42 Northern Woodlands / Autumn 2017
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.
Alcott Smith is a veterinarian, teacher, and naturalist from Lebanon, New Hampshire.
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:
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.
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.
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.