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Nostrils accomplish two things:

  • They filter out impurities and particles in the air. Thanks to the mucous membrane and hairs which act as a maze for the air to pass through. Snot is later drawn in to be digested or expelled.
  • They identify said particles for us to smell. Since they catch particles so easily they are the best place for sensory organs. The jacobson's organ (primary organ for chemoreception) is an essential survival tool for most animals.

Now the question is why a creature have multiple sets of nostrils?

In arid or cold conditions animals opt to have larger noses instead, which is where the difficulty lies in this question.

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    $\begingroup$ It looks like you're asking a question where any possible reason to have multiple nostrils is an equally valid answer, while also asking for any possible set of circumstances that could cause such a feature. We have a policy that states that questions with many valid answers are a poor fit for this site. This seems more like an attempt at getting help generating ideas of brainstorming than a question asking for help with a specific worldbuilding issue. $\endgroup$
    – sphennings
    Mar 24 at 17:35
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    $\begingroup$ "Reason for a creature to have multiple sets of nostrils": For example, tetrapods such as humans (and a handful of fishes) have two sets of nostrils: a set of external nostrils open to the outside, and a set of internal nostrils (also known as choanae) opening in the throat. Other fishes also have two pairs, but both are external: water comes into a pair of incurrent nostrils and goes out through a pair of excurrent nostrils. $\endgroup$
    – AlexP
    Mar 24 at 17:40
  • $\begingroup$ @sphennings That... wasn't the intention. Knowing which questions are too vague is subjective and therefore hard to predict. But it's good that you point this out to idiots like me. $\endgroup$ Mar 24 at 18:09
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    $\begingroup$ I now cannot shake the mental image of an animal that sounds like a pair of bagpipes. $\endgroup$
    – Flater
    Mar 25 at 10:11
  • $\begingroup$ are we dealing with earth life, otherwise you can have one set for in and one set for out, one way breathing is more efficient. $\endgroup$
    – John
    Mar 26 at 3:45

13 Answers 13

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Vocalization.

Indian bamboo flute

That's an Indian bamboo flute, but the principle can be extended to a whole range of musical instruments which use resonance and flowing air.

Why go to all of the trouble of evolving a larynx or syrinx when you can have a much more simple system that works like a flute or a recorder or bagpipes?

Maybe the animal uses its tongue to selectively close the whistle-holes in its bill or snout, or maybe it has special muscular flaps like the nostrils of marine mammals. Whatever the specific mechanism, it can whistle melodiously through its many nostrils for the purposes of attracting mates, threatening rivals or whatever else takes its fancy, assuming it has one.

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  • $\begingroup$ This seems so obvious ... Is anybody aware of an animal that actually does this IRL? $\endgroup$
    – fho
    Mar 25 at 8:38
  • $\begingroup$ One "downside" of the flute model is that for n holes you only get n tones (recorder players will object, but there are only some more tones that you get with combinations). I would propose having multiple "flutes" with one tone each that can be combined into chords. $\endgroup$
    – fho
    Mar 25 at 8:43
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    $\begingroup$ @fho: Flutes have rigid holes, but these animals could have some flexibility in their nostrils, and each pair of nostrils represents a different range that the previous pair of nostrils could not reach using only its flexibility. Much like having gears on a car, each gear can tackle a range of speeds (not just one specific speed), but one gear tackles a different range than another gear. I lack the musical theory vocabulary here, but different pairs of nostrils could represent different octaves/pitches/timbres (I'm not sure which is the best word to use here). $\endgroup$
    – Flater
    Mar 25 at 10:06
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    $\begingroup$ @fho: I reckon than it's biologically easier to increase flexibility in your given voice box (be they nostrils or larynx) than it is to grow another pair with independently controllable airflow (including all the needed internal pathways and required skill in using), which is why we don't have a ready example here; yet have many, many examples of animals having an impressive vocal range using their only voice box. $\endgroup$
    – Flater
    Mar 25 at 10:10
  • $\begingroup$ @fho, might well be more like an ocarina instead. Most ocarinas are designed with uniformly* sized holes for uniformly spaced notes, in which the number of notes the ocarina is designed to sound is roughly the same as the number of holes, but others, especially smaller ones, are designed with exponentially sized holes so that any distinct combination of closed holes generates a different note: e.g. 4 holes -> 16 notes. $\endgroup$ Mar 28 at 17:16
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Evolutionary Leftovers

A creature that originally had spiracles, like an insect, will have had multiple air inlet/outlet openings. Spiracles, howevr, don't offer enough oxygen transfer area to support a body larger than a few tens of grams (see largest beetles and spiders).

So, in order to grow larger, the air passages that the spiracles connect to have evolved into lungs -- air-filled, highly convolute (for large area in a compact volume), sacs with muscular connection to expand and contract them -- but instead of just two lungs like most vertebrates, your former insectoids have one lung for each spiracle. Then, when they switch from exoskeleton to endoskeleton to allow them to get even bigger (than a rat, say), those spiracles become a row of nostrils, each with its own independent lung. They might or might not breathe in unison -- there are advantages either way -- and will most likely have some level of voluntary control.

Species of this line found in dry or cold climates may have extended nostrils, like tiny elephant trunks, to provide better moisture retention and heat exchange, and such extended nostrils (as with elephants) might become auxiliary manipulators.

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  • $\begingroup$ In fact our own single pair of nostrils is, at an earlier stage of embryology, a double pair of nostrils. The second pair migrates inward until it becomes the Eustachian tubes. $\endgroup$ Mar 25 at 2:58
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Keep Bugs Out

Warning: image of centipede crawling up nose.

enter image description here

Large noses are better for smelling and breathing. Your species needs to smell and breath well.

Unfortunately a large nose is attractive to small burrowing insects. There is not a gadfly or centipede alive who does not relish the thought of flying or crawling into that schnoz and settling down for the winter.

The nose is a great place to live. It is warm and humid and there is a never ending source of food, provided you have the mouth parts to feed on blood. You are also near the brain, which is always good, in case you get bored.

Since there were too many bugs trying this, your species evolved to have many small nostrils rather than two big ones. This keeps the larger bugs out.

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    $\begingroup$ That picture is going to haunt my nightmares, thanks! $\endgroup$ Mar 24 at 22:51
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    $\begingroup$ Dude, a little warning before you drop nightmare fuel on us! (Well played, have a +1) $\endgroup$
    – Corey
    Mar 26 at 2:43
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    $\begingroup$ @Corey Always love it when a comment gets more upvotes than the thing it comments on. $\endgroup$
    – Daron
    Mar 26 at 16:31
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Three dimensional smell

Suppose you'd have two noses, and a relatively large, broad head. Like our two eyes do, the noses could provide a 3-dimensional "stereo smell image", the brain then being able to sense the position of the origin of a scent.

Why would it exist ? This creature could be a blind predator, or a night predator. These animals will gain an evolutionary advantage to competitors, because they can locate their prey in the dark.

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    $\begingroup$ This makes a lot of sense. It must exist. $\endgroup$
    – Willk
    Mar 24 at 22:38
  • $\begingroup$ Thx @Willk, btw I put it in a comment earlier, DWKraus answer about EYES the extra heads worldbuilding.stackexchange.com/questions/227070/… $\endgroup$
    – Goodies
    Mar 24 at 23:25
  • $\begingroup$ @Willk I don’t think there is a specific example for olfactory perception, but the sharks of the family Sphyrnidae (commonly known as hammerheads) evolved something like this for their electroceptive (and visual) perception with their rather distinctive flat, wide heads. $\endgroup$ Mar 25 at 12:28
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    $\begingroup$ Interesting idea, but this doesn't accomplish much that can't be done by simply moving a single nose around. Sights and sounds can be short, transient signals, so you really need multiple simultaneous detectors, since the signal might be gone by the time you'd be able to move a single detector in space. But smells don't come or go as suddenly - you can almost always move your head around to determine where a smell is coming from without it disappearing in the meantime, unlike a sudden noise or flash of light. $\endgroup$ Mar 25 at 15:19
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    $\begingroup$ @NuclearHoagie my idea relies on the assumption, the brains will get wired/trained from birth, to support these noses to work simultaneously. This predator does not need to pause and sniff around, it may be able to "know" the location of its prey at any time. That comes in handy when the creature is blind, night animal, or live underground. $\endgroup$
    – Goodies
    Mar 25 at 16:26
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Each set of nostril is specialized for a different environment, in a way that makes the use of single set less efficient than the multi-nostrils approach.

For example a set of nostrils for when the animal is on dry land and breathing air, another set for when the animal is underwater and air is not available, yet sensing is still important.

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They could have evolved different nostrils for different smells. So one set might be like ours now, and the other for something specific, such as pheromones.

They might also have different purposes altogether, such as one set for air with closeable flaps, and one set to act as gills, able to filter oxygen from liquid.

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Different placements for different environments

A Cetacean's nostril is on the top of its head to allow it to breathe while remaining mostly submerged. Alligators have upward-facing nostrils for a similar reason. For a being that routinely finds itself in different positions for long periods of time where much of the body is sunk into an area where oxygen is not readily available, it makes sense to have multiple sets of nostrils in different places so as to be able to continue to breathe in different orientations.

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  • $\begingroup$ Nice answer! Now to find what positions a creature would find itself in and why... more work. $\endgroup$ Mar 25 at 18:59
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This already exists

Most fishes, specifically all ray-finned fishes and most of the non-tetrapod lobe-finned fishes like coelocanths and lungfishes, have multiple sets of nostrils.

enter image description here

Image from n1outdoors.com

enter image description here

Image from https://alphynix.tumblr.com/

It's also thought that some of the extinct marine reptiles had multiple nostrils as well, though it's not entirely clear.

I'm not entirely sure why fishes have multiple sets of nostrils, but the explanation I usually hear is that it has something to do with how smell works in water. Namely that as the fish (or marine reptile) swims forward water is forced into the first pair of nostrils and then exists the nose via the second pair of nostrils. That way there is always water flowing over the sensory organs of the nose and the fish is constantly smelling its environment as it swims forward.

This works in fishes because unlike tetrapods, the nasal cavities of fishes generally do not link up with the digestive trace. In fact in tetrapods the rear set of nostrils ends up migrating into the mouth cavity over the course of evolution to eventually become the openings on top of our palate (choanae) that let us breathe through our nose.

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Redundancy.

Having multiple sets of nostrils lets you keep smelling even if one set gets clogged or inflammed.

You said this:

In arid or cold conditions animals opt to have larger noses instead, which is where the difficulty lies in this question.

But "opt" isn't really the right word. It's simply that these animals' ancestors tended to have larger noses, which provided them with an advantage in their environment, so they became more likely to pass their genes along. It isn't inevitable that that would happen; it simply happens to be what did happen in this case.

In other words, it's entirely possible that creatures which happened to develop extra sets of nostrils did better in their environments, allowing them to pass along those extra-nostril genes, propagating the trait. This would have taken many thousands if not millions of years, but it's plausible.

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    $\begingroup$ There would have to be some environmental pressure where the extra energy needed to grow and maintain a redundant organ would be balanced by the increased rate of survival due to having that redundancy. If the nostrils are located on different parts of the body then it might enable the creature to hide in shallow water for example. If they were on opposite sides of the body then perhaps it might allow the creature to breathe in something like sandstorms while facing into the wind. $\endgroup$ Mar 24 at 22:57
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Swimming

A whale has only one blowhole, and doesn't even really breathe through it's mouth (apparently). The blowhole is located on top of its body because it usually swims with that part up. In contrast, your critter may have two or more swimming modes, e.g.: travel, predatory, hiding(?).

In any event, in each swimming mode it has a different part of its body nearest to the surface of the water, which can allow it to conveniently breathe even while mostly submerged in that mode. Perhaps

  1. in travel the nostrils are dorsally located (like the whale's blowhole), for easy swimming,
  2. for predation the nostrils are on the feet or butt (to enable the critter to peer down in the water looking for game), and
  3. for concealment the nostrils are right next to their eyes (like humans), so that they can look up above the water to see when the coast is clear, leaving only their nose exposed above the surface.
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  • $\begingroup$ Sorry, basically a duplicate of Sean Duggan's answer above. $\endgroup$ Mar 25 at 19:36
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Your species has an incredibly sensitive sense of smell. One set of nostrils contains these scent receptors, tied very strongly to multiple parts of the brain. You can concentrate on a scent and experience the olfactory version of hyperfocus; your other sensory input fades into the background and the bulk of your brainpower is used for processing the scent.

The other set of nostrils has only the most basic scent receptors. This set is more like a horse's nose, designed to efficiently scoop in large amounts of air to fuel the body's aerobic processes while running or otherwise exerting yourself.

The body's default state is for the much smaller scent nostrils to be closed. You can consciously open them when you want to use them, but their intense sensory input can be overwhelming or distracting so they're "off" by default. You reflexively keep them closed when running, otherwise they'll be taking in so much air and generating so much sensory input that you can get disoriented.

Alternatively, your creature could be specialized for tracking, with two noses for differential scent analysis. It breathes in the scent of its prey with one nose, then closes that nose's nostrils to keep the scent in place. It can then track its prey using the other nose, with the brain cross-referencing against the reference sample for more accurate matching.

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  • $\begingroup$ Nice answer, thank you! Cross referencing scents sounds very cool, I’ll keep that in mind. $\endgroup$ Mar 26 at 8:10
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Low-oxygen atmosphere

If the atmosphere has little oxygen (or, almost-equivalently, if the atmosphere is normal but the creature has exceptional oxygen needs due to muscular effort), extra nostrils could provide an adequate supply of air.

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This question has the same answer as why most animals have two nostrils.

Having two nostrils allows for the smelling of left and right, for the same reason some reptiles have forked tongues. However, not all creatures exist in a 3 dimensional space, and of those that do, not all reside on a planar(or non-euclidean planar) surface. Some may need to be able to smell in more directions then just two, for the same reason that some sound systems have two speakers, left and right, while others have multiple, for surround sound(in this case, surround smell).
There may be a need for 4 nostrils, in order to smell up, down, left, and right.
A total of 6 is needed for all three dimensions.
But what if your species needs to smell forwards in time and backwards in time? Well that's eight nostrils at least...
Combined with the other answers to this question, with multiple sets for each direction(for redundancy), and any musical needs of the species, although those might be better done by internal pipes, see here, that's a whole lot of nostrils. Also many animals breathe through there skin, and all of those pores could be seen as nostrils.

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