Lynx eye: The word lynx originated from the Greek word “lychnos,” meaning “lamp” or “light;” it pertains to the iridescence of their eyes when illuminated at night. Photo by Eric Isselee/123RF
A predator’s physical senses determine the manner in which it hunts. Wild canids, for instance, favor their sense of smell – you can see this as they trot along the hedgerows, noses in the air, reading the wind. Bears, too, lead with their noses. Anyone who’s watched them ‘taste the air’ as they approach a food source won’t be surprised to learn that they’re nearsighted.
With the forest cats of the Northeast – the bobcat (Lynx rufus), the Canadian lynx (Lynx canadensis), and the once-common, now-extirpated cougar (Puma concolor) – eyesight is the most highly developed sense, followed by sound and, lastly, smell. This manifests in stealth. By stalking, slinking, sneaking, they position themselves for explosive, catapulting attacks of two to five bounds within a few seconds.
A cat’s general repertoire of senses is the same as ours, and our eyes are structurally similar. Light waves pass through the cornea, aqueous humor, pupil, lens, and vitreous humor to the retina in that order. Functionally, their eyes, like ours, behave as a camera with the pupillary muscles controlling the quantity and quality of light that reaches the retina. And yet, eons of nocturnal and crepuscular hunting have led to the evolution of some highly specialized and adaptive ocular features. First, a cat’s field of vision is 200 degrees, as compared to a human eye’s 180 degrees. This enables cats to recognize movement more quickly than we can. In hunting prey, avoiding detection, or escaping predation, this is a great advantage and explains why people rarely see a wildcat while they are walking in the woods. The cat most assuredly sees you first, allowing its avoidance by using cover and always staying in the shadows.
Secondly, while cats’ vision in broad daylight is comparable to our own daytime vision, in reduced- or dim-light conditions, cats see six to eight times better than we do. This superior night vision has allowed forest cats to flourish over the millennia as dominant – sometimes apex – mammalian predators.
Innumerable adaptations facilitate a cat’s keen night vision. The reflective yellow-green glow of a cat’s eyes when exposed to a light beam in darkness comes from a layer of reflective cells behind the retina, called the tapetum lucidum. These cells contain a reflective crystalline matrix of amino acids, minerals, and riboflavin (Vitamin B2). Light that fails to be absorbed by the retina’s photoreceptor cells on first-time exposure gets a second chance for absorption on the rebound. Thus, each photon of light becomes two photons in dim light. (You’ll remember from science class that the definition of a photon, where it relates to optics, is a unit of incident retinal illumination equal to the illumination from a surface area of one square meter with the brightness of one candle as seen through a pupillary aperture of one square millimeter . . . or maybe you won’t; the larger point here is that humans can only imagine the definitive visual detail these phantoms of the forest perceive as they peruse their domain beneath a darkened forest canopy with only a trace of starlight.)
In cats, as compared to diurnal mammals, the retina’s area centralis contains a greater number of modified dendritic photoreceptor cells known as rods. These rods are extraordinarily light sensitive and depict mostly shades of gray, as in black-and-white photography. These rods provide superb vision in low-light conditions.
This is why cats can see floating dust motes and the flickering of mouse whiskers in semi-darkness that we could not possibly see.
Science has shown that these rods are sensitive to a narrow range of short wavelengths, which allows cats to see some color, mainly blue and yellow, but not green, brown, or red. Cats’ retinas house some 400,000 rods per square millimeter, as compared to 160,000 rods per square millimeter in humans. By contrast, human retinas are heavily endowed with photoreceptors called cones. The cones respond to a wide range of wavelengths, which affords people good color vision, but requires large quantities of light, as in artificial or daylight conditions.
Another notable manifestation of our wild felines’ capacity for nocturnal hunting is the vertically elliptical conformation of their pupils. Only in darkness are these apertures wide open and round like humans’. In bright sunlight, cats’ pupils become barely discernible vertical slits. This limits retinal exposure to daylight, improving daytime visual acuity and depth of field. But far more importantly, when a cat enters darkness, these pupils open wider and faster than do our own round pupils, providing instant visual accommodation. This ophthalmic adaptation is unique to nocturnal predators.
If you own a cat, you’ve probably noticed that they have a third eyelid. Known as the nictitating membrane, or membrana nictitans, this organ serves to protect and lubricate the eye’s corneal surface. “To nictitate” means to wink. Located in the medial canthus of the eye’s conjunctival space, this membrane involuntarily and intermittently flashes across the cornea to cleanse, protect, and moisten its surface with tears.
You’ve probably also noticed that your cat can become transfixed by television. We can chalk this up to their superior flicker-fusion: Instead of a continuous flow of images on the screen, they are seeing a bouncy, interrupted series of individual frames. If you’re old enough to remember Charlie Chaplin’s silent movies, that’s what your cat sees. Once again, we have recognized another special feline adaptation facilitating small-prey capture in semi-darkness.
Cats are conveniently far-sighted, a trait that allows them to detect distant prey or danger. Their close up vision is poor; thus, prey seizure and execution is guided by the vibrissae (whiskers) on the face, chin, and feet. When your house cat plays with a captured mouse, escape attempts are registered via vibrations upon these highly sensitive whiskers. For the same reason, when a cat (wild or domestic) drinks from a pond, brook, or bowl, it first touches the water surface with its paw to determine how close it is to the source.
Perhaps these visual adaptations help explain why forest felids have been successful over the millennia. Until recently and for the past few centuries, bobcats have been the most widely dispersed mammalian predators in the continental US, and cougars the most widely distributed mammalian predators in the entire Western Hemisphere – with the exception of Homo sapiens. Only within the last decade has that status changed; today, the coyote (Canis latrans), with its propensity for omnivory, fecundity, and annual recruitment, has the greatest geographic distribution of any mammalian quadruped in the US and the Western Hemisphere.
The next time you spend a day in the woods, will you see a bobcat? Probably not. Bobcats and lynx lurk in the shadows and remain hermitic. Covertly clad in grizzled gray pelage with rufous tinting and melanotic flecking, they blend exquisitely with the arboreal domain. With a trained eye, sylvatic acumen, perseverance, and some woodsman’s luck, it may only be once in your lifetime that you spy this reclusive ghost of solitude.
To See or Be Seen
The pupillary conformation of a mammal’s eye may be round, vertical, or horizontal depending on the animal’s foraging habits, whether it is prey or predator, and the preferred circadian activity periods of the species. Following are three schematic examples:
Vertical Pupil: Felidae (cats), Crotalinae (pit vipers)
Wild felids have frontally placed eyes and tend toward vertically slit pupils, which facilitates binocular vision. These felines typically ambush their prey in darkness or semi-darkness. Reptilian ambushers like our timber rattlesnake (Crotalus horridus) also manifest this pupillary configuration.
Circular Pupil: Canidae (dogs), Ursidae (bears)
Like primates and bears, our wild canids have round pupils. Circular pupils, nearly parallel optic axes, and stereoscopic vision accommodate both diurnal and crepuscular foraging activity.Hunting and locating prey is guided mainly by olfaction or audition.
Active pursuit and prehension of prey is directed by sight.
Horizontal Pupil: Cervidae (deer), Bovidae (cows)
Herbivorous prey animals possess laterally positioned eyes that afford predominantly monocular vision with an expansive view of their surrounding landscape – in some species, in excess of 270 degrees. Additionally, they have horizontally oblong pupils to increase this expansive, wide-angle view. This facilitates recognition of approaching predators from the front, side, and rear. Furthermore, this pupillary attribute among ungulates enhances escape when fleeing over varied and uneven terrain. These prey animals also have tapeta that enhance the nocturnal surveillance dimension of their grazing and foraging patterns. Note that food detection and procurement by herbivores is guided primarily by olfaction.
Alcott Smith is a veterinarian, teacher, and naturalist from Lebanon, New Hampshire.