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Hey , all you folks who are interested in animal skulls...  I was looking at my skull collection and noticed: Every  prey animal I have: deer, antelope, sheep, goat, horse, camel  (but not rodents or marsupials) have enclosed eye sockets, including primates. Every carnivore skull I have, that I looked at  ( can't find the badger and skunk) has open eye sockets, except the meerkat and the ferret. The only other mongoose I have ( a binterong) and the other "weasels" I have ( related to ferrets) have open eye sockets too.

So I wonder why the difference? What about their lifestyle would require closed eye sockets?
Maybe because meerkats are prey animals as well as predators, they have some of the same characteristics as other prey animals??
"Open eye sockets" means that the place the eyeball sits in the skull is not completely surrounded by bone.
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:iconnotched-stag:
notched-stag Featured By Owner May 16, 2013  Student Artisan Crafter
Yeah, as far as I know it comes with needing to move the eyes differently-- birds have an unnattached ... I think it's called an 'occipital ring' that has to do with how they see, and what their eyes need to do. I rarely see my cat move her eyes- if she wants to look, she tilts her head accordingly, whereas I've seen wild birds look me up and down with their little eyeball. We have totally enclosed eyes and we as humans have pretty flexible eyes, so I imagine it's something like that.
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:iconstarhorse:
Starhorse Featured By Owner May 12, 2013  Professional General Artist
I'm glad I saved this journal to come back and read the comments :>
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:iconreptangle:
Reptangle Featured By Owner May 12, 2013  Professional Traditional Artist
Yes, I learned some ideas about what I wanted to know!
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:iconcozmicdreamer:
CozmicDreamer Featured By Owner May 6, 2013
*scratches head*.....Hmmm that is interesting.
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:iconkreepingspawn:
KreepingSpawn Featured By Owner May 4, 2013  Professional Digital Artist
No idea... better protection for the eyes, since prey species often spar via headbutting? ;p
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:iconbuckskinmare:
buckskinmare Featured By Owner May 4, 2013  Hobbyist Photographer
Having pulled apart enough dead animals in my career as a wildlife biologist, I can tell you the similarity between all those, and a postulation as to why the ungulates have closed eye sockets.

I don't know if you've noticed, but all those grazing herbivores rotate their eyes when they put their heads down to graze. The eyeballs literally rotate in the sockets so that the eye is level with the ground. Watch a sheep or horse grazing and you will see this phenomena. To do such a thing, there needs to be a bone support, otherwise the eye will pop out of the socket.

Carnivores by contrast, while they can move their eyes in the socket, they can't rotate them like the herbivores can. Thus the necessity for bone in that area is minimized. As it is, live animals do have an extremely tough cartilaginous tissue band bridging that area, which passes muster since they don't need to rotate their eyeballs.

Why do primates and other simians have this feature when we don't rotate eyes? Well. We don't rotate eyes... but we do move our eyes a great deal more, and have a finer musculature around the eyeball itself.

Just my two cents.
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:iconreptangle:
Reptangle Featured By Owner May 4, 2013  Professional Traditional Artist
I didn't know that sheep rotate their eyes when they graze!
-but I have observed that the goats,sheep and the bull I have had would rotate their eyes when they got turned over or laid on their sides. ( for vet care/ hoof trims)They'd get this crazy wild expression, because the whites of the eyes would show. The vet said that that was an automatic reaction to having their heads tilted.
How interesting!
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:icondonnabarr:
DonnaBarr Featured By Owner May 4, 2013  Professional General Artist
More stuff to throw in the discussion: chewing. The eye sockets are close to where chewing or shearing muscles attach. Carnivores just have to shear and gulp. You know, we're probably all right. Or... an ancestral form got on that road, and it stayed that way. Like external testicles in mammals - goofy, not actually dangerous, and turned into display, so stayed that way. Mosaic evolution can go down some very silly roads (As for sperm requiring cooler temperatures - result rather than cause?).
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:iconreptangle:
Reptangle Featured By Owner May 4, 2013  Professional Traditional Artist
Yeah, the external testicles always seemed like a strange adaption. Have you ever watched a ram run? Yike. What a place to stick 'em. Pigs, cats, and llamas at least have them stuck on in a better spot.
They say it is to keep them cool,because of the sperm production thing- but most birds have a higher body temp than mammals and they have an internal testis! -So what's with that?
Elephants don't have them externally do they? Oy, now I've got to google testicles. Hope my antivirus software is up to snuff...
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:icondonnabarr:
DonnaBarr Featured By Owner May 6, 2013  Professional General Artist
Yeah, I think it's an evolutionary "mistake" that wasn't lethal. It almost makes one believe in godlets - and female godlets at that. :D
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:iconequusfell:
Equusfell Featured By Owner May 3, 2013
I found a Journal of Morphology scholarly article by Heesy on this very subject!
[link]
The important bit is this:
Shifts in orbit orientation relative to the temporal fossa (increased protrusion or higher angle between the two) are correlated with the size of the postorbital processes, which replace the ligament. The allometric and ecological factors that influence orbit orientation vary across taxa. Postorbital bars stiffen the lateral orbital wall. Muscle pulleys, ligaments, and other connective tissue attach to the lateral orbital wall, including the postorbital bar. Without a stiff lateral orbit, deformation due to temporalis contraction would displace soft tissues contributing to normal oculomotor function.

I'm a functional morphologist in training (currently finishing my Master's degree) and also the Mammalogy lab instructor at my university, so I can help boil the language down for you!
Basically, what you are noticing when the postorbital bar (the part that 'encloses' the orbit) is present is a derived state in some mammals; the original state is just a ligament that encloses the orbit. Mammals will develop bone or increased stiffness in this ligament for a variety of reasons that are specific to the animals. Some features that have been correlated with a large postorbital bar or process (a bar that doesn't quite touch in the middle) are:
- a decreased angle between plane that the front of the orbit forms and the plane of the temporal fossa, which is the depression behind the orbit where the temporalis muscle attaches (one of the muscles used in closing your jaw). All placental mammals that this angle is greater than 114 degrees do not have a complete orbit.
-protrusion of the orbit from the face in animals with laterally directed orbits, which would be otherwise in line with the temporal fossa (such as in many of your herbivore skulls, think an antelope or bighorn sheep)
-large eye size relative to body size, which contributes to reorienting the orbit away from the temporal fossa as well
It is important to note that carnivores and flying foxes/fruit bats can possess incomplete or complete postorbital bars, so you may very well just have individuals or species that have incomplete bars...

Now, the functional reason for having a postorbital bar is another question entirely... When the orbit shifts in the ways described above, the attachment for the temporalis muscle (the temporalis fascia, which is a sheet of connective tissue, the edge of which is actually the postorbital ligament) shifts with it. Contraction
of the the temporalis muscle and tension in the temporalis fascia would deform the postorbital ligament. This would be a bad thing, because one of the ligaments that keep the eye stabilized is attached to the front of the postorbital ligament. Thus, if an animal with forward facing eyes tried to chew with just a postorbital ligament, the action would actually pull it's eyes off to the side! Therefore, because of the orientation of the eyes in these mammals, they must stiffen the postorbital ligament, usually by turning it to bone to varying degrees!

Now that we know what the function of the postorbital bar is and the orbit positions it occurs in, let's explore the ecological reasons some of your mammals may have adapted these positions!

Horses and ruminants: These are the ones with laterally facing eyes that are big relative to body size and protrude from the skull. They are indeed prey animals and generally have a very large field of view, capable of seeing 360 degrees. To do so, they must stick their eyes further out from their body and possess large eyes that are capable of taking in that range of view.

Primates: These have large, forward facing eyes with binocular vision. They have both the large eye and binocular eye developments. The large eyes are because primates rely heavily on their sense of sight, more than any other sense, and therefore they are well developed. The binocular eyes are because they are arboreal in origin, and binocular vision gives good depth perception, an important thing when catching that oncoming branch is the difference between travelling forwards or falling to your death!

Carnivores: Generally, carnivores have at least postorbital processes, if not bars. Again, this is for binocular vision, for depth perception useful for judging how far away the prey is! Also, their temporalis muscle is less developed than the others discussed.

Marsupials: Postorbital bars are rare, only occurring two extinct animals, which may mean that they are just much less likely to evolve postorbital bars because of their ancestry.

So there you have it! I hope I was informative, if you have any questions, please feel free to ask. Also, I have a pdf of that paper if you would like it, you may note me an email address to send it to!
~equusfell
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:iconspiritofthevixen:
SpiritOfTheVixen Featured By Owner May 17, 2013  Professional Artisan Crafter
Amazing read! Thanks so much for sharing this detailed information with us! :)
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:iconreptangle:
Reptangle Featured By Owner May 4, 2013  Professional Traditional Artist
Wow! Fantastic! I actually understood most of this.
One thing I don't understand: If a carnivore's temporalis muscle is considered LESS developed than the others discussed, what is the definition of "developed"? It seems as if the carnivores have much larger temporalis muscles (in relation to the masseter, anyway) than herbivores.In horses anyway, it looks like they have fairly small, weak temporalis muscles.
Also, when measuring the angle of the temporal fossa, what points on a skull represent horizontal?
Yes, and I have noticed that the postorbital bar in carnivores can come and go! I have a meerkat skull that has only one.
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:icondarkwingdrake:
DarkwingDrake Featured By Owner May 3, 2013  Hobbyist Traditional Artist
interesting! I think most prey animals have closed sockets because their eyes are set on the sides of the head, allowing them to to a far range around themselves for danger, and predators have open ones to allow the eye to face forward? I`m not entirely sure, but that`s my guesse XD
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:iconreptangle:
Reptangle Featured By Owner May 3, 2013  Professional Traditional Artist
Yeah, that seems to be true for the hoofed animals.They all have pretty protruding eyes.
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:iconhekatelesedi:
HekateLesedi Featured By Owner May 3, 2013  Student Filmographer
I remember reading somewhere they it has to do with muscle connections and the biological requirements of the skulls/teeth/eyes, etcetera.
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:icondraconiangem:
draconiangem Featured By Owner May 3, 2013  Hobbyist General Artist
I googled it and after poring though a ton of sites, the ONLY thing I could find was on Anthropoids and Prosimians... "Anthropoids have fused skull tissue while the prosimians do not. The orbit (eye socket) is open in the prosimians, closed in anthropoids." But virtually NOTHING else! Its mind boggling.
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:iconreptangle:
Reptangle Featured By Owner May 3, 2013  Professional Traditional Artist
Maybe I am using the wrong terminology to describe this!
My lemur skull does have what I am calling "closed eye sockets", meaning there is a ring of bone that surrounds the eye socket. Perhaps calling that a "closed eye socket" is the incorrect way to describe it, since they is a large open spot in the back, while a true primate has an enclosed cup-shaped eye socket.
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:iconeyproductions:
EYproductions Featured By Owner May 3, 2013  Student Filmographer
Can't say I really know why some animals have open sockets and some have closed, but I think it might have to do with attachment points for muscles. I know the enclosed eye socket is created by the post-orbital process of from the zygomatic arch and the post-orbital process from the frontal bones becoming big enough to come together (like how a stalactite and a stalagmite may grow together to make a column in a cave). Many animal skulls (like bats for example) have no post-orbital processes whatsoever, making no visible eye socket at all!

And don't be too impressed by the bone-name jargon; I had to look it up. The book I referenced is Animal Skulls: A Guide to North American Species by Mark Elbroch and I'd highly recommend it as it not only has lots of reference images, but the first 100 pages are all about the structures of the skull, their function, and other useful stuff for collectors.
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:iconreptangle:
Reptangle Featured By Owner May 3, 2013  Professional Traditional Artist
Yeah, I have that book!
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:iconeyproductions:
EYproductions Featured By Owner May 4, 2013  Student Filmographer
Cool! You officially know more about skulls than I do then (I don't have a collection, just the book).
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:iconreptangle:
Reptangle Featured By Owner May 4, 2013  Professional Traditional Artist
I know more now than I did, which wasn't much!
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:icontreyos:
Treyos Featured By Owner May 3, 2013  Hobbyist Writer
Well, prey animals are (shockingly!) more likely to come under attack, so maybe it's an evolutionary defense against having their eyes damaged by predators, which would render them largely helpless.
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:iconzhon:
Zhon Featured By Owner May 3, 2013  Professional Artisan Crafter
Are you talking about the orbital rim?
Most of what I can gather from my skull collection is that the prey animals could have a closed rim, not only because their eyes are set more outward, but because the jaw muscles arent as pronounced.
With any type of carnivore, they have open eye orbits because the jaws need to be more powerful because of their more substantial use.
Beavers....technically an herbivore.... have open orbits....but because their jaw muscles need clearance over their zygomatic arch.
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:iconreptangle:
Reptangle Featured By Owner May 3, 2013  Professional Traditional Artist
Oh I like your idea that the eyes in a prey animal need to protrude, that makes sense!
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:iconmirakhall:
MiraKHall Featured By Owner May 3, 2013  Hobbyist General Artist
'opened eye sockets'? What do they look like? :?
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:iconreptangle:
Reptangle Featured By Owner May 3, 2013  Professional Traditional Artist
Well, you know what a human skull looks like, with holes where the eyes go... those are closed eye sockets. Open eye sockets don't have a complete ring of bone around the eye socket. The eye sockets are kinda "c" shaped not "o" shaped.
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:iconmirakhall:
MiraKHall Featured By Owner May 3, 2013  Hobbyist General Artist
I should probably see one in person, one that is opened or closed, because I'm not sure what you mean ^^;
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:iconreptangle:
Reptangle Featured By Owner May 3, 2013  Professional Traditional Artist
Ok this skull , a coyote,[link] has what I am calling " open" eye sockets. The place where the eyeball would rest in the skull does not have a ridge of bone surrounding it

this skull , a llama does have a ring of bone surrounding the eye.[link]
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:iconmirakhall:
MiraKHall Featured By Owner May 4, 2013  Hobbyist General Artist
Hmm, I wonder why humans have a ring of bone on their eye sockets, even though we're carnivore more so than herbivore. Maybe that's irrelevant since we're technically omnivores.
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:iconreptangle:
Reptangle Featured By Owner May 4, 2013  Professional Traditional Artist
I do not know, but we certainly can move our eyes around a lot more than most animals, so it makes sense that we would have more bone for eyeball muscles to attach.
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:iconmirakhall:
MiraKHall Featured By Owner May 4, 2013  Hobbyist General Artist
Maybe so, but the damn octopus still got us trumped with the perfect eye :XD:

Hmm, I wonder, again. Does that mean that prey animals with horizontal eye pupils have opened sockets?
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:iconreptangle:
Reptangle Featured By Owner May 4, 2013  Professional Traditional Artist
No, they are the ones with what I was calling "closed" eye sockets.
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(2 Replies)
:iconkeight:
keight Featured By Owner May 3, 2013
My thought seems to have been covered, already: that is the amount of crushing pressure carnivore jaws can apply would apply muscle pressure, during a killing bite, that could damage eye, or eye neural tissue, if there was no "give" for the eyes.
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:iconarcticphoenixstudios:
arcticphoenixstudios Featured By Owner May 3, 2013  Professional General Artist
Another observation: the prey animals that you list are all ruminants with the majority known for head butting behaviour. Someone mentioned that the extra bone around the eye helps protect the eye. My guess is that if this is the case, that it was protection in fights within their own species as such duels tended to be what won mates over and thus the genes for that better protection would be more likely to be passed on. Also, protection if they're stampeding (another shared behaviour) and crash into stuff head on.

Why rodents and marsupials lack that encasing might be because they're also omnivores? They're also generally squishy creatures so maybe bits of their heads, too? :) They also aren't known for head butting, nor stampeding and same species fights tend to play out differently than with ruminant animals, using tooth and claw over horn/antler and hoof.

And then there are raptors (hawks, most noticeably. Owls too, though not as much), that sometimes have very long eyebrow ridges semi-enclosing their eyes.
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:iconhdjl:
hdjl Featured By Owner May 3, 2013
Fascinating
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:iconbluescuriosities:
BluesCuriosities Featured By Owner May 3, 2013
Also, so jelly of your binetrong. If you are ever willing to trade/sell it, please let us know :3
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:iconbluescuriosities:
BluesCuriosities Featured By Owner May 3, 2013
I have often wondered this myself, curious to see what others say about it! But my own guess would be the same as Fractal's, more for protection.
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:iconbladespark:
Bladespark Featured By Owner May 3, 2013  Professional Artisan Crafter
I wonder if it has anything to do with freedom of movement of the eye itself? I have no idea if this is the case, but prey's strategy is to have eye placement that lets them see a broad field, often with things like horizontal pupils that assist in scanning horizons, so they can more or less look everywhere at once without moving their heads or moving their eyes. Whereas most predator species have binocular vision, where they look at one single spot, but move their eyes to track that spot as needed. Ergo maybe a more open socket facilitates the muscle attachments that make this possible?

But with a meerkat, I've noticed that they seem to move their *heads* rather than their eyes. I know some species of owl can't move their eyes at all (just because they're too big) and thus have to track with their heads. And if a Meerkat doesn't move their eyes, then the open eye socket would have no benefit to them, and a closed eye socket provides more protection for the eye, so why not have one?

Just a random idea.
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:iconkaye00:
Kaye00 Featured By Owner May 3, 2013  Student Traditional Artist
Predators usually have forward facing eyes that allow them to judge depth, where as prey animals tend to have eyes toward the sides, which allow them to see predators approaching from the side as well as from behind. So... maybe in the case of open and closed eye sockets, it's a matter of peripheral vision. Predators that rely on forward vision may have open eye sockets because it allows a wider field of peripheral vision allowing them to see prey when it tries to escape, whereas with prey animals, because their eyes are more likely to be placed toward the sides of their head, they have no need for that extended amount of peripheral, so they have enclosed eye sockets.
Granted, this is all just guessing, so I have no idea if I'm right, it also doesn't explain primates who have forward facing eyes, but enclosed eye sockets. Unless having enclosed eye sockets gives better depth perception, which may be very important for primates, since they tend to be tree dwellers or in the case of humans, we relied on thrown weapons in our early development. Again though, these are just guesses. :P
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:iconsovriin:
Sovriin Featured By Owner May 3, 2013  Student Artisan Crafter
Hmm just a theory, and I could be wrong, but typically, prey animals will have their eyes on the side of thier head, and predatory animals have their eyes on the front. Maybe the different eye sockets simply allow the eye to be placed differently?
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:iconfractalxavier91:
fractalxavier91 Featured By Owner May 3, 2013
Well I'm in no way qualified to say for certain, but it may be to reduce damage to the eyes will making a kill. I remember watching a show on animal planet about a guy who'd been mauled by a puma. His wife had tried to get the puma off him by attacking it's eyes and it wasn't working. They had someone (biologist or zoologist or something) on talking about it who mentioned that their eyes weren't anchored in place and could move around some on the skull.

I don't know how qualified that guy was either, so everything I just said may be completely wrong. It seems like it would make sense, though, since you're only seeing it in the carnivore skulls. It's not so much that the prey animals require it, as it is the predators benefit from not having it.
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:iconleathurkatt-tftiggy:
Leathurkatt-TFTiggy Featured By Owner May 3, 2013  Hobbyist General Artist
Actually, that does make sense.
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:iconreptangle:
Reptangle Featured By Owner May 3, 2013  Professional Traditional Artist
Oh! Interesting! An open eye socket would allow the eye to get pushed around more without squishing it... huh!
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:iconfeatherfishes:
Featherfishes Featured By Owner May 3, 2013  Hobbyist General Artist
hmm. that's interesting.
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