The Air Conditioning in Your Nose

The nasal cavities are not mere holes running through the head. Anatomically, at least, that's broadly true at either end - in the vestibule immediately behind the nostrils, and in the nasopharynx above the throat. But in between, in the area above the mouth and separated from it by the palate, the air instead must pass through defined channels. 

These channels are formed by the "conchae", projections from the outer side of each nasal cavity stretching almost to the inner surface, so that most of the air is forced through the narrow slots between them. These conchae are, in turn, formed by the turbinate bones, delicate, paper-thin, sheet-like structures rolled up like a scroll, and covered in the same sort of fleshy lining that we find in, say, the trachea (windpipe). 

In humans, there are three: two arising from the ethmoid bone at the side of the nose, and one a separate structure that articulates with the maxillary bone in the upper jaw. There is considerable variation in other mammalian species, which can have multiple complex structures split up in ways that defy easy classification.

The obvious question to ask at this point is "why?" In fact, in mammals, there are two reasons, and they relate to different parts of the nose, although there is probably an overlap in some species. Those towards the rear are olfactory, with their lining packed with the nerve endings responsible for our sense of smell. By increasing the surface area of the olfactory lining this allows for more nerve endings, and thus improved smell. They can be especially important in animals with long snouts, such as wolves, mice, and deer, but they are present in all mammals with a sense of smell. 

Turbinates like these are found not only in mammals, but also in birds and some reptiles; even amphibians occasionally have fleshy folds inside the nose, although they lack the bones. Reptiles do not, however, have the other sort of turbinate.

Located at the front of the nose, these are the "respiratory turbinates". The lining of these has numerous small blood vessels close to the surface and a lining that secretes liquid mucus. Together, these warm and humidify the air passing them, increasing the ability of the lungs to extract oxygen from it. As the air is breathed out again, it passes the respiratory turbinates at the very front, which have just been cooled by the air that was breathed in, so that moisture condensates out, reducing water loss. They can also act as baffles to filter out dust.

They're basically air conditioners.

This is essential for effective breathing in warm-blooded animals, which is why birds have them, too. The presence of these structures does, however, have medical side effects. For one, the blood vessels being so close to the surface is what makes nosebleeds relatively common. Perhaps more significantly, inflammation of the conchae, while helping to produce mucus to flush out bacteria or allergens, necessarily swells their lining, forcing the air through narrower passages and creating the stuffy feeling of a blocked nose.

While toothed whales apparently lack turbinates altogether, they are known to exist in minke whales. Since they would have little use for a sense of smell, these are presumably respiratory, indicating the widespread importance of these structures.

Being small and fragile bones, turbinates have not historically been studied as much as other skeletal structures. This has, however, become easier since the development of cheap and effective CT scanning and over the last 15 years or so, there has been interest in how and why turbinate structures vary in different mammals. In the case of olfactory turbinates, for instance, we would expect them to have a larger area in animals with particularly sensitive noses and there has been some research into the way that turbinate structure in cats compensates for their short snouts.

For the respiratory turbinates, however, the expectation is that their size and structure should be more affected by the nature of the air being breathed. Since part of their function is to warm air, for instance, they should be larger in animals from colder climates. This may be further enhanced if the low temperature is due to high altitude, since the air there is thinner and has less oxygen to start with. And that can be important because, as the world warms, some species may be forced to climb to higher altitudes to stay cool, and it would be helpful to know just how adaptable to those changes in air pressure they might be.

But can we? The theory is that adaptation to colder, thinner air requires larger turbinates, but we're only just beginning to check how true that really is. A study published a few weeks ago makes a step in that direction. The authors chose to look at the question by using microCT to examine preserved specimens of closely related species living in an area of the world with dramatic variations in altitude.

Oldfield mice belong to the genus Thomasomys. Originally described by British zoologist Oldfield Thomas in 1882 (although he didn't come up with either the scientific or common names) these consist of at least 40 species, all native to the Andes Mountains from Colombia to Bolivia. Like many New World "mouse" species, they belong to the hamster family rather than that of the true mice, but there's not really any way of telling that by simply looking at them.

The study looked at 20 species, all from Peru and Colombia, and living at elevations ranging from 1,500 and 3,380 metres (4,900 and 11,100 feet). It showed that, relative to the size of the skull, the respiratory turbinates in high-altitude species are, indeed, larger than those of species living on lower slopes. They are also more complex, having tighter curls that increase their surface area, making them more efficient.

Surprisingly, however, the olfactory turbinates were also larger in high altitude species, such as the Inca Oldfield mouse (T. incanus), than in low altitude ones, such as Taczanowski's Oldfield mouse (T. taczanowskii). Perhaps as the respiratory turbinates grow over evolutionary time to cope with colder and drier air, the olfactory ones simply grow with them, a side-effect of the overall adaptation to thin air. 

It's also possible that there is some variation in diet here, requiring a keener sense of smell in the high-altitude species. This could be the case if the local climate affects the plantlife and associated invertebrate fauna, as it surely does... although it seems unlikely that this would be the whole explanation, or it would be less consistent. In any event, the study did show that seed-eating species such as the golden Oldfield mouse (T. aureus) had larger olfactory turbinates than those such as the montane Oldifield mouse (T. oreas) which have a more omnivorous diet and live on the margins, rather than in the heart of, the Andean cloud forests.

Clearly, there is more going on here than just one simple rule. Many different factors are interacting, as is often the case in biology. But this does lend support to the general principle, at least in one group of very closely related animals that should otherwise be broadly similar. It's another insight into an  anatomical feature that's often overlooked.

[Photo by J. Brito, R. Vargas, G. Encarnación and J. Velastegui, from Wikimedia Commons.]