# Really mundane observation about cat's eyes **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [December 4, 2009, 4:57pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000 "2009-12-04T16:57:14Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![KlondikeGeoff](https://avatars.discourse-cdn.com/v4/letter/k/3e96dc/32.png) [@KlondikeGeoff](https://boards.straightdope.com/u/KlondikeGeoff)\ **Post date:** [December 4, 2009, 4:57pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/1 "2009-12-04T16:57:14Z")
So, I was brushing one of my cats. There is nothing he loves more than to have his tummy brushed…he sort of zones out. Anyway, I was doing this, and when I was ready to put him down to brush the rest of him, he looked up at me. Only the very bottom tips of his elliptical pupils could be seen, but he stared at me adoringly. So, I am assuming, mundanely, that a cat can see out any tiny portion of its pupil, yes? Finally, why in hell do small cats have elliptical shaped pupils anyway, while larger cats have round ones? ---
**Author:** ![Alice\_The\_Goon](https://avatars.discourse-cdn.com/v4/letter/a/ed8c4c/32.png) [@Alice\_The\_Goon](https://boards.straightdope.com/u/Alice_The_Goon)\ **Post date:** [December 4, 2009, 4:59pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/2 "2009-12-04T16:59:01Z")
He wasn’t staring at you adoringly, he was thinking of tuna. The question itself might be better posed in GQ. ---
**Author:** ![MLS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MLS](https://boards.straightdope.com/u/MLS)\ **Post date:** [December 4, 2009, 6:06pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/3 "2009-12-04T18:06:22Z")
From [Wikipedia:](http://en.wikipedia.org/wiki/Cat) > [@](#): > > Another adaptation to dim light is the large pupils of cats’ eyes. Unlike some big cats, such as tigers, domestic cats have slit pupils.[50] These slit pupils can focus bright light without chromatic aberration, and are needed since the domestic cat’s pupils are much larger, relative to their eyes, than the pupils of the big cats.[50] Indeed, at low light levels a cat’s pupils will expand to cover most of the exposed surface of its eyes. Also from [here:](http://www.agarman.dial.pipex.com/bco/ver4.htm) > [@](#): > > In low light levels the cats pupil must be able to open as wide as possible, but also be able to contract to very small size to protect the sensitive retina in bright sunlight. In human eyes, this size variation of the pupil is controlled by a circular ciliary muscle, but this limits the amount of size variation. In cats however, the same process is controlled by two, shutter-like ciliary muscles, which gives the cat it’s characteristic slit-like pupil in bright light conditions. All cats pupils are therefore elliptical, however some, notable the ‘Big Cats’, appear more circular when dilated ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [December 4, 2009, 7:06pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/4 "2009-12-04T19:06:35Z")
Cats could have perfectly round irises like most other animals – there’s no fundamental reason for the mechanism to be different – round pupils would work just as well. I’ve long felt that the reason has to do with the optics of the situation. If your aperture is defined by a circular region, then shutting down that aperture to smaller sizes restricts the amount of light coming in at the cost of losing some spatial resolution along all axes equally. This is how our eyes work. If, instead, you close to a slit, you can restrict the amount of light entering as before, but you retain the same spatial resolution along the vertical axis, at the expense of losing more along the horizontal axis. _That’s_ a significant difference. So when the light gets bright, we lose resolution along both axes. A horizontal fine line, like, say., the tail of a mouse, gets lost the same as vertical fine lines, like blades of grass. It doesn’t matter so much to us – we’re above the grass looking down at the mouse. But to a cat, the mouse tail is still resolved, still seen against the background. The cat can’t resolve the individual blades of grass so well, but that doesn’t matter – the interesting, edible stuff is horizontal, and that’s still resolved, without the confusing vertical stuff you don’t need anyway/. And the cat _is_ looking through the grass, not down. I note that snakes have vertical slit pupils, too. They have the same advantage of being able to see fine horizontal lines, too. i say 'mouse tails" here to make it clear why this might be an advantage to cats and snakes, but it’ll be true for any mostly horizontal features – bird tails, insect bodies – that stand out from the general run of vertical “clutter”. I note, as the OP does, that large cats don’t have slit pupils. Look closely at a lion or tiger and you’ll see this is true. And I note that lion prey doesn’t necessarily have a lot of horizontal lines to it. I strongly suspect that T. Rex didn’t have slit pupils, as he is often represented. I suspect they had round pupils, as shown in _Jurassic Park_. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [December 4, 2009, 7:07pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/5 "2009-12-04T19:07:49Z")
Oh bag it, let’s just move this into GQ territory. Why are goat [pupils](http://www.google.com/imgres?imgurl=http://biology.clc.uc.edu/graphics/taxonomy/animals/mammalia/Domestic%2520Goat/JSC%2520970505%2520Goat%2520Eye%25202.jpg&imgrefurl=http://biology.clc.uc.edu/graphics/taxonomy/animals/mammalia/Domestic%2520Goat/&h=300&w=497&sz=28&tbnid=O1L3CYJimqTVnM:&tbnh=78&tbnw=130&prev=/images%3Fq%3Dgoat%2Beye&hl=en&usg=__9jn1f-fWlsGREUhHYV_MzUmcVNw=&ei=fV0ZS5_BIJC6swPt-_X3Bw&sa=X&oi=image_result&resnum=1&ct=image&ved=0CAkQ9QEwAA) shaped that way? (something about helping a goatse in the dark, I know…) I’ve read that it’s because they’re nocturnal, but so are deer and they have normal pupils. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [December 4, 2009, 7:11pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/6 "2009-12-04T19:11:51Z")
> [@](#): > > Why are goat pupils shaped that way? (something about helping a goatse in the dark, I know…) I’ve read that it’s because they’re nocturnal, but so are deer and they have normal pupils. I’ve thought long about goat pupils, and I can’t figure it. They’re rectangular with the long axis horizontal, so they’d have slightly better resolution of vertical features, but not enough to make a difference, I’d think. the “MTF Theory of Animal Pupils” I give above has ramifications for shark, skate, and gecko pupils\* that i can point out, but I still can’t figure it for goats. I haven’t seen this anywhere else, and i ought to publish it. \*Don’t go by GEICO TV ads – gecko pupils are way weird, and don’t look like the big Keane eyes that CGI lizard has. ---
**Author:** ![stpauler](https://avatars.discourse-cdn.com/v4/letter/s/9d8465/32.png) [@stpauler](https://boards.straightdope.com/u/stpauler)\ **Post date:** [December 4, 2009, 7:12pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/7 "2009-12-04T19:12:57Z")
> [@Inigo\_Montoya](#): > > (something about helping a **goatse** in the dark, I know…) :eek: What I want to know is why the cat’s pupils dilate at night even when the lights are on and just as bright as during the day. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [December 4, 2009, 7:19pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/8 "2009-12-04T19:19:39Z")
> [@stpauler](#): > > :eek: > > What I want to know is why the cat’s pupils dilate at night even when the lights are on and just as bright as during the day. Like any other critter, they dilate in response to arousal as well as in response to light stumulus. Piss off (or on) a cat and you’ll see what I mean. ---
**Author:** ![KlondikeGeoff](https://avatars.discourse-cdn.com/v4/letter/k/3e96dc/32.png) [@KlondikeGeoff](https://boards.straightdope.com/u/KlondikeGeoff)\ **Post date:** [December 4, 2009, 11:28pm UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/9 "2009-12-04T23:28:11Z")
> [@Inigo\_Montoya](#): > > Like any other critter, they dilate in response to arousal as well as in response to light stumulus. Piss off (or on) a cat and you’ll see what I mean. 😃 This is getting much more interesting than I expected with my idle OP musing. Goats, geckos, T-Rex? What about hamsters? ---
**Author:** ![E-Sabbath](https://avatars.discourse-cdn.com/v4/letter/e/e19adc/32.png) [@E-Sabbath](https://boards.straightdope.com/u/E-Sabbath)\ **Post date:** [December 5, 2009, 12:54am UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/10 "2009-12-05T00:54:56Z")
> [@Inigo\_Montoya](#): > > (something about helping a goatse in the dark, I know…) . Outside of a goatse, a book is man’s best friend. Inside a goatse, it’s too dark to read. Hamsters are, of course, optimized for dancing. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [December 5, 2009, 1:28am UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/11 "2009-12-05T01:28:43Z")
> [@CalMeacham](#): > > Cats could have perfectly round irises like most other animals – there’s no fundamental reason for the mechanism to be different – round pupils would work just as well. My memory is a bit vague, but I recall reading some years ago that an advantage of the slit pupil is that it gives them two methods of controlling light input; by shrinking the pupil horizontally, and narrowing the eyelid vertically. By opening both up all the way they let in more light than otherwise; by narrowing both they cut off light more than a shrinking pupil of the same basic size could. The vertical length of the pupil means the eyelid doesn’t need quite as fine a level of control to help with light input, and the addition of the eyelid to help means that the pupil can be wider than one that would be expected to shrink from a circle to a pinhole; it only needs to shrink along one axis. ---
**Author:** ![Terraplane](https://avatars.discourse-cdn.com/v4/letter/t/74df32/32.png) [@Terraplane](https://boards.straightdope.com/u/Terraplane)\ **Post date:** [December 5, 2009, 3:02am UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/12 "2009-12-05T03:02:21Z")
> [@stpauler](#): > > :eek: > > What I want to know is why the cat’s pupils dilate at night even when the lights are on and just as bright as during the day. The lights in your house at night aren’t anywhere near as bright as daylight. Your eyes adjust to the dimmer light so it seems brighter to you than it really is. ---
**Author:** ![Cat\_Whisperer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cat_whisperer/32/4839_2.png) [@Cat\_Whisperer](https://boards.straightdope.com/u/Cat_Whisperer)\ **Post date:** [December 5, 2009, 4:04am UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/13 "2009-12-05T04:04:33Z")
> [@Inigo\_Montoya](#): > > Oh bag it, let’s just move this into GQ territory. > > Why are goat [pupils](http://www.google.com/imgres?imgurl=http://biology.clc.uc.edu/graphics/taxonomy/animals/mammalia/Domestic%2520Goat/JSC%2520970505%2520Goat%2520Eye%25202.jpg&imgrefurl=http://biology.clc.uc.edu/graphics/taxonomy/animals/mammalia/Domestic%2520Goat/&h=300&w=497&sz=28&tbnid=O1L3CYJimqTVnM:&tbnh=78&tbnw=130&prev=/images%3Fq%3Dgoat%2Beye&hl=en&usg=__9jn1f-fWlsGREUhHYV_MzUmcVNw=&ei=fV0ZS5_BIJC6swPt-_X3Bw&sa=X&oi=image_result&resnum=1&ct=image&ved=0CAkQ9QEwAA) shaped that way? (something about helping a goatse in the dark, I know…) I’ve read that it’s because they’re nocturnal, but so are deer and they have normal pupils. Because they’re massive freaks of nature. Duh. 🙂 ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [December 7, 2009, 2:47am UTC](https://boards.straightdope.com/t/really-mundane-observation-about-cats-eyes/520000/14 "2009-12-07T02:47:54Z")
> [@](#): > > My memory is a bit vague, but I recall reading some years ago that an advantage of the slit pupil is that it gives them two methods of controlling light input; by shrinking the pupil horizontally, and narrowing the eyelid vertically. By opening both up all the way they let in more light than otherwise; by narrowing both they cut off light more than a shrinking pupil of the same basic size could. The vertical length of the pupil means the eyelid doesn’t need quite as fine a level of control to help with light input, and the addition of the eyelid to help means that the pupil can be wider than one that would be expected to shrink from a circle to a pinhole; it only needs to shrink along one axis. I don’t see the point – the cat could lose all that light simply by contracting radially, as most animals do. The point of a slitr pupil is that it retains the one long dimension, and if there is an advatange it must be because that long dimension is useful\*. Besides, this explanation won’t work to explain why many snakes have vertical slit pupils – snakes got no eyelids. \*I understand that not all evolutionary developments happen for “sound engineering reasons”, and that many changes happen because minor genetic changes only allow certain changes, or because some features are linked to other things controlled by that gene, or for a host of other reasons. I’ve read Stephen Jay Gould, too. But the fact that most animal with eyes of this sort have round pupils suggests that that is the standard model, and that develiopments along other paths strongly suggests some sort of evolutionary advantage. To an optics guy, like me, the primary effect of changing the shape of the aperture stop is to change the Modulation Transfer Function (MTF) of the system, so this seems an obvious reason for the change.