Catseyes

June 14, 2006 11:07 AM

How quickly do other animals react to changes in light, and why are we so slow by comparison?

If our cat is running about like a nutcase, and I turn off the lights, she can aparently still see where she's going. She doesn't run into anything or stop dead, yet I can't see a damned thing for a few minutes, and it's about 15 minutes until I can see really clearly.

Why is it that cats can switch so quickly from light sight to night sight? It's obviously not just a case of pupils dilating. And why have we aparently evolved to be so completely crap in the dark? What other animals are as good as cats or as crap as humans?

posted by twine42 to pets & animals (12 comments total)

I think its much more detailed than pupils dialating. Our night vision only uses our rods, no cones and I think it takes some time for the chemicals to purge. I hesitate to give any more details because I was just reading about this the other day but not really paying attention.

As far as why we evolved to be so completely crap in the dark, it might be because the ones of us that weren't so crap in the dark were out at night where there were cats who could run around like a nutcase after us.
posted by Brainy at 11:12 AM on June 14, 2006

Cats in the wild (of all sizes right up to the biggest ones) can operate in daylight but prefer to hunt at night, and are adapted to that. Primates are diurnal; we're not evolutionarily adapted to operating in the dark.
posted by Steven C. Den Beste at 11:35 AM on June 14, 2006

There is generally a trade-off between how accurately you can see (i.e. how small a detail you can make out at a given distance) and how well you can see in the dark (i.e. how sensitive you are to individual photons). Cats are optimised more for less detail/better night vision. We're optimised for better detail/worse night vision.

An example: In cats, there is a reflective layer ('tapetum') behind their retina (that's why their eyes flash when they catch the light). This catches light that has passed through the retina, and reflects it back onto the retina for a second chance. [The retina is where the light is detected.] This helps cats make the most of every bit of available light, improving their night vision. However, the light doesn't follow exactly the same path back, so it makes cat vision a little blurry. We don't have this reflective layer, so we are less good in the dark, but also less blurry.

Many nocturnal animals have tapeta like the cat. Diurnal animals general don't.

Another example of the accuracy/sensitivity tradeoff (though I'm not sure exactly how it affects the cat/human difference): Rods are big photoreceptors, used for night vision. Being big makes them sensitive (bigger area to collect light over). Cones are small photoreceptors, used for daylight and colour vision. Being small makes them accurate, but less useful at night. We (and cats) have a mechanism that switches our vision between rods and cones depending on light level, i.e. rods for the dark, cones for the light.
posted by beniamino at 11:44 AM on June 14, 2006 [1 favorite]

There's lots of info about how the eyes of various animals operate differently than ours in the book Animals in Translation. Kid's eyes adjust more quickly to light changes than adults, too, I think, but that may be about the flexibility of lenses.
posted by tula at 11:46 AM on June 14, 2006

Consider also that your cat has a better sense of spatial orientation of itself and its place in the house and where all the furniture is. If I flicked all the lights off on you, I bet you could still find the couch, the table and the lightswitch. The cat is significantly better than you at this, and combined with better night vision as mentioned above, it contributes to this sense of "Instant Nightvision Cat Ninja."
posted by frogan at 12:26 PM on June 14, 2006

15 minutes to adapt to darkness sounds extreme. You might read up on nightblindness, and eat more carrots.

As for the cat in addition to the things mentioned by the other posters, it has whiskers, and better hearing.
posted by Good Brain at 2:04 PM on June 14, 2006

I was disconcerted looking at Lion's pictures a few years ago, to notice their pupils are as round as mine or y-- er, mine; and a few minutes with Google has convinced me the same is true of Tigers as well, though I wish I could have gotten a better look. I tried to make a determination with Hyenas, too, because they have such a reputation as nocturnal hunters, but I could not tell.
posted by jamjam at 3:13 PM on June 14, 2006

15 minutes to adapt to darkness sounds extreme. You might read up on nightblindness, and eat more carrots.

Not sure if this was in jest or not, but eating carrots actually has no effect on eyesight! See snopes here.
posted by ranglin at 4:01 PM on June 14, 2006

Good Brain makes an important point. The fact that a cat seems able to move around without waiting for its eyes to adapt is in part due to its whiskers. Basically, it has a much better ability than we do to maneuver even if it can't see at all. That's another adaptation of the nocturnal predator.
posted by Steven C. Den Beste at 4:15 PM on June 14, 2006

Good Brain writes "15 minutes to adapt to darkness sounds extreme."

As many amateur astronomers will tell you, it takes about twenty minutes to fully dark-adapt. But I suspect it's an asymptotic approach to full adaptation, so you can see pretty well rather earlier.

tula writes "Kid's eyes adjust more quickly to light changes than adults, too"

Any links on this (is it in the book you linked)? I'm curious. My son's too young for me to ask him about his vision but I have definitely noticed that my own eyes adjust more slowly than they used to in one respect: very bright sights leave after-images in my field of vision that last much longer than they once did.
posted by Songdog at 5:44 AM on June 15, 2006

Read the whole snopes entry that you linked to ranglin, particularly the part about Vitamin A deficiency and night blindness.
posted by Good Brain at 11:30 AM on June 16, 2006

Coming in late with this, but I thought I'd mention that horses are extremely ill-equipped to handle sudden changes of light, despite the fact that their night vision is far better than humans'. It takes some 20 minutes for them to fully adjust to a quick, extreme change of light (i.e., flipping on the lightswitch in the barn in the middle of the night). But again, because of their other senses, they tend not to have too many problems with it. Just don't ask them to gallop about and jump in bright daylight immediately following being in a dark barn.

(Information from EQUUS Magazine - I'll source it completely on request, but can't be bothered to look through my massive collection of back-issues at the moment.)

Not all animals are better than humans at this.
posted by po at 12:12 PM on June 20, 2006