye and is located behind the retina. Most primates, including humans, lack a tapetum lucidum, and compensate for this by perceptive recognition methods.

True eyeshine is caused by a layer of tissue in the eye called the tapetum lucidum (meaning ‘bright tapestry’ in Latin), and is present in many vertebrates.  It lies immediately behind the retina and reflects visible light back through the retina, increasing the light available to the photoreceptors, though blurring the initial image of the light on focus. The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal; especially carnivores that hunt their prey at night, while others are deep sea animals.

While there are numerous mammals that possess tapetum lucidum in their physiology, very few primates possess this feature. Prosimian primates do exhibit this feature, like the Lemurs (superfamily Lemuroidea) and the Aye-Aye (genus Daubentonia), who use their tapetum lucidum to hunt and navigate the forest at night. The diurnal Brown Lemur (genus Eulemur) and Tarsiers (family Tarsiidae) also exhibit this trait.

The tapetum lucidum structure is iridescent, and reflects light roughly on the interference principles of thin-film optics, as seen in other iridescent tissues. However, the tapetum lucidum cells are leucophores, not iridophores. It is a retroreflector of the transparent sphere type. Because it is a retroreflector, it reflects light directly back along the light path. This serves to match the original and reflected light, thus maintaining the sharpness and contrast of the image on the retina. The tapetum lucidum reflects with constructive interference,₂ thus increasing the quantity of light passing through the retina. In the cat, the tapetum lucidum lowers the minimum threshold of vision 6-fold, allowing the cat to see light that is invisible to human eyes.

The ‘Cat's Eye’ retroreflector is modeled after their optical system. The combination of the eye's lens and the cornea form the refractive convergence, while the tapetum lucidum behind the retina forms the spherical concave mirror. Because the function of the eye is to form an image on the retina, an eye focused on a distant object has a focal surface that approximately follows the reflective tapetum lucidum structure, thus providing good retroreflection.

The eyeshine reports attributed to Sasquatch varies from red to green. I have personally witnessed light green eyeshine only. Based on the reported and witnessed nocturnal capabilities of a Sasquatch to navigate and hunt in near total darkness, I believe it would be a strong indicator and reasonable expectation to theorize that this hominid species possesses tapetum lucidum to enhance their night vision.


Some have theorized that the eyeglow witnessed in Sasquatch is attributed to bioluminescence of the eye itself. Based on the known attributes of the retina, along with the form and function of the tapetum lucidum, it would seem counterproductive for a nocturnal being to bioluminesce the eye.  And yet, there are hundreds, if not thousands, of witness reports that indicate similar or nearly identical characteristics of these glowing eyes. As stated earlier, as a first-hand witness in search of answers, I know what I witnessed and seek an explanation.

Bioluminescence is a form of chemiluminescence where light energy is released by a chemical reaction. Animals and insects that are bioluminescent typically use this ability as warnings, prey attraction, mating rituals and communication. The firefly (family Lampyridae) is a common bioluminescent insect we would collect in jars during muggy summer nights as children. In recent years, scientists have noted that large groups of them would synchronize their lights, termed phased synchronization₃. While the true intent is unknown, it is speculated that this collaborative community effort allows for long distance communication with other firefly communities.

Bioluminescence is a fairly common characteristic among squid and cuttlefish. In fact, 63 out of the 100 genres contain species with this capability. Light production in cephalopods can be either autogenic, produced intrinsically by the organism itself, or bacteriogenic, produced extrinsically by bacterial symbionts. It is possible for a species to display one or both of these types of luminescence, depending on their physiology.

Flashlight fish (families Anomalopidae and Stomiidae) are named for their large bioluminescent organs. These are located beneath the eyes and contain luminous bacteria. Two methods are used by different species for controlling light emission, either a shutter-like lid is raised over the organ or the organ is turned downward into a pouch. The light is used for predator avoidance, to attract prey, and for communication with other flashlight fish₄.

The primary and common purpose for bioluminescence with all species seems to be communication. Man has used light to effectively communicate throughout the centuries. In the 1880’s, heliograph stations were constructed throughout Arizona mountain peaks to allow instant communication across great, rugged distances to convey, via Morse code, Apache warrior movements and subsequent orders using mirrors and the sun. However these stations were ineffective after sunset.

The structure of the eye is an ideal reflector. Retroreflectors, as discussed previously, are used in our everyday lives on the roads we drive. One recent retroreflector innovation was a device to provide secure communications between two stations in a line of sight. It was modeled after the combination of the tapetum lucidum and the bioluminescent "flashlight" in flashlight fish. 

For a Sasquatch, being able to covertly communicate with another nearby Sasquatch would be an invaluable tool for maintaining coordinated surprise of their prey. As numerous researchers and scientists have stated, no other animal has the ability to bioluminesce their eyes at will, nor would it seem be beneficial to their night vision if it was physically and biologically possible. However, what if that bioluminescence occurs externally to the eye?

Vestigial Remnants

The human body has numerous remnants of our evolutionary past. Some of these are as follows:
  • Our ears have vestigial muscles intended to direct them towards a sound.
  • Our maxillary sinus has a drainage position designed for a quadruped that stooped over. That being the case, it often turns into a cesspool of trapped fluid and bacteria.
  • Most people’s jaws no longer have space for all of their molars, especially wisdom teeth.
  • We have canines, teeth that were once beneficial for capturing prey and for display.
  • Our retinal nerves and blood vessels go over our retina, thus blocking out certain portions of our vision that our brain blurs over to create a seamless image. You may be familiar with this, as it is known as our “blind spot.”
  • We have the leftovers of a nictitating membrane in the corner of our eye called the “plica semilunaris of convunctiva.”

The plica semilunaris is of interest to this discussion in particular. It is a transparent or translucent third eyelid, or haw, present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. 

Many animals have functional nictitating membranes. All primates have vestigial remnants of the plica semilunaris, however the Lemurs and Lorisoid primates do have partially functional third eyelids.  The Calabar angwantibo (Arctocebus calabarensis) Lorisoid primate does have a fully functioning nictitating membrane. It is primarily arboreal and nocturnal. 

Modern man evidently evolved away from it, due primarily to environment. Nonetheless, man did at one point possess a fully functional third eyelid.

I am theorizing that the Sasquatch may have a fully functional nictitating membrane, and this third eyelid may have bioluminescent properties. In many ways this makes sense. Their environment dictates navigating dense brush, thickets and forests on a daily basis; much of which is accomplished nocturnally. Having a protective haw to cover the eye would be an advantage to life in this environment. In addition, while the third eyelid is luminescent, the pupil could contract to reduce the amount of light entering the eye.

From a biological standpoint, the cartilage and tissue in the nictitating membrane would be able to support a cell structure that could allow bioluminescence. 

Karl Sup is a software architect, developer and analyst, and an avid Bigfoot researcher working in the mountains of Arizona for many years.  During this research and in other states including Maryland and Wisconsin, he observed eyeshine and eyeglow from different subjects. Karl also has had decades of audio analysis and editing experience, and assisted in helping M.K. Davis clean up and enhance audio from VHS tapes he has been studying and discovered the presence of infrasound within those recordings.

1.     Pages 578-581 of Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). "Chelicerata: Araneae". Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 571–584. ISBN 0-03-025982-7.
  1. Locket NA (July 1974). "The choroidal tapetum lucidum of Latimeria chalumnae". Proceedings of the Royal Society B 186 (84): 281–90. doi:10.1098/rspb.1974.0049. PMID 4153107.
  2. Murray, James D. (2002). Mathematical Biology. I. An Introduction (3rd ed.). Springer. pp. 295–299. ISBN 978-0-387-95223-9.
  3. Howland HC, Murphy CJ, McCosker JE (April 1992). "Detection of eyeshine by flashlight fishes of the family Anomalopidae". Vision Res. 32 (4): 765–9. doi:10.1016/0042-6989(92)90191-K. PMID 1413559.


  1. It was not that long ago that this theory would have been labeled as the lunatic fringe of bigfooters. Then is one of the Finding Bigfoot episodes this past season Matt Moneymaker spoke of bioluminescence. Because of this it became a topic on the Blue Forums. I listened to several researchers telling of their experiences and having witnessed the eye shine with absolutely no background sources, and some had seen it numerous times as Karl Sup has. I have since found several others coming forward with this thesis also. One gentleman, I believe Freeman Young,thinks the different colors of red, yellow, and amber/whitish are used at least in part as communication to other members. Red being high alert or danger, Yellow being cautious, and the white being all is well. He did not mention the blue or green colors. It is amazing how this field is starting to evolve.

  2. Interesting theory, but a problem arises with it when you realize that many "monster" sightings are accompanied by brightly glowing eyes. Not just hairy hominids- phantom dogs, phantom cats, flying creatures, some in broad daylight. In fact, the only real physical proof that the creature known as Mothman ever existed was after Connie Carpenter saw him during the day WITH glowing eyes and got bad eye burn. Her sore and swollen eyes were later witnessed by investigator John Keel.

  3. I saw the eyeshine of Bigfoot from a distance of approximately 30-40 ft. I clicked on my LED flashlight and it looked exactly like 2 LED flashlights shining back at me. It reflected my flashlight EXACTLY. Perfectly round and brilliantly bright white. I caught it off guard but the encounter lasted long enough for the eyes to have time to adjust to such light. It was startled and scared. The color and appearance remained the same the entire time. Everytime thereafter, I saw it from a distance with the same flashlight and it always appeared the same as the first time. Not sure if my experience is of any help to you, but the eyeshine of Bigfoot was by far the most intriguing thing I've ever seen in my life. Thank you for sharing your information.


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