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Lots of animals have their eyes more on the side of their head, like an octopus or a parrot. Are animals with eyes more on the side able to tell the depth of objects at different distances? It seems like the line of sight in each eye wouldn't be able to cross to a singular point to triangulate distance.

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    $\begingroup$ Good examples of animals with eyes 180 degrees apart are the hammerhead sharks. Here is a good read with very clear illustrations, explanations and quantification across sister taxa. And to address your question, the fields of view of each eye must overlap to produce stereoscopic, binocular vision that allows for (instantaneous) depth-perception. This is called stereopsis. However, many animals perceive depth through head turning - parallax and holding images for comparison - but this is obviously not instantaneous, nor as accurate..! $\endgroup$ – S Pr Feb 22 '19 at 12:31
  • $\begingroup$ Yes. Animals can also calculate the distance to object based on the senses of smell, color, and hearing. $\endgroup$ – Kaka Loko Feb 23 '19 at 20:29
  • $\begingroup$ Nope, depth perception specifically refers to sight. en.wikipedia.org/wiki/Depth_perception but depth sensation would qualify that. $\endgroup$ – Vane Voe Feb 23 '19 at 23:28
  • $\begingroup$ hammerhead vision actually overlaps on the front and back, so in theory they have two different depth perceptions. $\endgroup$ – John Feb 24 '19 at 14:52
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Many such creatures do overlap in part of their field of view. Some don't. Presumably, the former still have some range sense. It's generally accepted that range judgment it's far more valuable to predators while herbivores focus on movement over a wide range. (at least in the sources I've read on this.)

That said:

Focal length can provide some amount of depth information, but not like comparing two angles. ("accommodation " in the wiki article on depth perception)

Parallax can provide some useful depth information. This is a function of basoc geometry. (In the same article, see "motion parallax", "occultation", and "perspective")

Theoretically, if a creature is precisely enough aware of its movement, it could pick up depth by rotating its head. This is because of the previously mentioned geometry. (https://en.wikipedia.org/wiki/Depth_perception#cite_note-5)

Not sure if any animals actually do this, but some birds seem to.


This answer is, fundamentally, just a summary of elementary information. The wiki entry on depth perception chops up the list of factors a lot more fine than I did off the top of my head.

https://en.wikipedia.org/wiki/Depth_perception

https://en.wikipedia.org/wiki/Equine_vision#Visual_field

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  • $\begingroup$ The rotating of the head in order to improve depth perception is pretty common amongst prey animals. I have seen this countless times in lab rats of nervous disposition. When you have them in a box with an open top and are moving around above them, it is very common for the rat to crane its neck so that one eye is looking up at you and then start rolling its head back and forth, presumably to improve its ability to see when you are moving toward and away from it. As you mentioned, birds do the same thing, and do so even when not under stress, whereas I've only seen stressed out rats do it. $\endgroup$ – Jeremiah Feb 20 '19 at 16:46
  • $\begingroup$ True. My uncertainty is whether that's figuring distance. I'm not sure how you'd test that, but it sure seems likely. I'd sure be interested in any clever studies that demonstrated or disproved this, since I'm not aware of any. $\endgroup$ – The Nate Feb 20 '19 at 19:36
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    $\begingroup$ Hmm. That's an interesting question. I suppose I just never thought too much about it. Depth perception improvement just always seemed like the obvious reason for that behavior whenever I've observed it. I just looked more into it. There doesn't appear to be any rigorous study of this behavior. Rats have often been observed doing this same thing before jumping long distances and albino rats with poorer vision roll their heads more often than dark-eyed rats so this seems to lend some credence to the depth perception hypothesis in their case. $\endgroup$ – Jeremiah Feb 21 '19 at 22:00
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    $\begingroup$ Birds seem to be more complicated and have a number of reasons for head rolling, including the inability to move their eyes and the existence of two fovea, which allow switching between binocular and monocular vision. Interesting stuff. $\endgroup$ – Jeremiah Feb 21 '19 at 22:01
  • $\begingroup$ I didn't know that any birds had multiple fovea. That's interesting. I'll have to look deeper into this. Yeah, that rat note really is pretty compelling, albeit not proof. $\endgroup$ – The Nate Feb 22 '19 at 2:59
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Worth noting, even you can perceive depth with one eye closed if you slide your head left and right. It's the same reason we can have 3d gifs that just rely on sliding the camera. Look up stereoscopic 3d lke these:

https://i.gifer.com/FQvL.gif

enter image description here

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    $\begingroup$ Welcome. tanks for your nicely illustrated answer. Can you add some references to your answer, and can you add web links to the images you shared? It allows people to background read on your material. +1 $\endgroup$ – AliceD May 25 at 18:21
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In the paper "Rats Maintain an Overhead Binocular Field at the Expense of Constant Fusion", the authors monitor rats eyes while they are doing a task that definitely required visual depth perception: jumping across a gap. A more accessible write up is here. As @the-nate stated, you don't need overlapping visual fields for depth perception. Another depth cue, which he didn't mention, is size. If i know that an object is 1 meter tall, then the visual angle tells me how far away it is.

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