What are the advantages of having two nostrils?

[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 because 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.