Spawned by an acquaintance’s series of thoroughly disturbing pictures of himself posing with glass taxidermy eyes. o.O
But really: Most every critter I know almost never shows the whites of its eyes. Cats, dogs, horses, deer, lions, tigers, bears, dolphins, donkeys, giraffes, elephants, leopards, mountain lions, rhinos, okapis, cows… I stop here only because I might repeat myself. But I think I’ve made my point: Our fellow mammals do not have visible sclera. We rather definitely do. What’s the deal?
Indeed, the white of many critters’ eyes are not normally visible. Except . . .
You’ve heard of being “wide-eyed” with fear, excitement, or other stressful stimuli. This seems to be a widespread (across many species) phenomenon: The eyelids open up wide, exposing the sclera, when an animal is emotionally aroused.
Dolphins do this. Dolphin trainers learn to recognize that as part of a dolphin’s facial body language. It often means you’ve got a pissed-off dolphin who might be liable to chomp you if you try to pet it.
Originally Posted by LawMonkey View Post
Most every critter I know almost never shows the whites of its eyes. Cats, dogs, horses, deer, lions, tigers, bears, dolphins, donkeys, giraffes, elephants, leopards, mountain lions, rhinos, okapis, cows…
2 thoughts:
Do the animals mentioned have much range of motion of their eyeballs? I think humans might have a wider range of motion and this may be a reason.
Could visible whites (ie bigger eyes) be a form of neoteny?
I don’t know the answer but it would make sense for porpoises, canines and hominids
to show white sclera because they are group hunters. It is important to know in which direction
a fellow hunter is looking. Visible sclera make the eye a better signalling device.
I think most larger animals can move their eyes pretty well, and certainly many (if not all) monkeys can. (In general, eyes need to be almost constantly on the move in order to work, but small animals can get enough movement by moving their heads or whole bodies, so they do not necessarily need a separate eye movement system.)
I just googled images of monkeys. I got a mixture mainly of photos of monkeys, with the occasional cartoon monkey, and I noticed that although the real monkeys do not have whites to their eyes, the cartoon ones almost all do. I think this adds some weight to the suggestion that the whites play an important social communicative role in humans. (Cartoon monkeys are drawn to look more human than real ones, and to appeal to us emotionally.)
Iti s not just a matter of signalling emotions, either. It is well established that noting (and often following) another’s direction of gaze plays an important role in human social interaction, and the white often provides a more easily seen cue to someone’s gaze direction than does the pupil itself. It is certainly a much more salient cue than one would get from a dark pupil set in a completely dark surround, like most animals have.
Not only that, but according to a Nat Geo documentary on dogs I saw last year, one of the evolutionary differences between dogs and wolves is that dogs have learned to read human intent and reactions by looking at our eyes, which wolves do not do.
The experiment they described to illustrate this was done to compare the behavior of wolves raised in captivity and fully habituated to humans from birth with that of dogs. They had people work with the two sets of animals like so: first, they were given food treats by hand, to understand that This Person Gives Goodies. Then, they were shown this person (who they now expected to have treats for them) in a room with two boxes turned over, one of which had the treat underneath. The trainer then pointed to the box that had the treat under it. The dogs followed this cue far more than the wolves, who basically ignored it (i.e., it was 50/50 as to whether they went to the box that was being pointed at or not).
Then they repeated this experiment not with pointing, but where the trainer simply LOOKED at the box with the treat. The dogs followed this cue just as well as the pointing! (The wolves also did not.)
This documentary about dogs is where I first heard of this “humans have sclera to give background context to our eyes so we can silently see and follow where other humans are looking” - as an intro to going on to say, “…and dogs are a species of wolf that has domesticated so closely with us that they pick up this too”.
One step further, it appears that humans have the rare characteristic of non-round eye openings. Most animals have eye openings that approximate the shape of the round iris, so the white part of the eye remains hidden.
So, I have just changed the question to “why do humans have almond-shaped eye, instead of round?”
Huh? I would be more impressed if the wolves followed the pointing but ignored the human when he just looked at the box. It just seems to indicate that wolves are much less likely than dogs to pay attention to any human cues.
Yes, a good question is are our eyes bigger, or animals’ irises bigger? How about pupil/iris size?
Maybe animals have bigger pupils, and hence bigger irises because they need to see better at night?
Maybe animals have bigger irises for a wider range of pupil sizes?
Maybe humans just have bigger eyes due to neoteny?
Comically enough, this question was the reason that I joined the SDMB in the first place. And I’m really not sure.
The most likely answer I’ve found is that humans use it for non-verbal communication, whereas other species don’t because they don’t want predatory species tracking their eye movement. I’ve had my doubts about this answer since, as it’s been mentioned, animals are notoriously bad at eye tracking anyway.
So… my theory is that it’s the reason we’re sentient. Yup. Makes no sense, but whatever.
Slight hijack but…
Does anyone know a good list of anatomical features either outright unique to Homo sapiens, or rare outside of the homo genus?
Whenever I google this I get either pages that focus on our similarities to other animals, or focus on our cognitive abilities (rather than anatomy).