What are the advantages of having two nostrils?

[Monique]

Something struck me as weird yesterday, every animal that I know of has two nostrils. Why? Why is a single large nostril not sufficient?

Nasal congestion might be an answer, but in my experience the congestion affects both nostrils at the same time, so you're still not able to breath through the nose.

 

[mgb_phys]

I assumed it was to tell the direction of a smell - like two eyes or two ears

 

[Andy Resnick]

Ha! Sesame Street recently had a segment where they asked if every animal had two nostrils.

*Dolphins* have one nostril. Whales, too.

 

[Borek]

I can be wrong, but from the videos I have seen I feel like whales - even if they have technically one nostril - seem to have something like nasal septum, so their airduct is still split in half.

 

[Monique]

Ha! Sesame Street recently had a segment where they asked if every animal had two nostrils.

*Dolphins* have one nostril. Whales, too.

You watch Sesame Street?

A whale has a single blowhole through which they breathe, but do they also have a sensory organ in there that detects scents?

mgb_phys, if it were for determining the directionality of smell, wouldn't it be very inconvenient to have them right next to each other, only separated by a septum? The eyes and ears are separated for a reason. I don't know of any animal that has nostrils that are located far apart, they are all part of the same nose. Dogs are supposed to have a really good sense of smell, but if they follow a trail they move their head in order to sense directionality.

 

[mgb_phys]

Presumably whatever worm type thing we evolved from had a nostril on each side and in the water that mattered. It's less of an advantage in air - and we (and dogs) can always move our heads to find the source.
It's probably the mechanics of keeping them open that stop them merging completely.

 

[Moonbear]

Embryologically, our entire head develops as two separate halves, and then fuses in the middle later. So, we have two nostrils for the same reason we have two eyes. This is also the same process that fuses the palate and lips together from both sides (that's why you have that little indentation in your upper lip just below your nose), and why disruption of that process leads to defects like cleft lip or cleft palate.

 

[Monique]

But a single nostril would also be symmetrical. A nose with a single opening could be created in the same way that a mouth is, right?. There are embryonic defects that result in the creation of a single nostril or a single eye, so it is biologically not impossible to create one.

 

[Q_Goest]

Embryologically, our entire head develops as two separate halves, and then fuses in the middle later.

No kidding? I've always wondered if my head fused properly...

But a single nostril would also be symmetrical. A nose with a single opening could be created in the same way that a mouth is, right?. There are embryonic defects that result in the creation of a single nostril or a single eye, so it is biologically not impossible to create one.

Interesting question. Can I ask a stupid one? Are our X chromosomes symetrical? I've always assumed they are not symetrical at all, but never really thought to ask.

 

[mgb_phys]

But a single nostril would also be symmetrical. A nose with a single opening could be created in the same way that a mouth is, right?.

But you would have more difficulty breathing since there wouldn't be a septum to keep it open,
It would have to evolve to be smaller or have more cartilage in the nose part - which aren't really evolutionary advantages.

 

[Andy Resnick]

You watch Sesame Street?

<snip>

Absolutely- I have a 2.5 year old. Seems like you should watch it, too. :)

 

[Andy Resnick]

I can be wrong, but from the videos I have seen I feel like whales - even if they have technically one nostril - seem to have something like nasal septum, so their airduct is still split in half.

according to http://www.enchantedlearning.com/subjects/whales/anatomy/Blowhole.shtml

"Why do some whales have one blowhole and some have two?
Most mammals have two nostrils (blowhole equivalents). One of the nostrils (air-passages) of toothed whales evolved into their echolocation system (the sensing system in which they make and receive high-pitched sounds in order to orient themselves, catch prey, and communicate), leaving them with only one blowhole. "

 

[Proton Soup]

Something struck me as weird yesterday, every animal that I know of has two nostrils. Why? Why is a single large nostril not sufficient?

Nasal congestion might be an answer, but in my experience the congestion affects both nostrils at the same time, so you're still not able to breath through the nose.

we have different experiences.

my guess (and that's all it is) is that since a primary purpose of breathing through the nose is to warm and humidify the air, a septum increases surface area and makes the process more efficient. evolving a single nostril, unlike a single mouth, would be maladaptive.

i think mgb_phys' "worm" is actually the http://www.npr.org/templates/story/story.php?storyId=875863" . and contrary to that story, they are a lovely thing to have in a home aquarium, where they really aren't pesky at all.

 

[mgb_phys]

i think mgb_phys' "worm" is actually the http://www.npr.org/templates/story/story.php?storyId=875863" . and contrary to that story, they are a lovely thing to have in a home aquarium, where they really aren't pesky at all.

Sea squirts are also known as "tenured worms"
As juveniles they are free swimming with a backbone and simple brain, when they find somewhere permanent to land they become fixed to the rock, their backbone and brain dissolves and they develop a thick skin.

 

[Proton Soup]

Sea squirts are also known as "tenured worms"
As juveniles they are free swimming with a backbone and simple brain, when they find somewhere permanent to land they become fixed to the rock, their backbone and brain dissolves and they develop a thick skin.

lol. i think i originally heard that joke as a sea cucumber, tho.

 

[Borek]

As juveniles they are free swimming with a backbone and simple brain, when they find somewhere permanent to land they become fixed to the rock, their backbone and brain dissolves and they develop a thick skin.

Are you sure it is about worms? I know a lot of people that fit the description.

 

[croghan27]

Embryologically, our entire head develops as two separate halves, and then fuses in the middle later. So, we have two nostrils for the same reason we have two eyes. This is also the same process that fuses the palate and lips together from both sides (that's why you have that little indentation in your upper lip just below your nose), and why disruption of that process leads to defects like cleft lip or cleft palate.

Interesting point - I did not know that. Still the question remains why two nostrils - we do not have two mouths and supposedly they (it) developed out of the separate halves you mention.

If the nose retained the double orifice, and the mouth did not there must be some other explanation for our current situation.

 

[hypatia]

While we only have one mouth, just about every thing in our mouths are a mirror image of each side from teeth, taste buds and tonsil's.
The sense of taste {sweet, sour, salty} is very complex, requiring both nose and mouth. But we really only need one way to fill our stomaches.

 

[DaveC426913]

But a single nostril would also be symmetrical. A nose with a single opening could be created in the same way that a mouth is, right?.

Bilateral symmetry.

I expect that the mouth parts developed very early in evolution, before bilateral symmetry, when creautres were very simple in design. But specialized nostrils developed later, after bilateral symmetry was adopted by the more complex organisms.

 

[croghan27]

While we only have one mouth, just about every thing in our mouths are a mirror image of each side from teeth, taste buds and tonsil's.
The sense of taste {sweet, sour, salty} is very complex, requiring both nose and mouth. But we really only need one way to fill our stomaches.

hypatia, that occurred to me as I made the comment - that the mouth, without a mouthy septum, is very symmetrical and balanced. But that still does not answer the original question. Howcome we got two nostrils?

If we are talking about historical priority - then smell has it all over the others, as it is connected to the oldest part of the brain - just about the top of the stem (backbone). We could smell before we could see. Certainly it has had time to get involved in many other things beside just identifying odours - as you say, taste being just one. But could this not be accomplished with a single (maybe whale-like) nose hole?

oh - hypatia - how is your father Theon? Is he still in that little place in Alexandria?

 

[DaveC426913]

If we are talking about historical priority - then smell has it all over the others,

Right so smell may have developed first, but still after bilateral symmetry. The mouth, on the other hand, was developed even earlier.

 

[Ouabache]

Embryologically, our entire head develops as two separate halves, and then fuses in the middle later. So, we have two nostrils for the same reason we have two eyes.

Bilateral symmetry.

We can understand this in terms of their morphology, but it does not explain any advantage over having a single nostril.

Nasal congestion might be an answer.

You may be onto something Monique. Although I've experienced congestion simultaneously in both nostrils; more often when one nostril is congested, the other remains clear. (e.g. I am currently recovering from a cold and while my left nostril is congested, the right is clear).

 

[alxm]

We can understand this in terms of their morphology, but it does not explain any advantage over having a single nostril.

Why would there be an advantage?

Or put it this way.. Given the overall symmetry and so on going on, it seems reasonable to think that it was easier to evolve two symmetrical nostrils rather than a single one. So what benefit would a single nostril give to warrant this change?

 

[Monique]

Why would there be an advantage?

Or put it this way.. Given the overall symmetry and so on going on, it seems reasonable to think that it was easier to evolve two symmetrical nostrils rather than a single one. So what benefit would a single nostril give to warrant this change?

I really don't understand this argument, why would it be easier to create two nostrils than one? We have a single nasal cavity, a circle has a perfect bilateral symmetry.

 

[Monique]

I've found this: http://www.nature.com/nature/journal/v402/n6757/full/402035a0.html". It is a bit old so there may be newer studies, but it is interesting.

 

[Borek]

Right so smell may have developed first, but still after bilateral symmetry.

I think it may depend on what we would call smell. For sure it is one of two chemical senses - the other one being taste.

In the case of air breathing animals smell seems to be connected with detection of molecules dispersed in the air. Here nasal cavity of some kind is a must.

Is there a smell in the case of water animals? If not, smell could not evolve before life left water, that's relatively high on the evolution tree. At the same time in water chemical senses (taste? smell?) were probably present from the very beginning, wiki states that they are aleady present in Cnidaria - that's before bilateral symmetry. Do we classify them as smelling animals?

--
methods

 

[croghan27]

I've found this: http://www.nature.com/nature/journal/v402/n6757/full/402035a0.html". It is a bit old so there may be newer studies, but it is interesting.

That is a good and salient article, Monique - the boots on the feet of a Vandal in a rugby game solved the problem of switching from one nostril to another for me and deviated my septum - as well, to make a pun, a big problem it isnot. (so to speak).

It may be to the point or not, but whales and proposes (porpi) have only one orifice for breathing (along with the mouth) - but the utility of a sense of smell to them would be marginal, as their milieu is aquatic, not gaseous.

However, they are mammals and as such, as has been pointed out before, their 'heads' must have developed in way similar to ours: separate entities that came together - so their single hole and our twin nostrils would seem to have a Darwinian genesis. Their shape or number is the result of natural selection.

 

[DaveC426913]

Bilateral symmetry.

We can understand this in terms of their morphology, but it does not explain any advantage over having a single nostril.

Why do you assume there is an advantage? Not everything in our morphology is a result of recent evolutionary change. Many things we take for granted are intrinsic to our deep evolutionary past.

 

[DaveC426913]

Is there a smell in the case of water animals?

Absolutely!

Smell is a critical sense in most marine animals, arguably even more than land animals, due to the issue of reduced vision and sound.

Ask any shark or other fish.

 

[Borek]

Is there a smell in the case of water animals?

Absolutely!

Smell is a critical sense in most marine animals, arguably even more than land animals, due to the issue of reduced vision and sound.

I think we are not talking about the same thing. I have no doubts that chemical senses are extremally important in water, however, I am not sure how to differentiate between smell and taste in this case. So, when I am asking whether water animals have a sense of smell it is more semantics than biology.

However, assuming they have smell, it is older than bilateral symmetry, as obviously it is present in Cnidaria - and as far as I can tell they predate bilateral symmetry.

--

 

[DaveC426913]

I think we are not talking about the same thing. I have no doubts that chemical senses are extremally important in water, however, I am not sure how to differentiate between smell and taste in this case. So, when I am asking whether water animals have a sense of smell it is more semantics than biology.

What does taste have to do with anything? The mouth does not exist to provide a sense of taste, the mouth exists to eat food. I don't think anyone will argue that eating evolved pretty early...

 

[xxChrisxx]

What does taste have to do with anything? The mouth does not exist to provide a sense of taste, the mouth exists to eat food. I don't think anyone will argue that eating evolved pretty early...

There is always that thing where you smell something, and you can kind of 'taste it' just from the smell.

No idea what that phenomenon is, is it just becuase what you breathed in has a taste or, is it a trick of the mind/senses?

 

[Borek]

Both taste and smell are kinds of chemoreception and they work together. I can be wrong, but I think if you move down the evolutionary tree at some point there was only one chemoreception system. No idea if it should be classified as smell or taste.

 

[Ouabache]

Why would there be an advantage?
Or put it this way.. Given the overall symmetry and so on going on, it seems reasonable to think that it was easier to evolve two symmetrical nostrils rather than a single one. So what benefit would a single nostril give to warrant this change?

Why do you assume there is an advantage? Not everything in our morphology is a result of recent evolutionary change. Many things we take for granted are intrinsic to our deep evolutionary past.

Whether recent or historic, selection pressures allow useful traits to be passed on while others are discarded. Two eyes gives us depth perception, a distinct advantage over creatures with a single eye. With this added piece of information, we can detect and escape predators with greater ease than those with one eye. We are also less likely to walk off a cliff. With only those two examples, there would be a distinct selection pressure for keeping two eyes. Similarly with our ears, we perceive direction information because our ears receive incoming sounds stereophonically. We perceive which direction a threatening predator is approaching and thereby have a better chance take evasive action over those with a single ear.

Monique raises a good point, that a single circular or oval nasal passage would also be bilaterally symmetric. Animals could have just as easily, generated this phenotypic variant. So begs the question, what benefit is there having two nostrils over a single one? For example, does it empower animals to detect food or a mate more efficiently?

 

[alxm]

With only those two examples, there would be a distinct selection pressure for keeping two eyes.

I'd seem so. But the fact that there appears to be an advantage doesn't in itself always mean we evolve or keep that trait. It would seem to be an advantage if I had better sense of smell (most mammals are better than us at it), but I don't - even despite the fact that I've got the genes for it. We have genes for a lot more smell-receptors than are expressed.

Everything comes with a price-tag attached, and while the benefit might be obvious the 'cost' usually isn't.

Add to that vestigial traits, from organs all the way down to genes and the countless eccentricities that don't exist for practical reasons as much having to do with how we evolved, or even specific events in our history. Example: Human Cytochrome c Oxidase has 13 chains, 3 of which are encoded in mDNA and 10 in nuclear DNA. There's no particular benefit to that, but it's entirely consistent with our idea of how the mitochondria evolved.

So I think it's very hard to answer this kind of question unless we've got all the details about what the 'options' were, so to speak.

 

[Andy Resnick]

Whether recent or historic, selection pressures allow useful traits to be passed on while others are discarded. Two eyes gives us depth perception, a distinct advantage over creatures with a single eye. With this added piece of information, we can detect and escape predators with greater ease than those with one eye. We are also less likely to walk off a cliff. With only those two examples, there would be a distinct selection pressure for keeping two eyes. Similarly with our ears, we perceive direction information because our ears receive incoming sounds stereophonically. We perceive which direction a threatening predator is approaching and thereby have a better chance take evasive action over those with a single ear.

Monique raises a good point, that a single circular or oval nasal passage would also be bilaterally symmetric. Animals could have just as easily, generated this phenotypic variant. So begs the question, what benefit is there having two nostrils over a single one? For example, does it empower animals to detect food or a mate more efficiently?

You have to be careful about this type of argument. For example, surely there would be an advantage to being able to see a full 360 degrees (or 4*pi steradians), but not many animals have that trait. It's similar to asking "Why is Ca++ used as a second messenger molecule?" Evolution (at the molecular level) tends to explore a very large solution space, and often finds several equivalent solutions- homologous proteins.

 

[DaveC426913]

Whether recent or historic, selection pressures allow useful traits to be passed on while others are discarded.

Yes, that is the tendency. But it is not a rule.

There are untold numbers of traits we have that have never been selected out. In fact, anything short of the perfect organism means that traits have not been bred out.

We still have three separate brains, including the most primitive "lizard brain" at the base of our skull. We can still choke on food because our trachea and esophagus are joined. etc.etc.

Monique raises a good point, that a single circular or oval nasal passage would also be bilaterally symmetric. Animals could have just as easily, generated this phenotypic variant. So begs the question, what benefit is there having two nostrils over a single one? For example, does it empower animals to detect food or a mate more efficiently?

It is wrong to assume that everything has to be an advantage. It simply does not work that way.

Morphology is a balance between the initial starting configurations and how much evolution can change any trait.

 

[vivanenko]

Without two separate airways through the nose, our sense of smell wouldn’t be nearly as sharp as it is.

Let's say you're out for a walk and you stop to sniff a flower. You inhale, and air containing tiny flower particles rushes up your nose. Sensors in the nose absorb the particles and send smell signals to the brain. Now, although they may look identical, your nostrils go through regular cycles of swelling and shrinking. When the right is swollen, the left is normal, and vice versa. The result is that the swollen nostril inhales a bit more slowly than the non-swollen nostril, which affects how quickly the flower particles reach the inside of your nose.

This is important because there are two basic kinds of smell-producing particles that enter your nose. Some dissolve slowly and others more quickly. When you sniff a flower, the slow-dissolving flower particles need time to settle down inside your nose in order to really register. Breathe them in too fast and many just pass right through without stopping to contribute to that great flowery smell. Meanwhile, the fast dissolving flower bits like to charge in and take over. Caught on a too slow air stream, these particles will dissolve before they really have a chance to make their mark.

And that's why having two nostrils comes in handy. Slowly-dissolving particles do better when inhaled through the swollen, slow breathing nostril. Fast-dissolving particles thrive when taken in through the non-swollen, fast breathing nostril. Working together, your nostrils allow you to smell both kinds of particles.

 

[croghan27]

vivanenko: thank you, what a great response - it covered just about all the bases.

Can I take it that this applies to all mammals with two nostrils? Dogs, cat and elephants?

 

[AyazM]

Good question so here's my contribution. I cannot answer why in the first place there are two nostrils but I can add some interesting dimensions to it. Usually it's just the nostrils that are two; the two cavities unite in the nasopharynx which is the direct continuation of the nostril spaces and continue as one entity into oropharynx and thereby dividing into larynx and oesophagus. The two nostrils are divided by a nasal septum. Diseases inflicting damage to nasal septum cause more damage than expected. Removal of septum is associated with turbulent airflow which leads to crusting of the mucosa and atrophic changes or infectious ones. On the other hand, atrophic rhinitis has the capability of "Enlarging" the nostril spaces and it has the same effect: turbulent air currents in the nostrils. Septal perforations cause the same effect: crusting and damage to the margins of the perforation.

It appears the airflow should be directed in a linear motion straight to hit the roof of nasopharynx where the mucosa would warm it, humidify it and of course the sense of smell. Any variations in the architecture of the nasal cavity, especially the two cavities becoming "one" is very troublesome and a common presentation to ENT surgeons.

I guess that might just be one way of looking at this problem, but evolutionary biology can add some other interesting perspectives to it.

 

[DaveC426913]

Without two separate airways through the nose, our sense of smell wouldn’t be nearly as sharp as it is.

Let's say you're out for a walk and you stop to sniff a flower. You inhale, and air containing tiny flower particles rushes up your nose. Sensors in the nose absorb the particles and send smell signals to the brain. Now, although they may look identical, your nostrils go through regular cycles of swelling and shrinking. When the right is swollen, the left is normal, and vice versa. The result is that the swollen nostril inhales a bit more slowly than the non-swollen nostril, which affects how quickly the flower particles reach the inside of your nose.

This is important because there are two basic kinds of smell-producing particles that enter your nose. Some dissolve slowly and others more quickly. When you sniff a flower, the slow-dissolving flower particles need time to settle down inside your nose in order to really register. Breathe them in too fast and many just pass right through without stopping to contribute to that great flowery smell. Meanwhile, the fast dissolving flower bits like to charge in and take over. Caught on a too slow air stream, these particles will dissolve before they really have a chance to make their mark.

And that's why having two nostrils comes in handy. Slowly-dissolving particles do better when inhaled through the swollen, slow breathing nostril. Fast-dissolving particles thrive when taken in through the non-swollen, fast breathing nostril. Working together, your nostrils allow you to smell both kinds of particles.

Is any of this based on fact? Where is your information coming from?

 

[Sorry!]

Yeah, it's based on science Dave.
[quot]...most of us have some degree of rhythmical change of air flow from one nostril to the other. This is called the Nasal Cycle. It seems that this Nasal Cycle gets weaker as you get older.

Also, if you lay down on one side, then after about 12 minutes, the erectile tissue in the nostril on that side will begin to engorge and swell. This might be due to sensors in your chest and pelvis.

Now if you look at the odour chemicals that land on your olfactory epithelium, you can break them down into two types - the ones that dissolve quickly, and the ones that dissolve slowly.[/quote]

http://www.abc.net.au/science/articles/2000/11/01/198395.htm
It's not definitive though as far as I know.

 

[arildno]

I would assume that the increased surface/to volume ratio in the two-nostril case relative to the one-nostril case has some adaptive advantage(s).

Two immediately suggest themselves:
a) Since smell receptors sit on the surface, rather than inhabiting the volume, the two-nostril case allows for more receptors.

b) Since irritants/invaders get glued to the surface rather than to the volume, the two-nostril case is a more effective barrier to airborne infectants than a single nostril comprising the same volume.