by Bob Church
I'm away from home, but Elizabeth clipped this for me to answer, with the words, "Isn't it your goal to bore people to death. Just do it so I can have some relief." Bore people? Me? Hummph.
How do ferrets' eyes work? Do they have pupils and iris like us (I understand ours to some degree) that I just can't see? Do they move independently of the head or does the ferret move it's whole head? How do they focus? My brother and I have been wondering for awhile now, and we haven't been able to figure it out through observation.
The eyes do look like little black beads, don't they?
They are like all mammalian eyes in that they have a retina, lens, iris, etc. Their pupil is round (like ours) when fully opened, and slits (like cats) when closed down in bright light, but unlike cats, the slits are horizontal, not vertical. As far as I know, all mustelid eyes are fully muscled and they can turn their eyes independently of their head much like we do, albeit with less range (Besides, most people will turn their heads to look at something instead of cranking their eyes around. That is, unless you're scoping out the fox/hunk next to you). There are some significant differences, however, in the overall shape of the eye, the ability of the eye to focus, and of course, in the structure of the retina. Because ferrets are predators, the eyes mostly point forward (right down the nose), but because they are also prey from larger predators, the eyes bulge somewhat (for a wider view, like a wide-angle lens) and are somewhat centred to the side and top of the head when compared to large predators or humans (again, for a wider view). So the ferret basically looks forward to see stuff, but they have a wider view of their surroundings than we do. Cool, eh?
The vast majority of ferret's eyes are either red (in albinos) or black. There are some ferrets with various other colours, but most owners just hear about them, or only notice them when the light is right. The red eyes are red because the eye completely lacks pigments, and the colour comes from the blood circulating in the vessels of the retina. Most other ferrets have eyes so dark you can't see the pupil unless you look from the side while shining a penlight into their eye. Then you can see the pupil without a problem. Ferret eyes also have the white part (sclera), but it is behind the eyelids and is not normally seen. If you lift the eyelid (be very careful) you can see some of it in the corners of the eye.
The retina is different from human retinas in that it has very few cones (cones see colours) and a high density of rods (rods see tones of grey). Also, they have a reflecting layer behind the retina that bounces light forward, which is why ferret eyes seem to glow in the dark (biologists looking for black-footed ferrets wait until it is dark, then shine lights and look for green spots. Yep, BBFs....well when they can find them).
Finally, the lens is structured so that the ferret can see very well up close, but like the stereotypic bookworm, can't see clearly across the room. That is not to say it can't see anything; ferrets are actually better than people in noticing movement or shadow, they just can't read the fine print until they get closer to see if the box says raisins or prunes, that's all. But they don't need to, because they are specially adapted to hunt animals that live in burrows, which are dark places without much visual distance. So their eyes are perfectly suited for their job; that is, hunting rodents in dark burrows.
And that is why the eyes are so dark. Basic light physics says light coloured things reflect light and dark coloured things absorb light, which I learned at about three when I would walk barefoot across parking lots. I would race across the asphalt yelling bloody murder, then stand on the white painted lines until the fire went out. I crossed parking lots from line to painted line. The reason the asphalt was so hot compared to the white stripe is because dark things absorb light. Same with eyes. You want to see well in the dark, you need dark eyes. Now, cats get away with light eyes because their eyes are so big and their pupil is constructed in such a way to allow it to open quite large compared to round pupils, but ferrets run around underground, and thus have tiny eyes (helps keep the dirt out, helps keep facial injuries to a minimum from fighting rats, and don't have much use for them in the dark anyway). So the eyes have to be dark to do their job. They only see in tones of grey because rods ar far more sensitive to light than cones (they can actually see a little red). The reflecting part of the retina bounces light forward so the same light photon can stimulate the same rod twice, giving it a double stimuli and increasing its ability to see in the dark tremendously. And they see best up close because they don't have to see the rat from across the street--just down the tunnel a few feet.
Now, while the ferret's eye is basically the same as ours, they are not nearly so dependent on vision as we are. Instead, they use their nose to find their way from place to place. Foster, my 12-year-old, has recently lost 90% or more of his sight. I'm not sure of what he can see any more, but it doesn't matter much because he gets along just fine using his nose. I have noticed he never walks in a straight line any more, but sort of zigzags, using his nose as a homing device. He startles if you grab him, but I just call his name and let him sniff my hand first and he is fine. Even blind, he can still beat Carbone to the raisins.