A couple of days ago, I ran into a friend I hadn’t seen since 1995 (someone I went to high school with). Talking about this and that, somehow, the question of goat’s eyes came up, and she wondered why a goat’s pupils are that rectangular-squarish shape. I had to admit that I’d never wondered about it previously, but I suddenly became interested in the question as well. After some searching online went nowhere quickly, I decided to turn to the Teeming Millions. Does anybody know? Is there something, evolutionarily, that led to this shape? This enquiring mind wants to know!
Damn…there is a web site on everything
http://www.goatworld.com/articles/goatslife/goatfacts.shtml
While this doesn’t explain the shape…it does offer a reason why.
I don’t know – I’ve wondered about this. I have suggested several times on this Board that the vertical slit pupils of cats and snakes are due to the tradeoff in Modulation Transfer Function (MTF) – In bright light, these creatures maintain excellent horizontal resolution (at the expense of vertical resolution) and can see fine horizontal details (like, maybe, mouse tails). Certain sharks have annular pupils, and retain excellent high-frequency resultion in all directions. But the rectangular pupils of goats, longer horizontally than vertically, baffle me. If there’s an MYF explanation for them, I can’t see it.
Hmmm…my husband used to have a goat. I’ll have to ask him when we get home tonight. I never knew a living thing could have rectangular pupils. I need pictures. This is very weird indeed.
Here’s a picture of a goat pupil. It’s the second one down.
Hmmm, the octopus also has a rectangular pupil but I haven’t found what it does. Maybe the speculation that it helps night vision is true…aren’t octopuses (or octopi, if you prefer) nocturnal?
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*Originally posted by Sue Duhnym *
Hmmm, the octopus also has a rectangular pupil but I haven’t found what it does.
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Duh. It says right there, "The rectangular shape pupil is always horizontal and enable the Octopus to make the difference between horizontal and vertical lines. "
Not the best photo, as the pupil is quite dilated.
That’s maybe the weirdest simulpost I’ve been involved with.
Just a WAG: Goats are prey animals, as are horses, whose pupils also contract to horizontal bars. It’s possible that there’s some natural advantage in seeing a predator coming with a “wide angle” view. (I don’t know how other artiodactyls and perissodactyls contract).
Similarly, a tiger, whose eyes contract to small circles, and house cats with their vertical slits are predators, and it’s an advantage when you’re hunting down your meal.
Here’s some info. (but not the whole story):
http://www.schools.net.au/edu/lesson_ideas/optics/optics_wksht1_p1.html
So … why a slit pupil vs. round? Goats (or their ancestors) must be somewhat nocturnal. Why horizontal vs. vertical? Still no friggin’ clue.
The darker the environment the wider an animals pupils must open to let in enough light. But a bigger pupil means a blurrier view. (In bright daylight I can see and read quite well without my glasses - my constricted pupils act just like pin-hole camera lenses and everything appears in focus. As things get darker my/our human vision gets progressively less sharp - in all directions.)
My guess is that cats being nocturnal hunters, who would most often be hunting small creatures on flat surfaces - and would need to be able to see motion moving across their field of vision (left to right or right to left) and judge the accurate location of animals moving that way rather than going up or down. So when the light starts getting dim a cat’s eye could sacrifice seeing clearly along the vertical axis and still work pretty well as a tool for catching mice in dim light.
So the first thing that happens when a cat’s pupil starts to open is that the vertical slit opens up higher and lower - and the cat’s ability to sort out layers, for example, becomes more and more blurred as this happens. But its ability to see mouse motion moving across its field of view would be the last thing to get blurry. (If a cat would really need to see the layers in front of it - it could simply tilt its head a bit to shift its axis of clearest vision - and cats will do this sometime when they are looking intently at something.)
Goats aren’t night hunters - but they are rock climbers; and for them the ability to see small ledges on cliffs is probably pretty important. So for the same reason as the cat - when the light starts to fade a goat would find loss of clear vision along horizontal lines less of a problem than the loss of the ability to judge accurately the vertical distance between ledges and the size and depth of ledges they are looking at as gauged by how thin or thick the darker lines appear before them.
Another way to think of it is that as the light got dim a cat could still ‘read’ a standard UPC bar code - unless it tilted its head to the side (which would make the code appear blurry.) Whereas a goat in dim light would lose its ability to read that same code - unless it tilted its head way over to one side or another (so the code appeared as a bunch of small black and white lines stacked one on top of another.)
Probably because their predators are not going to come from above, better to have a wider view in the horizontal rather than the vertical. From here: http://www.environmentalgraffiti.com/news-eyes-have-it-coolest-eyes-animal-kingdom?image=6#QA67ymKRYjGgd2rM.99
I’ve read that one advantage of a vertical slit pupil like a cat is that it has the ability to further alter the effective size/shape of its pupil by half lidding its eyes; half lidded eyes block off the top and bottom of the slit pupil.
As long as this old thread is new again, it wouldn’t be complete without a mention of the weird eye of the dolphin.
Eyes in the air do most of their focusing with the cornea (due to the high refractive index between the cornea and the air), with the lens providing only the fine-tuning to adjust for distance. But under water, the cornea is nearly useless, because of the similar refractive index between the cornea and the water. For animals evolved to be in the water, the lens takes the major role in focusing.
Dolphin eyes are adapted to work well both in the air and under water. The pupil is weirdly shaped, commonly described as either kidney-bean shaped or even horse-shoe shaped. Picture a kidney-bean shaped swimming pool. That’s roughly what a dolphin’s pupil looks like, in dim (mainly underwater or night) light. In brighter light (typically in air), the central narrow part of the pupil contracts so the pupil is more horseshoe shaped. In really bright light, this continues until the pupil is little more than two very narrow slits, with the central narrow part completely closed by the surrounding iris.
This is claimed to have desirable optical properties that allow the eye to focus well in bright (in-air) light, even though the eye is otherwise best adapted for underwater use.
Brief summary article:
Here’s a page from google books that shows a close-up photo of a dolphin’s eye, with contracted pupil as it would be in bright light.
Don’t toads also have rectangular horizontal pupils?
I know the HypnoToad on The Simpsons does.
HypnoToad never appeared on The Simpsons. You’re thinking of Futurama.
They make it easier for zombie goats to find braiinnss.
But … but … what about that episode with the animal show?
Darn it, I’m getting senile and confusing my Matt Groening properties.
OK, so why are the pupils of a mongoose horizontal like those of a goat?
