Yes, I'm serious about this.

May 8, 2011 4:02 AM

Why do humans (and most mammals) have two nostrils instead of one? Seems redundant.
posted by bardic to Science & Nature (18 answers total)

What's wrong with redundancy?
posted by jon1270 at 4:06 AM on May 8, 2011

So you can blow your nose?
posted by Not Supplied at 4:08 AM on May 8, 2011 [1 favorite]

I mean if you had a uninostril, it could cause a dangerous pressure if you tried to blow it.
posted by Not Supplied at 4:09 AM on May 8, 2011 [1 favorite]

So you know which direction smells are coming from.
posted by kisch mokusch at 4:13 AM on May 8, 2011 [4 favorites]

Because stereo smelling is an actual thing.
posted by ceribus peribus at 4:14 AM on May 8, 2011

Best answer: Actually, you only breathe through one nostril at a time. Maybe the obstruction isn't as total as I've been led to believe, though? Might also be interested in the debunking of a NYT article about how the different nostrils smell things differently.
posted by soma lkzx at 4:22 AM on May 8, 2011 [2 favorites]

Best answer: Because stereo smelling is an actual thing.

Yup.

And then, to hear the pranayama afficionados tell it, there's this. (PDF)
posted by likeso at 4:24 AM on May 8, 2011

Best answer: Also some detail here:

But if the same scent molecules drift up both nostrils when we take a sniff of, say, a rose, why isn’t one enough? It turns out that tissue lining each nostril shrinks or expands with blood flow. When one side is slightly swollen, the other is free-breathing. This cycle goes on day and night, making the airflow through each nostril slightly different. (Test this for yourself by pressing a nostril shut and sniffing through the other side. Now switch.)

In the last few years, researchers think they have solved part of the "two nostrils" puzzle. Some scent molecules dissolve quickly in mucus; others take longer. So slow-dissolving molecules that are whisked too quickly through the nose don’t have a chance to register. Meanwhile, fast-dissolving molecules have their biggest scent impact when they are swept into contact with a large swath of neurons. When we inhale a scent, having two nostrils with different airflows allows us to better detect both kinds of odor molecules, giving us a more complete smell picture of an object.

posted by ceribus peribus at 4:28 AM on May 8, 2011 [8 favorites]

It's also worth noting that humans are bilaterally symmetric because of the genes that govern our early development. Even if there wasn't a huge benefit to stereo nostrils we might still have them.
posted by atrazine at 4:52 AM on May 8, 2011 [8 favorites]

Best answer: Nostrils evolved a very long time ago in land vertebrates, before even limbs.
posted by exogenous at 5:27 AM on May 8, 2011 [2 favorites]

"Most mammals?" Out of curiosity, which mammals have only a single nostril?
posted by ricochet biscuit at 7:33 AM on May 8, 2011

Cetaceans.
posted by likeso at 7:45 AM on May 8, 2011

Or at least, two external nostrils. See sperm whale.
posted by likeso at 7:49 AM on May 8, 2011

Best answer: QI dealt with this. (starts at 2:10)

David Mitchell and Stephen Fry!
posted by you're a kitty! at 8:59 AM on May 8, 2011 [2 favorites]

Best answer: Also, it seems that two nostrils have a larger surface area than one nostril encompassing the same volume, because the septum also has a surface area - a larger surface area will expose more molecules to absorption... it's the same principle behind gills, the more the merrier, within limits. Further, it may be that perhaps the presence of a septum has additional benefits because the septum may contain a different tissue type (this is pure speculation, I have not studied this particular physiology), and you wouldn't have a septum in a mono-nasal passage.

This also allows for the following bit of trivia: in the 1961 Italian film "Il Posto", there's a nice bit of dialogue "Trust everyone, except those who have two nostrils". So another advantage to two nostrils is that it allows us to judge trustworthiness, at least according to an Italian saying :).
posted by VikingSword at 1:16 PM on May 8, 2011

It's very rare for me that both nostrils are ah, clear at the same time. Usually I can only get any air to come in through one of them, and that one not very well either. If I only had one nostril I'd probably die in my sleep when it closed up every night, as they seem to want to do.
posted by jenfullmoon at 9:46 PM on May 8, 2011

VikingSword: Presumably even in a mono-nasal setup you could develop turbinates to fulfil those functions (except for deceit).

What animals have more widely separated nostrils? Horses etc have somewhat-separated nostrils but I can't think of any animals with really wide "stereo" nostril separation (except for animals that don't combine breathing and olfaction, like moths).
posted by hattifattener at 12:22 AM on May 9, 2011

It's not that you have just two nares, you also have two of everything else back up in there. The septum separates the two halves.
posted by pwb503 at 12:18 PM on May 20, 2011