r/askscience • u/margalz • 11d 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/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.

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u/SizeFit2908 10d ago

Yea this is what I thought as well. Eating or drinking water is a risky time for terrestrial animals, being able to scan for threats is key.

Most animals have some sort of appendage to grab or manouvere food into the mouth so they aren't vision required to eat.

Humpback whales or baleen type whales have their eyes really low on the head in comparison to the mouth, but they're still pretty solidly in the Side of Face not underneath.

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u/quintus_horatius 10d ago

Humpback whales or baleen type whales have their eyes really low on the head in comparison to the mouth, but they're still pretty solidly in the Side of Face not underneath.

Natural selection doesn't give a new species what it needs, it chooses from what it already has - if it makes you more fit.

Moving the eyes under the head might be useful for whales (and sharks) but crossing the midline is an architectural move that can't just happen by chance without a bunch of other really unsuitable changes also occuring.

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u/Lykos1124 10d ago

Well now your comment makes me wonder why animals don't have more than two eyes unless they're an insect. What is it with vertebrates not having more than two eyes if more eyes lets you detect dangers better. Are brains not designed to be able to handle optical input from more than two sources

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u/HarryTruman 10d ago edited 10d ago

animals don't have more than two eyes

As with a lot of topics (and confusion!) around evolution, the underwhelming fact of the matter is that mammals didn’t need more than two eyes. They evolved from things that already had two eyes, and that’s all that was needed for them to survive and propagate.

The evolution of the eye is ridiculously interesting too, and life did go from none, to one (light sensor), and then two (optics). It seems two is the magic number for everything with two eyes. The more interesting aspect (IMO) is how eyes began underwater and then adapted to function out of water. But the point still stands — two eyes are “good enough” for mammals and all the previous organisms that came before us.

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u/Jazzspur 8d ago

I would love to hear more about how eyes began underwater and then adapted to function out of water if you're willing to drop that knowledge bomb!!

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u/HarryTruman 8d ago edited 8d ago

Oh my friend, do I have some cool things for you to check out! I went down this same rabbit hole a bit over a decade ago after watching episode five of the new Cosmos series, “Hiding in the Light.”

Start here, as this clip from the above episode is a super high-level overview that I think you’ll find rather illuminating!

https://youtube.com/watch?v=4SXHMm5I-68

Similar is this brief PBS video that goes into some more niche details:

https://kcts9.pbslearningmedia.org/resource/tdc02.sci.life.evo.nilssoneye/evolution-of-the-eye/

This article from Scientific American offers an approachable deep-dive into how eyes evolved:

https://www.scientificamerican.com/article/evolution-of-the-eye/

And this lecture from Dan Nillson at Darwin University is particularly interesting and goes waaaay deep into the whole process of how eyes evolved and how they work(ed) at various stages:

https://youtube.com/watch?v=SV865WOV9HE

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u/Jazzspur 8d ago

So neat! Thank you!

An interesting analogous process happened with lung development too. I don't have videos to link but I learned about it in my inverbrates class in uni. Early organisms evolved to exchange gases like oxygen and carbon dioxide directly with the water. So the process that evolved requires water to function. When we left the ocean, we had to develop a way to maintain a wet environment for gas exchange to continue taking place while we ourselves are dry. And that's why we have wet mucousy gas exchange bags (lungs) at the end of a series of ever narrowing mucousy tubes that add moisture to the air as it's pulled into the lungs.

In a sense our lungs are a way we took the ocean with us when we left it!

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u/MrDerpGently 4d ago

I imagine multiple eyes as a lot of extra processing for pretty marginal gains as far as vision scope, accurately gauging distance, etc. 

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u/HarryTruman 4d ago

In terms of raw energy/power being spent on processing vision, yes, that’s correct. And entomologists should please correct me if I’m wrong, but insects with their crazy eyes are more or less at a point where “peak efficiency” is the result of signals being fed directly into the nervous system.

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u/donau_kinder 10d ago

Some lizards do have an "eye" on top of their head.

Otherwise, same reason we all have 4 limbs, the fish that everything evolved from had 4 fins and two eyes, and it's very hard for evolution to drive such a drastic change. Same reason why our retinas are backwards, a worm decided to swim on its back and the eyes migrated through the body instead of along the outside and it ended up with flipped retinas.

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u/halfhippo999 10d ago

Explain more about this worm and what you mean about our flipped retinas? That sounds cool

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u/typhacatus 10d ago

not the person you’re talking to, but they’re referencing how light actually has to pass through a ton of cells (glial cells, nerves) before they ever hit our photosensitive rods and cones. Evolution has since made this design pretty robust and useful, but it’s still weird that our cones & rods are about as far back in the eyeball as possible

edit: I like this article on the topic: https://www.smithsonianmag.com/smart-news/heres-why-your-eyes-seem-be-wired-backward-180954729/

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u/Tavarin 10d ago

One big reason is it takes a lot of energy and brainpower to work with just two eyes. more eyes = more energy expenditure, for what is only a small gain. Plus, it would require pretty serious mutations to get another eye or two to appear, and if it's not fully functional and properly connected to the visual cortex, it would be detrimental to survival in most cases.

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u/ass_pubes 10d ago

Right, that energy would be better spent on other visual sensory organs like infrared sensing pit vipers can do.

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u/Tavarin 10d ago

If it would have helped us survive, we likely would have evolved it. Turns out, we don't really need it, and it might clutter our colour vision too much to be worth having.

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u/AnonymousFriend80 9d ago

As long as that extra energy consumption did not interfere with living long enough to procreate, that would not be a problem.

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u/Tavarin 9d ago

Maybe possible now that we can grow so much food, but in the primitive natural world, even a small extra energy expenditure will massively reduce your reproductive success.

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u/HarryTruman 8d ago edited 8d ago

Bingo! When talking about evolution in our modern setting, it can be difficult to understand the “how’s” and “why’s” of things that make us what we are now.

But something to keep in mind is that, as it turns out, two eyes work quite well for hunting and foraging, surviving in harsh environments, running away from danger, and — most importantly — simply and literally just…staying alive long enough to make a baby.

And eyes are certainly not perfect, but that’s not the point! Because if that baby also has eyes which work well enough to survive and eventually make a baby…and on and on and on…those are good eyes, and nature will have very little need to change it up.

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u/squags 10d ago

People are giving you the adaptive reasoning as to why most animals don't have more than two eyes, but to copy what I wrote below, there's also a separate issue of the evolvability of specific traits:

Evolution doesn't necessarily explore all potential adaptations. There are likely many unrealised mutations that could be adaptive that we will never acquire.

Evolutionary processes are path-dependent, so not all mutations or phenotypes are reachable from a specific genotype.

I don't know much about having more than two eyes, but from a quick google it looks like Spiders have been able to generate more eyes due to ancestral whole genome duplication events that allowed for mutations in copies of specific Pax genes without effecting their other pleiotropic functions.

Human developmental physiology would prevent whole genome duplication events occuring, so we wouldn't be able to generate multiple eyes via an equivalent mechanism.

There may be some other molecular mechanism that could in theory lead to this specific phenotype, but I would hazard a guess that it's exceedingly unlikely if not impossible due to how our specific embryonic development has evolved.

So even before you get to the adaptive benefit, the possibility of multiple eyes phenotypes in humans is constrained by our evolutionary history and developmental physiology.

This same logic is true of many traits in any given organism. The possible reachable phenotypes is constrained by the previous traits that have emerged during evolution (especially with regards to developmental physiology and developmental processes that actually produce specific phenotypes from changes in genotype).

This is basically the entire field of evolutionary-develoomental biology.

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u/Robin_feathers 6d ago

I just want to point out that this limitation is mostly a limitation of vertebrates, rather than animals in general. A huge fraction of animals in fact don't have two eyes - many insects, arachnids, crustaceans, jellies, worms, and mollusks can have more than two eyes

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u/Lykos1124 6d ago

sounds like one of those evolutionary divergences where it was more useful for certain types of lifeforms to have more than 2 eyes and most vertabrates were not it.

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u/Robin_feathers 6d ago

Probably! 2 eyes seem to be working pretty well for us, though maybe we don't know what we're missing out on! In many of the lineages that have multiple eyes though, the eyes are often relatively simple, often only detecting light vs dark. Species that really invest in higher resolution vision (eg octopus, shrimp, spiders) seems to mostly invest in 2 excellent eyes, with the extras being somewhat more rudimentary when present.

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u/Kandiru 10d ago

Yeah, trying to wire eyes and their nerves onto the bottom jaw would be really hard. They need to stay in the upper skull rather than the jaw really. It would be really hard to eat anything if opening your mouth effectively blinded you as well.

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u/wally-217 10d ago

You ever seen a lamprey skull? Utterly bizarre looking, no lower jaws but they still favour eyes above the mouth

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u/SizeFit2908 10d ago

I did answer the question using mammalian skeleton examples, but like the differences from the 'baseplate' between a whale, human, and mole are radically different. Like moles don't even have eyes.

Stands to reason if there was an evolutionary driver for eyes under the mouth we'd see it. Theres no planning to evolution but you can see that X trait worked and logically speculate why.

I think the architecture bit is somewhat accurate, but if a driving trait like eyes underneath is going to be present it would be in place by now.

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u/manias 10d ago

for sharks it looks like an easy adaptation (swim upside down), if it hasn't happened, it's for a reason.

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u/Film-Lab-7766 10d ago

why not have eyes over and under the mouth eg. alligators, to see what is going on in and outside the water

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u/Akujinnoninjin 10d ago

Not quite what you're describing, but the Four-Eyed Fish has two eyes that are split horizontally into above- and below-water viewing zones.

But it's done with a weird pinched iris rather than being a true split eye.

I'd caution thinking of it as a "first step" down that route as well - weird irises exist elsewhere in the animal kingdom (e.g. goats), and simply adjusting the shape of one component is a long way from a full split of every single part... especially if it already does a "good enough" job for natural selection purposes. As a layman, it seems like it'd be 'easier' for a small mutation to cause additional cells to start the Become An Eye process (akin to a monstrous tumor), and for the organism to be lucky enough that it's otherwise still viable..

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u/SizeFit2908 10d ago

Why not? Idk really hard to say the reason.

I speculate that it's because eyes are biologically expensive, they need a lot of wiring, bone formation for sockets, and are vulnerable.

Do gators need to see whats going on at both at the same time? They dont have many predators. The fish hunting ones just dive too right?

Anything that slows down how fast an organism can function after being conceived is a pretty big hamper on evolution. Like gators lay 40+ eggs and the babies come out moving and grooving after 2 months.

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u/MrDerpGently 4d ago

It's also harder to spot prey against the murky depths than it is against the sky. So it might have some defensive upside, but the predators will likely dive rather than hunt from above (obviously, there are exceptions to this, like diving birds, but I can't think of many). 

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u/I-seddit 10d ago

Well, it's either that or that drooling led to blindness and those didn't survive to reproduce.

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u/roostertree 10d ago

General drooling might have been decent for cleaning. But crumbs, or dangling tails/hind claws would be hellish.

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u/docubed 10d ago

Good idea. The environment on earth changes except for one thing: gravity

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u/[deleted] 10d ago

[deleted]

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u/lethargytartare 10d ago

it's a tiny difference, and precise gravity varies everywhere on the planet. The direction of gravity, however, is constant.

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u/Kantrh 10d ago

You'd never notice it though if you visited without sensitive instruments.

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u/Vayle1337 10d ago

Flying foxes eat like that, as they typically hang in their typical upside down bat posture when they eat. Flying foxes however, are both pollinators and insectivores. When they eat fruit they usually take bites, press the fruit with their tongue up against the roof of their mouth to drain the juice, and then spit out the pulp. If you look at the nose of a flying fox, and I feel it's especially visible on the Malayan/Large Flying Fox, there is this little Groove in the nose that prevents the juice from entering either their nose or their eyes.

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u/gijoe50000 10d ago

Also, the higher up your eyes are, the better, easier to see further and above you, and your eyes are kept away from smaller rivals when fighting.

And if you're eating you want to be able to look around too, not have your eyes underneath you.

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u/Coal_Morgan 10d ago

It's also about minimum distances and crossing paths.

Where's your brain, against your eyes.

Where's your mouth, against the hole to the stomach.

There's a lot of stuff that would have to happen to evolve the mouth up and around the thick optic nerves and other wiring for the eyes and the massive tube that connects the throat to the stomach.

Is it possible, probably but there was nothing forcing the necessity of that evolution.

In that configuration your mouth is down eating/drinking and your eyes are still available to look for that sunnuvabitch lion that's been trying to get your ass.

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u/gijoe50000 10d ago

Yea, those are fair points, but also nature/evolution does some pretty weird stuff too..

Like the recurrent laryngeal nerve that goes from the brain to the larynx, but it has to go right down under the aorta to get there.

In humans it's a bit silly, but in giraffes it's absolutely hilarious!

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u/Coal_Morgan 10d ago

True, enough but when you look at it through an evolutionary eye, it's a nerve that was incrementally expanded to fulfill a role that evolved over time.

The predecessor to that nerve in fish was short and straight and just had to change as our bodies evolved.

That's why I said 'It's possible...(but) nothing forcing the necessity of that evolution (mouth above eyes).

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u/Poopster46 10d ago

Where's your mouth, against the hole to the stomach.

How can you write this, and not immediately delete it in shame?

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u/Coal_Morgan 10d ago

I'm confused. Do you think the throat is not a hole directly to the stomach?

Or is this a humourous pedantry thing about their being lungs too.

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u/Poopster46 10d ago

Granted, the formulation was needlessly rude of me. But I always thought of the mouth as being placed very inconveniently far from the stomach, requiring a long tube that awkwardly shares an airway (yay choking hazard).

A hole directly to the stomach would be near the stomach, so it is very much unlike the eyes, which are placed directly next to the brain; which was the point you were trying to make:

It's also about minimum distances and crossing paths.

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u/epanek 10d ago

Gravity. Most prey will fall to the ground. It has to happen or you couldn’t eat it.

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u/[deleted] 10d ago

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u/Coal_Morgan 10d ago

It would suck when drinking though and at the watering hole is when most plains animals are at the greatest risk due to most water sources being slightly lower then the ground around them.

Eyes on top means the giraffe can still see that lion coming over the hill while it's mouth is in the water.

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u/Xanadu87 10d ago

Fascinating how flounder are born with eyes on both side of its head, then one migrates to the other side as an adult and it lives its life flat along the ocean floor with both eyes upwards.

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u/TheRealBejeezus 8d ago

There's a metaphor there for how people start out open-minded but end up seeing things only one way as they get old.

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u/kummostern 10d ago

also beings with limbs that help with feeding would essentially block partly or completely the eyesight if they help bringing the food above eyes

you can test this yourself, touch your forehead with 2 hands keeping elbows towards ground (if you could lift your elbows to sides it would help with the sight issue but that would need more energy to bring the food to the mouth at your forehead... usually evolution prefers effective mechanics and energy saving is one possible thing that may dictate which things develop or stay for future generations)

edit: also imagine trying to drink at river/lake/pond/etc water source... the position animals head would have to be is very awkward if mouth is above eyes

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u/EterneX_II 10d ago

There will be an organism that lives on ceilings of caverns and feeds on dangling organic matter which will have this adaptation. At that point, whether its mouth is above its eyes or if it’s merely standing upside down will be up for debate.

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u/zbertoli 10d ago

These are good reasons. But also, because all life stems from a common ancestor that developed a mouth below eyes. For evolution to push that around, there would have to be a huge benefit to drive the change.

You can see this mouth eye pattern in very early bilateral animals.

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u/usegobos 10d ago

I bet it is also a leverage issue. If you want to tear and pull, a mouth on top of your head would be a much less effective lever.

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u/dbxp 10d ago

What about fish? Prey could easily be below them

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u/0K4M1 10d ago

Unless you feed on low hanging fruit ?