Case #1.) Close your eyes and fold your hands in front of you on your desks. Now, listen attentively as I walk around the classroom, lightly clapping my hands.
Not much trouble telling where I am, right? Your two ears working together tell you when I’m next to the door to your left or by the windows in back. Now I’ll just pop off my shoes and tiptoe quietly about the room in my socks for a bit. …
Could you tell what path I took around the room?
If you were a bluegill sunfish and this room your pond, you’d have known, even in complete darkness. You’d have felt my presence with your skin as I passed by.
Case #2.) Ever watch a movie on TV with your dog? Yes?
If you have an older television, the answer is no — at least not in the way you may think. You might have been enjoying “Marley and Me” with Owen Wilson and Jennifer Aniston, but your best friend was only seeing an irritating sequence of thousands of flickering still images.
Without giving it much thought, we generally assume animals perceive the world a lot like we do. Maybe dog noses pick up more odors and cat eyes see better at night than their human counterparts, but for the most part, that’s just a matter of scale, isn’t it? We have noses, eyes, taste buds and ears, and so do they.
Yes and no. Cocoa and l both have eyes, but the way they work differs enough that we have vastly divergent perceptions of reality. In several ways my eyes are a good deal better than his; by some criteria, it’s the other way around.
Human eyes have three types of cones (color-detecting cells) in their retinas for picking up red, green and blue. Dogs’ eyes are missing the red-detecting cones. Although it’s sometimes said their vision is similar to that of a person with red-green colorblindness, that doesn’t mean dogs are colorblind. Their natural color vision simply means they can only see the world in blue and yellow hues. (It’s still common to hear the quite false statement that dogs only see shades of gray.)
Dogs also are nearsighted. Several years ago, a group of researchers at Linköping University in Sweden developed a method for comparing human and canine vision, reporting that in both bright and dim lighting, human visual acuity (sharpness of vision) is about three times that of a dog. If Cocoa’s dog license allowed him to drive, he’d need glasses thicker than mine.
On the other hand, dogs, cats, horses, deer and many other mammals have many more rods in their retinas than we do. Although rods only detect shades of gray, unlike cones they can function in dim lighting (which is why colors seem to disappear in the dark). They commonly also have a mirror-like tissue behind their retinas called a tapetum lucidum.
Giving a new meaning to the phrase “double vision,” some of the light entering the eye passes through the retina, bounces off the tapetum lucidum and back through the retina again, giving the rods a second shot at detecting a dimly lit image. That’s why Cocoa’s eyes shine like yellow light bulbs when caught in the flash of a camera.
And the flickering images on the TV screen? Dogs take in visual information at least 25-percent faster than humans. The pictures being broadcast on the screen are coming at us fast enough that we perceive them as moving — moving pictures … movies. Not so for dogs; in at least this, their daytime vision is sharper than ours. Newer high-resolution digital screens, however, refresh much more quickly, fast enough for even our pets’ eyes to be fooled into seeing continuous motion too.
There’s all sorts of curious ways animals’ perception of the world differ from ours. Bears can smell carrion 20 miles away, elephants smell water up to six miles away. Sharks detect electric fields generated by their prey through gel-filled pores on their snouts; so does the duck-billed platypus. Many birds and insects make use of the earth’s magnetic field in navigating and see ultraviolet-lit patterns on flowers we totally miss. Rattlesnakes can detect a mouse on a moonless night from a meter away just by the heat it gives off.
But imagine how life would differ if, without looking, you could literally feel the presence of structures and people yards away from you. Most fish have a series of pores running along their sides and extending onto their heads that make up its lateral line system. Each pore contains an array of tiny “hair cells.” Changes in water pressure around the fish — say when passing some structure or another animal — cause the hair cells to trigger nerve impulses which inform the animal about its surroundings.
Experimentally blinded tropical fish have no problem schooling, but not if the nerves connecting the lateral line are also severed.
Ken Baker is a scientist and a retired biology professor. If you have a natural history topic you’d like the author to consider for an upcoming column, email your idea to firstname.lastname@example.org.