Why do we , and nearly all animals and birds have two nostrils ?
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I’m a fish biologist, so I’m going to start with fish nostrils.
Jawless fish (lampreys and hagfish) have only one central nostril. In lampreys, this dead-ends into a sac. In hagfish, there’s a duct connecting to the mouth/throat. It’s not clear to me if these nostrils are the same “sort” of thing, and either way they are quite different from nostrils in other fish.
Most fish have _four_ nostrils. Actually they have two nostrils, but the nostrils are U shaped, with two openings at the top of the U. The reason they have this shape is so that water can flow _through_ the nostril…it flows in the front and out the back. This is good because it brings scents into the nostril past the cells that pick up smells. These nostrils are entirely used for smelling (more on that later)
Finally we get to land animals…birds, mammals, reptiles, amphibians, etc. These are known as “tetrapods”, and they are all very much the same sort of animal. At some point during the transition from fish to land animals, it seems that one of those two nostril openings of each fish nostril migrated down the side of the head and around the lip and into the roof of the mouth. So instead of being a U shape, our nostrils lead from snoot to the mouth. This is very useful because it lets us take in air from nostrils and bring them to the lungs while the mouth is closed. This may not seem like a big deal to you, but it’s great if you are a semi-aquatic critter that likes to hang out near the surface of the water with just the tip of your nose out to breathe.
So once we have the basic tetrapod “two nostril” pattern, it’s going to stick around. It’s useful for various reasons like redundancy and smelling (more on that in a sec), but more importantly, it’s just hard to have a mutation that drastically changes nostrils…most mutations that would do that just screw up development enough that the animal wouldn’t survive at all. So tetrapods, from birds to frogs to you, have two nostrils.
**Smelling: the actual reason**
Now that I’ve covered all of the above history, I want to get to the actual reason…smelling. Why have two nostrils instead of just one in the middle? Because it lets an animal know which direction a scent is coming from. If you are a fish swimming around looking for food, you can compare the amount of smell in your left and right nostril and turn toward the side with the greater scent. That’s the great advantage that two nostrils provide over a single nostril. But having extra nostrils beyond that don’t really provide much individual information, because you already have basic directionality. I guess you could argue for “high” and “low” nostrils but even most species of fish are searching across a surface (the bottom of the lake or ocean, or the water near the surface) rather than in pure 3d space of open midwater.
And many land animals continue to make use of nostrils in the same way.
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