By Paul Donovan
There has been much written regarding the extent to which reptiles rely on vision as a primary sense of their surroundings, with many people believing them to be blind or having poor visual perception at the best. It is true that some snakes and lizards, such as burrowing snakes of the family Typhlopidae, Thread snakes, Leptotyphlopidae, and Worm lizards in Amphisbaenidae, are blind or have very rudimentary eyes capable of detecting perhaps only minor changes in light patterns, but by and large reptiles have good vision.
Living a subterranean lifestyle would make the need for vision redundant: A) because light levels would be so low that unless the eye was highly evolved with the equivalent of night vision capability, it would be incapable of picking out enough light to form an image, and B) there would be nothing to see anyway.
This then brings us to the remainder of the reptile orders – those living on the surface (terrestrial) and those living in the trees (arboreal). Almost certainly, within the thousands of snake, lizard, chelonian and crocodilian species, there must be great disparity in visual acuteness. From examination of the makeup of the eye, arboreal snakes and crocodilians have the most sophisticated eye structures.
Structure
Outwardly and although the reptilian eye would bear a resemblance to the mammalian eye, they are structurally very different. One of the major differences of the reptilian eye, is the lack of muscle structure that physically alters the shape of the lens to bring about focusing. Focusing is then achieved by contractions of the surrounding ciliary muscles which increase pressure on the vitreous body and physically moves the fixed focus lens backwards and forwards away from the retina which lacks in all but one species, (Ahaetulla), no fovea.
In effect, the lens works in a similar fashion as to focusing a magnifying glass. In this respect vision could be described as being poor. However, the makeup of the retina is highly sensitive, as it is in humans, to movement and so could technically be regarded as being very responsive. Furthermore, a snake’s eyes can focus on an image falling on the retina, but the detail of that image is not as clear as that of higher vertebrates.
Within the different reptilian orders, vision will vary greatly, from those only being able to distinguish light from dark, to those capable of seeing color. Many snakes have both rods and cones in their eyes as humans do, but there are some diurnal species such as the European Grass snake, Natrix natrix, which have just cones that are sensitive to blue, red and green, though not in any significant numbers. Although sensitive to color, the Grass Snake’s eyes lack the range of colored oil droplets, as do many other snakes, in the photoreceptors that further filter the light, thereby restricting the range of colors seen detectable.
It is thought that the absence of this oil was due partly to early snakes being subterranean, where the need for vision was superfluous. As they evolved to a terrestrial way of life, vision began to evolve parallel to this, but in some species the oil droplets remained absent.
To help protect the eye, many diurnal snakes have a yellow filter inside the lens that helps absorb ultraviolet light; in crepuscular species this yellow filter is absent. The presence of rods and cones in crepuscular species does indicate that vision is good even at very low lighting levels.
As for nocturnal species, they have the capacity to see exceptionally well in the dark, due in part to the presence of a vertical pupil (technically called a stenopaic pupil), a clear lens, and a retina rich in rods. The light gathering ability of nocturnal species is further enhanced, by the fact that because the pupil is like a vertical slit running from the top of the eye to the bottom, as it opens, it allows proportionally more available light to penetrate the eye.
In contrast, the photoreceptors of lizards and turtles show a high degree of multi-colored oil droplets affording superior color vision. Within the cones of the retina are proteins called opsins. These proteins are sensitive to varying wavelengths, and afford some species the ability to see into the UV spectrum range, far beyond the capabilities of the human eye.
Tiny Mirrors
Vision in crocodilians on the other hand, is even more intriguing. I am sure that you have all watched documentaries where crocodile researchers go out at night with spotlights and shine them in the water. If it picks up a crocodile, its eye will glow. This is due to the presence of ‘mirror-reflecting’ cells called tapetum lucidum. Although popularised in crocodilians, the presence of tapetum lucidum is widespread in many vertebrate species, particularly nocturnal hunters, as well as deep-sea dwelling animals.
The tapetum lucidum lies directly behind the retina and reflects light back through it thus maximising the exposure time to the photoreceptors. This light gathering property significantly enhances low-level light vision, but does have one drawback; because the light is reflected back into the eye, the image seen can be blurred. The retina is also occupied predominantly by a mass of cone photoreceptors that are sensitive to varying wavelengths of light. Given this, the crocodile has first-rate daytime vision, and may even have superior color vision even when compared to that of humans.
Interestingly, ‘eye-shine’ in those animals where tapetum lucidum are present, may actually exhibit a wide range of varying colors. However, because the resulting color is brought about through iridescence, the angle at which the light is shone into the eye tends to give rise to a silvery appearance. That being said, the color of the bulb within the torch or spotlight can also have an impact on the eye’s color.
Do not be under the misconception that a similar effect takes place in humans when you take a photograph – a condition called red eye. The redness you see is caused by the camera’s flash bouncing off the retina, which is rich in blood vessels.
Location
The location of the eyes on the head is dictated to a certain degree by the type of habitat the reptile lives in. For example, many desert snakes and crocodilians have large eyes positioned more towards the top of the head, whereas many reptiles have smaller eyes positioned lower down. The positioning of the eyes on the side of the head, opposed to being in the front, as is the case with humans, opens up the reptile’s depth of field quite considerably.
A number of snakes may actually have what is commonly referred to as ‘binocular vision’. The Vine Snake, Ahaetulla nasuta, is amongst several snake species that have very distinct pupils that are neither circular nor vertically slit-like as they are in many nocturnal species, but horizontal and shaped like a figure eight. In accompaniment to this, the region anterior and posterior to the eye is channelled. This channel enables light to be directed, say from the rear of the snake, onto the extreme anterior region of the retina where there is a fovea-like area and thus maximise the image seen. It also enables the snake to see potential predators or prey more readily. Binocular vision allows these snakes greater perception of depth and distance giving them a field of vision in the region 130-degrees.
When we talk about vision and eyes, we cannot fail to mention the chameleons. Surely, a group of lizards with the most bizarre eyes of all reptiles. The extraordinary cone structure, which covers the eye, is formed by the fusing together of the upper and lower eyelids. This not only protects the rather bulbous eyes, but also prevents them from drying out. The eyes can be moved independently from one another, and in any direction. This gives the chameleon a 360-degree field of vision; greater than just about all other animals. Therefore, when one eye is looking up, the other can look backward. This is of great benefit to the chameleon, for it can search for prey with one eye, while looking out for predators with the other.
Although the aperture formed by the eyelids is very small, and therefore only a limited peripheral field of vision is available when static, this is more than compensated for in the ability to roll the eye through 360 degrees. Furthermore, the size of the eyes, and the way the eyelids form a small aperture, give each eye the equivalent of a telephoto lens with a focal range of around 100 to 150mm. These features provide the chameleon with exceptional visual perception in the detection of food or predators, which few other animals can compete with.
This is an overly simplified view (no pun intended) regarding the state of color vision in reptiles, as there are many exceptions to the examples I have given. Of course as with any animal species, beit vertebrate or invertebrate, the eyes respond to movement, and in snakes how they respond to this will vary from species to species. Take the cobra for example. Its response to movement is to rear up off the ground and display a wide hood as a warning. Stay still and the snake drops back to the ground and moves off. Move again and it will rear up. Others will simply move off without giving away their presence before you even knew they were there. Although some snakes may remain motionless and wait until trouble has passed, this is not the case with lizards. At the first sign of movement, the lizard scurries off to the most accessible place of safety. Only when it is confident that the threat has passed, does it emerge from its retreat.
It is true to say then, that while movement is of obvious importance to the snake, it does not play as important of a role as say the olfactory sense does. This can be witnessed by the way the snake uses its tongue to ‘smell’ the air. Still, vision rarely works alone, and either proceeds, or is accompanied by other senses; i.e. heat detection in rattlesnakes and some pythons and boas.
Protection
One difference you will notice between snakes and other groups of reptiles, is that they lack moveable eyelids which offer protection and a method, blinking, of keeping the eye moist and free from irritants. It would be foolhardy in the snake’s evolution to omit some method of protecting the eye from damage, and this has come about by the evolution of a transparent scale called a brille or spectacle covering the eye. This is not a separate scale, like a contact lens, but actually forms part of the snake’s skin and is shed periodically when the snake sloughs. The brille is particularly evident when the snake approaches a slough when it turns a milky white color due to secretion being released to separate the old skin from the new.
Many aquatic or semi aquatic species such as the crocodilians, have a third eyelid called the nictitating membrane which covers the eye when the animal is submerged beneath the water. This semi-transparent membrane allows the animal to see while submerged, almost as though it were wearing a pair of goggles, though the image detected will not be in such clear detail.
I want to finish off by talking about another type of eye, and that is the parietal eye, or third eye; sometimes inaccurately referred to as the pineal eye. The parietal eye is visible on top of the head of many lizard species such as Iguanas, as a small whitish ‘spot’. It is a light-sensitive photo-sensory organ present in many animal species, including, but not limited to, lizards, frogs and some fishes. It is associated with the pineal gland and functions in regulating the circadian rhythm, hormonal production for reproduction, and ultimately thermoregulation. Structurally it resembles a primitive eye in that it has a small rudimentary lens and retina, though it is not capable of visual perception. In some species, such as the Tuatara, it is well developed.
The presence of a parietal eye may account for the reason why many lizards are very skittish. The eye is responsive to shadow movement, so as you move towards the individual the shadow you or your hand casts, alerts the individual to your presence and it will act accordingly.
Vision, then, in all reptile groups is far superior than many people imagine. Adaptations to the eyes in the form of distinctive pupils have enabled snakes, lizards and crocodilians, to evolve a diurnal or nocturnal way of life. And while we humans may not have good night time vision, nocturnal species, with their reflective retinal cells, do. In this regard, they sport a far more superior sight than we do.
