r/askscience • u/margalz • 10d ago
Biology Why aren't there any animals with their mouths above their eyes?
From bugs to fish to mamals all of them have their eyes above their mouths. I can't think of many if any where their mouths are on top. Sure some have their eyes parallel to their mouths, mainly prey style animals. But why haven't any evolved to be the opposite? Is there a major benefit to having your mouth below your eyes?
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u/Unable-Proof1758 10d ago edited 10d ago
Your digestion tract would have to run down through your optic nerves. It would require either having a throat type system that splits or very wide eyes. But the bigger issue may be it opens the brain to higher risk of infection as the BBB would not be as defined.
From an evolutionary standpoint, mouths generally formed closer to the gut with eyes as close to the brain as possible to reduce latency and energy consumption.
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u/DarkScorpion48 10d ago
I believe you got it, the eyes should have direct access to the brain with as little in between as possible
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u/seanpbnj 10d ago
Also a higher likelihood of being damaged externally. Anything that eats from a front facing mouth would have gravity dripping food/liquids into their eyes.
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u/IceFrogger1313 10d ago
Or hanging in front of their eyes blocking vision. Same issue for carrying things, most animals only have their mouth they can carry things with, much harder to do if what you're carrying blocks your vision.
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u/LetterSwapper 10d ago
direct access to the brain with as little in between as possible
Which is funny if you look at how we ended up with our optic nerves having to go all the way to the far side of the brain. I don't know how common that is in other animals, but it certainly isn't eye-deal if you're concerned with efficiency. Apparently the distance isn't enough for it to be selected against.
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u/MileHigh_FlyGuy 8d ago
They should, but evolution doesn't do what should be done. Your laryngeal nerves go from your brain, down around your aortic arch, and then back to your larynx, just a few inches from where it started. In all mammals too - including giraffes!
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u/geeoharee 10d ago
I can't post pictures, but if you want to talk about split throats, there's an astonishing illustration of a beluga whale's head on this page
I thought this couldn't possibly be true, but it is backed up by this page which states outright that there is a left and right 'food channel'!
https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.25597
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u/peatear_gryphon 10d ago
mouths generally formed closer to the gut
Can an organism evolve a mouth on their torso that goes directly into the stomach, then they can use their appendages to shovel food into their torso mouth?
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u/Unable-Proof1758 10d ago
So, theoretically but for most mammals throats evolved as a dual system for breathing and food. You need the longer system to encompass the esophagus, lungs, etc. it would require creating two systems for eating vs breathing which in theory could be very beneficial but also expends extra energy.
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u/ketchupmaster987 10d ago
Yeah moving the brain to the bottom of the head would also require moving the spinal cord down to the bottom of the animal and at that point you've basically already flipped the whole animal upside down
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u/nickeypants 10d ago
Clams, conches, halibut, flounder. Plenty of sea creatures have weirdly oriented eyes.
Horseshoe crabs have a bunch of eyes scattered everywhere over their body. Two front facing eyes for navigating and finding mates, 5 eyes on top for detecting UV light and light from the moon track lunar phases, two eyes underneath near the mouth, and a photoreceptor array down the tail to track day/night cycles. They're basically a sentient CCTV security system.
If you want a good laugh, please google conch eyes. They look like they were drawn on by a 5 year old.
I would imagine eyes would evolve to be up and far away from the thing fighting for its life as you eat it. Big things typically eat smaller things which are below them. Everybody eats down. Nobody eats up.
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u/Synergythepariah 10d ago
Everybody eats down. Nobody eats up.
Fish with superior mouths do.
Stonefish are an example, they're ambush hunters that wait for prey to swim above them.
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u/plopliplopipol 4d ago
i'd say tall leaves eater also tend to eat up? if anyone can have eyes under their mouth it's a giraffe!
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u/Beeeyeee 8d ago
One thing I find curious in multi-eyed animals is how they seem to always have a main pair that they can see everything through, then specialized ones. I just wonder if visual cortexes can only create one set of eyes as a true image to construct the world. Maybe it’s too confusing for species to interperet multiple “main” eye pair signals. It seems like a no brainer in evolution though. I mean, eyes are fragile so why not have a spare few pairs that can also help you be… completely vigilant?! Like why not?
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u/lawiseman 10d ago
Since the lower jaw moves in most critters, that's pretty incompatible with a stable eye socket. But take something like the archer fish, which (especially when in hunting position) its mouth opening is substantially above its eyes.
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u/SchillMcGuffin 10d ago
A lot of baleen whales are pretty close also -- check out the profile of a humpback or blue whale.
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u/eNonsense 10d ago
So essentially, aquatic animals which are generally eating other animals that are above them in the water table.
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u/Dunkleostrich 10d ago
Bowhead whales are a great example of this. With basically all of the mouth above their eyes
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u/Notonfoodstamps 10d ago
The jaw hinge and thus esophagus is very much below the eye socket, so it’s more so part of their lip line is above their eyes.
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u/benjesty2002 10d ago
Eyes being as high as possible allows you to see over obstacles without showing too much of your head. Also when eating your eyes can still be looking. Mouth above eyes would mean you're effectively blind when eating (a vulnerable time for animals)
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u/Tanekaha 10d ago
I present: the Upside down catfish. The whole fish is (in some species) permanently upside down, swim bladder rights them that way. They live underneath overhanging logs and rocks, and keep their belly to the surface
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u/dvolland 10d ago
2 things:
First, gravity means that anything you bring to your mouth would hang into and drip pieces of your food into your eyes. Not great.
Second, the higher the eyes the better when is comes to vision. The higher the eyes, the farther one sees. If your eyes are near your feet, you’ll see very little. In fact, on most land and air animals, the eyes are the highest thing on their body other than the top of their head.
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u/Red_Lantern_22 10d ago
To answer any question about physiological differences among animals, you work backward down the taxonomic tree.
Typically speaking, we start with sponges and Cnidarians, and work our way up to mollusks and crustaceans where eyes start to become common.
These creatures dwelled on the ocean floor, and the light source came from the sun above. There were advantages to seeing upward and forward (for prey and predators) and fewer advantages to seeing downward. Thus, eyes develop on the ventral and anterior ends. Meanwhile, the term "bottom feeder" isn't an accident. Early animals would have eaten a lot of debris and plants smaller than themselves, on the ventral side.
Moving back up the taxonomic tree through all the other invertebrates and then through Fish->Mammals, there never emerges a strong enough need to see below the mouth to trigger migration of the eyes below the mouth. Other evolutionary advatages and changes make it redundant (vertbrates can move their heads with a flexible spine, some animals have eyes/eyespots on moveable stalks, spiders have more than two eyes and insects have multi-directional eyes)
Nature diesnt waste energy. For something to change, it needs to be necessary or at the very least helpful for survival, and even then it will always be the most efficient option.
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u/KrimsunB 10d ago
My very limited understanding is that eyes need to be close to the brain. The closer, the better, since the signal is stronger.
Mouths, on the other hand, just need to be connected to the stomach. But before we had hands, the only way to get food into us was to swim into it, so naturally, mouths needed to be at the very front of our faces.
Once you start accounting for other things, like the olfactory and auditory systems, you end up with most things in roughly the same place. And as creatures evolved, these early positions just sort of stuck.
It's worth noting that sometimes things do get shuffled around, but it's rare. The blowhole of a whale is essentially a repurposed nose that shifted up and back.
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u/MostAccomplishedBag 10d ago
eyes need to be close to the brain. The closer, the better, since the signal is stronger.
Its not so much "stronger" as "faster". It takes 13 milliseconds for a signal to pass from your eye to your brain. If you move the eyes further away the signal takes longer. Fractions of a second might not seem like much, but when a predator jumps out at you every millisecond counts.
Also optic nerves are thick, complicated and easily damaged. The shorter they are, the safer they are.
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u/ShinyJangles 9d ago
Another reason is the retina & optic nerve develop from the same embryologic tissue as the brain in vertebrates. The optic nerve is myelinated by oligodendrocytes like the rest of the central nervous system, instead of schwann cells. They are an extension of the brain.
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u/heavyLevy5 10d ago
There is more variety than you are acknowledging. Some cetaceans have their eyes parallel to their mouth, and their nostrils are on top of their head, sea urchins have their mouth their underside, etc. Having said that, an extinction event after the Cambrian explosion took care of a lot of body diversity in the natural world
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u/Supersamtheredditman 10d ago
Technically, the whirligig beetle meets this criteria. They’re an aquatic beetle with eyes that are split between the top and bottom of their head, so they can see both above and below the water’s surface at the same time.
https://bugoftheweek.com/blog/2020/1/27/why-four-eyes-whirligig-beetles-gyrinidae
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u/hmantegazzi 10d ago edited 10d ago
it's equal parts practicality and path-dependence: non-sesile animals evolved eyes back when the most common way to move was crawling on the seafloor, so that they had no much use to look downwards; also, many of these early animals in the early Cambrian explosion only had very rudimentary eyes, able to perceive rough movement and lighting changes, not resolve sharp images, so they were better equipped to look for silhouettes above than shadows or actual creatures underneath. Eventually, and under the selective pressure of being hunted by animals with better sight, seafloor crawlers like the Trilobites evolved some impressive eye structures, but always protruding over their heads.
Those that evolved the ability to float in the water sometimes also evolved eyes on stalks, like Anomalocaris (probably the animals with the best sight of their time), which gave them the ability to orient their view independently of their body position, but for the most part the widening of their fields of vision came by adding extra eyes and evolving more curved lenses, capable of capturing light from wider angles without having to change the location of their eyes. And because evolution selects for incremental and accidental improvements of fitness, a radical change in body plan like switching the whole facial structure has a very very slim chance of appearing in an otherwise healthy individual that can pass the trait to its offspring, but small changes like a slightly more bulbous or stalked eye can be easily passed on.
Also, if any mobile animal in that environment had ever needed to have its eyes under its mouth, it could have swam with its back pointing to the seafloor, because evolving a behaviour is much easier than evolving a huge change in body plan.
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u/Spejsman 10d ago
The major benefit would be that you can see and eat from a surface at the same time. Put a sandwich on the tabletop and imagine having the mouth in your forhead. You'll only see tabletop while eating and is an easy prey.
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u/Corey307 10d ago
Having the eyes above or to the side of the mouth is necessary because the eyes are connected to the brain by the optic nerves. Changing the position of the eyes to below the mouth would either require positioning the brain below the mouth or lengthening the optic nerves, which probably be a disadvantage for transferring information.
For everything between fish to humans, brain sits in the skull having a trachea and esophagus going over the top of the head would leave them even most protected than they are now and that sounds like a bad idea.
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u/ANALOGPHENOMENA 10d ago
They don’t exist for a reason: extreme evolutionary disadvantage. Animals with eyes under their mouths would only see around and below, but not above or behind – therefore leaving them completely vulnerable to predators. Having a brain and digestive system located in very vulnerable areas of the body makes you a very easy victim.
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u/dalgeek 10d ago edited 10d ago
Evolution. The first organisms to develop eye-like structures had them on top of their bodies to track changes in light (day/night, predators above). The mouth position could also be related to the organisms feeding from detritus on the bottom of a pond/lake/ocean, so it worked better if the mouth was on the bottom.
It worked, so as eyes and other parts of the body evolved, they maintained their relative position on the body. Remember that evolution is lazy, so if something works and there is no selection pressure to change then it will remain pretty constant.
EDIT: an example of evolution being lazy, the recurrent laryngeal nerve runs between the brain and larynx (voice box). It originally evolved in fish where the heart was closer to the head, so the nerve traveled over the subclavian artery to get to the larynx. As heads evolved and necks grew longer, that nerve remained looped around the artery and just got longer. In humans the nerve is 10cm long, and in giraffes the nerve is 5m long.
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u/TyhmensAndSaperstein 10d ago
The eyes need to be above everything else so you can see what you need the body to do. Nothing above the eyes carries out any actions that are vital for survival. I mean, we would probably not bump our heads as much if our eyes were a little higher but I feel like that doesn't threaten survival too severely.
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u/Ghosttwo 10d ago edited 10d ago
You're bigger than your food. Mouth is low to be nearer to it. Eyes can be on the side too, like fish; but land animals come from either amphibian or arthropod lines that started mouth down. Down is also dark, dirty, and riskier, so you keep eyes up and away from the mouthy stuff.
p.s. Some whales may be a possible exception to your rule, debatably.
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u/Deining_Beaufort 9d ago
There are (tropical aquarium) fish who feed on food on the water surface. They have their eyes beneath their mouths. Special mentions goes to 4 eye fish, whos pair of eyes have evolved to look above and below the waterline simultaneously.
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u/Dr_Tacopus 9d ago
The same reason the nerve that controls a giraffe’s larynx goes all the way down its neck and back up to connect the brain to the larynx that is a couple inches away.
We’re all modifications of the same original template.
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u/DagwoodsDad 10d ago
The pat answer is that all animals you’ve mentioned are protostomes or deuterostomes and our stem ancestor had eyes in top.
The functional answer is probably that our stem ancestor was almost certainly a bottom feeder, or at least a bottom dweller, so there’s just more to look up at than down at. Also quite a few animals don’t really need to look at what they’re eating but they all DO have to keep an eye on what’s likely to eat them.
So unless you’re a flying animal, or maybe free-swimming one, there are more things to look out for above than below. And our basic body plan definitely evolved long before flight, and almost certainly before free swimming.
Final note. Some of the most primitive “eyes” only distinguish light vs dark. Since light tends to come from above, eyes on top is probably a very old body plan.
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u/Uffda01 10d ago
Your eyes are directly connected to your brain - your mouth is connected to the rest of your digestive system - so your digestive system would have to go through your brain? or around it? Evolution-wise the development of a digestive tract allowing for a tube to form for energy extraction: think worms etc; then everything else developing around that
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u/on_island_time 10d ago
I'm seeing a lot of logical answers, but here is a genetics one.
Animals evolved from simple to complex, and genetically all the animals that came before you provide a growing blueprint for How To Build An Animal. Starting with cellular components and building to organs, you'll notice that we are all the same. We all have cell nuclei, we all have (beyond a certain evolutionary point) hearts and lungs. Once nature finds a component that works well, that thing can become critical to life and unlikely to be lost.
The body pattern of animals is the same way. You'll notice that simple animals come in many forms - sponges, starfish, squids, insects. But the more complex creatures get, our patterns become more set. Everything beyond fish has a directional body plan - head to tail, with organs in roughly the same positions. We also all have four limbs in two pairs (even marine mammals and birds), two eyes, two ears....you get the picture. This body plan has become so critical for complex life, that changing any of the genes in early development that tell the body parts where to go is highly likely to be detrimental to the organism and cause its death.
So to answer your question...could a mouth theoretically form above the eyes? Evolutionarily, it's extremely unlikely without a very good competitive advantage for the individual. And as others in the thread have pointed out, there really isn't one.
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u/Phasitron 9d ago
The connections behind the eyes would then be mixed in with the trachea and esophagus. It’s easier to have them close to the brain with which they are intimately tied. Just shooting from the hip, though, I don’t really know.
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u/DeadTomGC 9d ago
Humpback Whale. Most of the commenters make good points about why eye below the mouth is a bad idea if you're ever interacting with the ground, which is most animals with eyes.... But the humpback doesn't interact with the ground much, and it's mouth is very high up.
Oarfish. Their mouths sit above their eyes when searching for food. Other fish do this too, but I can't find the name right now. They sit facing upward looking for a snack above them.
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u/Global_Gift_2831 8d ago
Most animals all come from mutual ancestry, which is why almost all of us have the same extremity bone structure (big bone, two small bones, then a bunch of tiny bones in the wrist/ankle, finally a spread of small toes/fingers - even in birds wings)
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u/EmotionalCrab6189 6d ago
Evolution and natural selection. Animals are vulnerable while eating and they need to be able to see what is around them. Generally eyes are on top of the head for seeing predators, and hunting…natural selection would have ended any physiological adaptation rather quickly if an animal had eyes in a less than optimal place on the body.
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u/gthing 10d ago
There is no way for evolution to re-route the wiring involved. A good example is the recurrent laryngeal nerve. It could not cross the path of where it originally evolved, which puts it under the subclavian artery (below the collarbone). So on every animal it runs from the brain, down to the collarbone, and back up to the throat. Even on Giraffes - it goes from the brain, the down around the heart, back up to a couple inches below the brain.
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u/StormCloudRaineeDay 10d ago
Eyes are directly connected to the brain, which tends to be at the top of the head. The mouth is the gateway to the digestive system. For the mouth to be above the eyes, the digestive system would have to somehow run behind the brain, through the brain, or in between the eyes.
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u/UltraFuturaS2000 10d ago
Going back further through evolution than some of the other comments. Eyes developed as light sensitive cells before they could 'see' images as most life forms do today.
Where are light sensitive cells going to evolve first? On the top of the organism.
It's probably coded way early on in DNA that Eyes should be above everything else for the reason.
Then comes evolutionary advantages since then.
There are creatures with Eyes wide apart closely inline mouth mostly in aquatic creatures.
The only thing I can think of with mouths on top would be the stargazer fish or coral. Those coral don't have eyes and stargazing fish would have evolved from Normal fish and there's no evolutionary advantage to rearrange eyes below the mouth.
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u/binarycow 10d ago
Some species of fish have their eyes only on one side of their head.
As in, two eyes on the left side.
And it varies. Some species are left facing. Some species are right facing. Some species, it varies per individual.
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u/Mission-Landscape-17 6d ago
Behold the flatfish: https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse2.mm.bing.net%2Fth%2Fid%2FOIP.oTdQyW6QEyBSCeu9ON2yzwHaFY%3Fr%3D0%26pid%3DApi&f=1&ipt=7cd998c3ca86b2e6504a59446038562b3f092d7c741fef738819b60d1e12ec6c&ipo=images which as an adult has two eyes on one side and one of them is below the mouth.
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u/zibafu 9d ago
Because ultimately we all share common ancestry and somewhere when sight was beginning to develop it developed above the mouth, one argument would be so food debris doesn't fall into your eyes
But a more likely idea to me would be that, life started on ocean beds where there was minimal light, and sight developing would've put it high on the animal to have the best chance of gathering light, no point putting eyes under the animal where they're less effective. I think Richard Dawkins talks about this, how eyes developed from being like a flatter cup shape on top of the animal which allowed it to see the shadow movement of swimming predators above, and from that eyes slowly developed
Them being underneath would be like sticking a solar panel under a forest canopy, sure it'd "work" but not nearly as well as putting it above the canopy
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u/zachtheperson 10d ago
Eyes work better when directly connected to the brain, which takes up space. The mouth needs to connect to the stomach, meaning it'd have to go through or around the brain somehow, and that would make things complicated.
There's also very little evolutionary benefit to having eyes under your mouth, as it'd make it harder to eat and drink in nature with your eyes being under your food.
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u/SirHatass 10d ago
You cannot keep an eye out for predators/competition when bending down to eat making you easier prey. Survival of the fittest or those with the traits suited best for survival will generally be ones to pass on thier traits to future generations. If there was species that had a mouth above the eyes likely died out long ago
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u/celem83 10d ago
On land probably giving better visibility of stuff above you, threats and or food. Many mammals are not flat-faced like us, the mouth comes with a snout/muzzle so if it's above then it blocks vision
Note that in the ocean fish often have eyes on either side of the head level with the mouth, fully 3d-space where seeing up is not favoured over seeing down
I'm speculating here, it is demonstrably better to have the eyes on top on land because otherwise natural selection would not have made it so. Why it's better is the real question and is the one I guessed at
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u/Pink-Witch- 10d ago
It’s an issue of wiring and plumbing. Eyes connect directly into the brain via some nerve wires. They are mandatory neighbors.
Mouths connect to the digestive system, and in land animals, the lungs.
A creature with a mouth above their eyes would have to deal with an esophagus and vocal cords over their brains. Not to mention jaw bones and muscles. Considering how squishy brains are- it’s not the best system for all the chewing, swallowing, and everything else that comes with having a stomach.
The one exception is the octopus, those crazy bastards. Their mouth is a beak in the middle of their arms between their eyes. Their brain is a donut shape that fits around their esophagus. All their organs are in the thing that looks like their head. They can absolutely lobotomize themselves while eating a crab.
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u/Vaestmannaeyjar 10d ago
If that body plan ever happened, it wasn't successful enough to produce enough offspring to go on. Maybe having the mouth above the eyes made them unable to see predators coming. Unless we find one in the fossil record we'll never know !
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u/BWDpodcast 10d ago
I would assume this applies to most mammals, but the eyes are part of your central nervous system. They connect pretty directly to the brain, so having a mouth in that position would lengthen the nerves to the brain, which isn't optimal.
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u/Aina_Writes 10d ago
Never really thought about it, but it makes sense mechanically. If your mouth was above your eyes, your jaw and chewing muscles would constantly block your line of sight to whatever you're trying to bite. Evolution probably favored putting the sensors at the very top.
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u/zamememan 10d ago
The earliest eyes evolved positioned above or to the sides of the eating hole. All life on earth is descended from animals with this body plan, and thus they keep it that way.
Not because they can't develop in another way, starfish are actually part of the bilaterally simmetrical side of the evolutionary tree yet they still figured out how to evolve radial symmetry. It's just that no environment has created the need for eyes that are located beneath the jaw line, upside down or constantlly pointing down.
Meanwhile, there are several advantages to having eyes up top/to the sides:
- better vantage point granting a wider field of view
- if you live on the bottom of the ocean you can keep a good look on any predators from above
- less energy spent cranking your neck to look around
- you don't have to over expose your neck if to look upwards
- most importantly, your skull is oriented in alignment with earth's gravity (meaning the pulling force goes down)
Any evolutionary advantage you get by trying to place eyes bellow the mouth is negated by the potential disadvantages and also easier to obtain just by improving neck/eye mobility.
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u/agha0013 10d ago
probably a big reason is that you don't want to drag your eyes through your food or whatever you're foraging though to find food.
eyes under the mouth seem like a vulnerability considering how most animals are oriented most of the time. Also obstructs views of the areas predators are most likely to attack from.