Nature gave us the ability to distinguish red fruit from green, but it deprived us of night vision. Cats, on the other hand, can’t detect red, but their glowing eyes can see perfectly well at night.
The faint-hearted don’t venture into the Central American jungle after dark. In our temperate climate zone, especially away from the city lights, it is easy to sink into the warm darkness of July nights, fragrant with honeysuckle and drying hay. Though dark, these nights are full of life, vibrating with sounds and movement, inviting, promising a moment of respite and calm. But in Honduras or Costa Rica, the jungle by night is a foreign environment for man. Deadly. An inky gloom in which our day-trained eyes go blind. A labyrinth of rustling plants among which the dance of death is constantly played out. Poisonous jararacas slither lazily through the undergrowth, and armed spiders roam among the leaves, ready to inject intruders with their toxic cocktail. And then there’s the light. Or the lights, in fact.
Though submerged in darkness, the jungle at night shimmers with lights all around, flickering on and off. Some of them seem to surpass the darkness and the black contours of the objects immersed within it, like the arteries of a circulatory system that is invisible during the day. Green-glowing strings wind around rotting trunks and cover piles of leaves crumbling in the humid heat. Each of these threads is part of a larger web that spans this entire ecosystem, made up of tangled paths of mycelium permeating dead matter. Some of them shine, some even grow small, glimmering caps, creating mini gardens of surreal luminosity straight out of Alice in Wonderland. Fireflies glide silently through the trees, their brief, fast-paced lives literally burning up in the celadon shimmer produced by their bodies. Incapable of feeding, they expend all their accumulated energy within a few days, their only goal being to pass on their genes––and with them, the secret of taming the neon glow. But there are other lights as well. Green, sky-blue, yellow, and brownish-gold. They appear and disappear in a strange rhythm against the background of the black forest. They hang motionless in the air; often in pairs, sometimes in larger groupings. Clouds and swarms of little lights, like hundreds of attentive eyes.
During one such late-evening walk in the Java River valley near San Vito, Costa Rica, I was accompanied in the traditional way by garlands of tiny points of light scattered across the blackening background of the jungle as it sank into the night. And everything would have been fine––this was not my first night walk through the thickets of Las Cruces––if it weren’t for one particular pair of golden firelights that suddenly appeared a hundred feet in front of me, right in the middle of the forest path. Two specks, unyielding, patient. I knew how such meetings in a tropical jungle could end. If there’s just the one speck of light, and especially if it’s performing unpredictable acrobatics and flourishes, it’s probably a firefly. However, if there are two dots, an immediate assessment needs to be carried out: how close are those dots to each other? Less than an inch and it’s probably a rodent. A couple of inches or more: time to beat a quiet but hasty retreat. That’s what I did. For a brief moment, I stood staring at the creature (creatures?) before me, and for some reason, my mind painted an absurd picture. I imagined that just below the two little lights, as would be fitting in Wonderland, I could almost make out a wide, perfectly white, feline grin. And although the owner of those golden lanterns could not have been the Cheshire Cat, my imagination was not far off.
An Evolutionary Error
The next day, I discovered quite quickly what the two “fireflies” had been––and the general excitement at the research station was palpable. It turned out that shortly before my night walk, a camera trap mounted on the path leading to one of the gates had picked up a puma lazily strolling through the jungle. Both the time and the dimensions of the potential owner of the glowing specks matched perfectly. So it was confirmed: I’d made the right decision in retreating. What’s more, my wild imagination had not, in fact, gone overboard in producing a vision of a cat’s smile hanging in the air.
Whether the puma actually smiled at me, I don’t know. What was amazing, however, was the vivid brightness of those eyes, stoically and calmly following my every step––I was a strange animal clearly having trouble moving around in the darkening forest. That event was an unusual meeting of two strategies for dealing with the world through the senses. On the one hand, there I was: an animal of the day, a mammal habituated to collecting food in the sunlight, my eyesight adapted to distinguish subtle variations in color. This is the vision ancient humans required––the ability to recognize fruits, ignore the unripe ones and focus on those that were tasty and ready to eat. According to researchers of the prehistory of mammals, these were the evolutionary forces that led primates (including humans) to develop “tricolor” vision, based on the stimulation of three color receptors in the eye. This ability made it possible to effectively distinguish the inedible green fruits from the tasty red ones. So human vision has evolved to perform better and better during the day, when enough light reaches the eye and the color-sensitive cones are responsible for most of what we see. When darkness falls, we become visual klutzes. The cones retreat from lead position on the sensual front and become practically useless. At this point, the image of reality is largely formed by the rods, which are sensitive to weak light, but less numerous and more spaced out in the retina. This means that in the dark, we can see things, but poorly and out of focus.
The structure of the vertebrate eye is also partly responsible for this state of affairs. The retina of the human eye consists of many layers––the most important and most light-sensitive layer is the one that contains the “bodies” of the rods and cones. For reasons that are still debated by biologists and embryologists, this layer is located at the base of the retina. It’s a bit like putting the film into a camera backward, with the photosensitive side facing out. It would still be possible to take a photo (the surface of the film is transparent, so it would let a lot of light through), but the photo would be out of focus, hazy. The suboptimal structure of the human eye means that in the dark, extra layers of the retina absorb valuable photons destined for the photoreceptors, which also impairs the eye’s capabilities.
Evolution is rarely idle, so since it made an “error” in the creation of vertebrate eyes, it decided to correct it––at least partially. Reversing the evolution of an organ as precise and intricate as the eye was out of the question––it would be too easy to break something rather than mend it. Therefore, it had to be “redesigned”––and the Costa Rican puma, along with most other felids, is an example of the structural revolution that partially fixed an evolutionary dud.
Luminous Tapestry
Tapetum lucidum. These two Latin words, which translate literally to “luminous tapestry,” represent the evolutionary answer to the ocular needs of cats, dogs, and many other animals. Predators often hunt at night, so they need perfect vision in the dark. The reflective membrane––that is, the tapetum lucidum––is a brilliantly simple solution to the problem of the small amount of light available at night, which requires no revolutions in the structure of the eyeball. Built in a slightly different way in different animals, the reflective membrane is basically a retroreflector––a structure that reflects the light falling on it in the exact opposite direction from its point of origin. In pumas, as in other predatory mammals, this layer is located between the retina and the choroid. It is made of special cells containing transparent guanine crystals arranged in a way that gives the reflective membrane a characteristic, iridescent sheen. Light entering the animal’s eye first passes through the retina and its light-sensitive receptors. What remains of this light falls on a deeper layer equipped with a tapetum lucidum––there, the rays are refracted and reflected in the opposite direction, thus passing through the light-sensitive receptors a second time. Much of this light then escapes from the eye to the outside, and since its rays are focused by both the concave fundus and the lens, that light appears to be extremely bright, even if the animal’s eye originally received relatively diffused light. An additional effect provided by the tapetum is what is known as constructive interference: on their return route from the reflective layer, the reflected light waves meet the next incoming photons, and they join together, amplifying the weak signal carried by the faint light from the animal’s nocturnal environment.
These “super-eyes” sound splendid, so the question arises as to why humans weren’t equipped with this invention. How amazing it would be to have razor-sharp color vision during the day, and night vision provided by a retroreflector lining the fundus by night. But few things in nature are free. The cat’s eye is almost fifty times more sensitive than the human eye to visual stimuli at night, but having a shiny membrane that reflects light also leads to a decrease in visual acuity, especially when light is plentiful. In humans and other mammals that are active mainly during the day, the evolutionary need for sharp color vision prevailed. As a result, the human retina not only contains no reflective membranes, but it is also very dark: the pigments inside absorb as much light as possible so that no unruly photons are reflected inside the eyeball, compromising the precise work of the cones recording light stimuli and enabling vision of the entire color spectrum.
Color Blindness Welcome
Cat eyes, although they outclass humans’ in the night vision department, have one basic limitation that applies to both felines and canines, as well as many other non-primate mammals. Unlike human trichromatic eyes (which see color thanks to a combination of three types of color receptors), feline retinas contain only two types of color-registering cones: long-wave and short-wave. This means that the perception of these animals is limited to dull, grayish mixtures of blue and yellow-green (from the human point of view; of course, we have no idea what colors cats actually see). Most notably, their world is lacking in the warm tones: reds and oranges.
I’ll admit that the realization of the limitations of feline vision was quite a surprise for me. How is it possible that the tiger, unable to perceive the color orange, uses this very color to create the iconic pattern of its fur? In developing their coloration, shouldn’t animals use those colors to which they are able to react? The story of the tiger’s coloring and the shortcomings of the cat’s eyesight bothered me for a long time and only made sense when I started to untangle the complex paths of the evolution of animal coloration. As in the case of the brilliant solution to the problem of the “faulty” inverted retina, nature turned animal color blindness into an asset.
To start with a simple observation, a tiger sneaking through dense, tall grass is clearly not a master of disguise. Despite the oft-repeated theory that its striped fur is supposed to guarantee invisibility in its natural habitat, it is hard to imagine a worse camouflage than bright orange streaks against the lush green of the Asian jungle. But couldn’t this be a case of stumbling into the trap of anthropocentric, self-absorbed thinking? Yes, the tiger is a smudge of fiery orange, especially among lush greenery, but only to eyes like ours, equipped with three types of color photoreceptors. For most mammals, anything in the spectrum of visible light that lies to the right of yellow-green shades is practically non-existent: it merges with the green part of the spectrum, because those animals simply don’t have receptors capable of registering the “otherness” of colors warmer than yellow. For even-toed ungulates––the tiger’s typical prey––its coat is therefore almost the same color as the surrounding plants. Vertical, dark stripes make the camouflage even more effective––in an environment of perpendicular green stalks, the coloring of the tiger turns out to be a masterpiece of concealment.
But wouldn’t it be much easier to just be green? Then the tiger wouldn’t have to count on the visual deficiencies of its prey, and its fur would be universally “invisible,” regardless of the degree of sophistication of its potential dinner’s color vision. The problem is that, apart from plants that use chlorophyll, green is a rather scarce pigment in nature. Few animals can produce it, least of all mammals (the green sloth is merely an interesting oddity here––its fur takes on a green hue thanks to the algae growing on it). Mammals’ fur owes its color to melanin pigments, which can cover a whole range of shades: from black to brown to fiery red. Unfortunately––from the tiger’s point of view––these pigments don’t include green. So, in the case of tigers, evolution did what it does best. It turned a problem––a limited repertoire of skin pigments––into an asset, taking advantage of the color blindness to which many species of mammals are condemned.
Theodosius Dobzhansky once said that “nothing in biology makes sense except in the light of evolution.” Cat’s eyes––wonderfully flawed, perfect only as far as needed––are a textbook example of this. Do we really have the right to call limited color vision in animals “color blindness”? Or let’s reverse the logic: I wonder what specialist term the Cheshire Cat would come up with to describe the weakness of our poor human eyes in the darkness of night. Whoever heard of eyes that can’t cope in complete darkness? What’s more, eyes that don’t even shine in the dark with an eerie, golden-green glow? Outrageous!
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