Like a Hole in the Head: What's the Point of Sinuses?

We're probably all generally familiar with the existence of what are technically termed paranasal sinuses. That's largely because the inflammation of them, known as sinusitis, affects as many as 1 in 8 Americans, and presumably a similar number of people elsewhere, at least in the west. The term 'sinus' just means an empty space in something, and in the case of the paranasal sort, those spaces are inside the bones of our skull, connected by narrow passages to the nasal cavities.

The existence of these passages is important, ensuring that the sinuses are drained of mucus and filled with air... and also allowing any germs that you might have breathed to get into them and make what would otherwise only affect your nose into something worse. But why do we have sinuses at all?

Humans possess four pairs of paranasal sinus and it turns out that this pattern is broadly followed in other mammals, too. (A few non-mammals, such as crocodiles, also have sinuses, but these have presumably evolved separately and possibly for different reasons). Each of these pairs are typically named for the particular bones of the skull in which they are found.

The frontal sinuses are above the eyes, in the lower mid part of the forehead, while the maxillary sinuses are located to either side of the nose in the cheek region. The ethmoidal sinuses are a set of relatively small spaces behind the bridge of the nose, while the sphenoidal sinuses are further back and, possibly because of this greater distance from the nasal cavities, are less likely to be affected by sinusitis, at least in humans.

Obviously, the exact location of these pairs, never mind their size and shape, do vary between mammal species, not least because the shape of skulls vary so much - the exceptionally flat face of humans and our high forehead are both significant here, when comparing us with other species. In fact, there's a fair degree of variation even between members of the same species, and one recent research paper took a detailed look at this variation in the case of a species where some aspects of the cavities have been used diagnostically: the common warthog (Phacochoerus africanus).

A couple of years ago, as part of my series on the pig family, I discussed the warthogs, including some details of their taxonomic history. Two different species of warthog were identified back in the 18th century, largely on the basis of the shape and number of their teeth. In the 20th century, however, these differences were deemed to be insignificant, and the two species were merged into one. Come the 21st century, and modern genetic sequencing, and it became apparent that the two kinds of animal had diverged at least 4.5 million years ago - long before even the beginning of the Ice Ages - and it has since been widely recognised that there are two living species after all.

The thing is, palaeontologists had suspected this for a long time, even though it was thought that one of the species had gone extinct in the late 19th century. The reason for this is that the skulls have a number of differences. As I mentioned in my warthog post back in 2018, the most obvious of these concern the teeth, but there some other, more technical, differences. 

Among these technical differences are the exact shape of a couple of pit-like depressions in the sphenoid bone of the skull. These are much larger and more open in the desert warthog than in the "common" species and this is said to be one way of telling them apart, if a skull is the main thing you have to go on. As you might guess from the name, these pits are in the same bone as the sphenoidal sinuses, but they are supposed to be entirely separate structures; the pits are on the outside of the lower surface of the bone, and don't actually go anywhere.

At which point, I should probably explain what your sphenoid bone is.

Location of the sphenoid bone in humans

While the word 'sphenoid' literally means 'wedge-shaped', this is hardly doing the bone's appearance justice. Unlike many skull bones, there's only one of it, consisting of a central plate that forms the front part of the floor of the braincase, with two large flaring 'wings' stretching away on either side and forming part of the side of the skull behind the eyes. 

And that's really simplifying things. This complex shape is largely due to its evolutionary history since, in reptiles, it is represented by multiple bones that are partially or wholly fused in mammals. But, at any rate, all mammals do have one and, like many other species, warthogs do have sinuses inside them, which connect to the nasal cavity in the usual way. (They also, as it turns out, have the other three pairs of sinuses found in humans as well, all broadly speaking where we'd expect them to be).

This wasn't really a great surprise but what was new was that it turned out, after poking about with some bits of flexible wire in a number of warthog skulls, that those unusual pits do, in the majority of individuals, connect with the sinuses after all. Since the usual access to the sinuses is still present at the other end, what we have here is effectively a hole (the paper uses the term 'aperture') running clean through the bone.

It's worth noting here that domestic pigs, like humans, only have maxillary and ethmoidal sinuses when they are born; the other two pairs, including the sphenoidal sinuses, don't appear until a few months later. In the study, the skulls of the younger animals also tended to lack the pits, suggesting that the two form together. (They were missing in some adults, too, though, perhaps because the samples used came from inbred animals).

This would point to a clear difference between the two species of common and desert warthogs. Paranasal sinuses of all kinds form when a small pit begins to form on the surface of the relevant bones, where it lines the nasal cavity. This steadily gets deeper, and eventually opens out into a larger space or, more accurately, a set of interconnected spaces inside the bone, as specialist cells literally eat away at the solid structure, carving out the interior. The cavity becomes lined with the same type of tissue that lines the interior of the nose (which is why it can produce mucus) and the hole through which the cells originally entered from the 'outside' of the bone becomes the duct through which it drains - and through which infection can enter.

In the case of the common warthogs, the spaces are effectively being dug from two different directions, one from the usual place inside the nose, and the other from where the back of the nasal cavity joins the top of the throat. So far as we know (and studies are likely incomplete) the only other living animals to have anything similar are the babirusas, also members of the pig family. The most likely reason for the presence of this extra passage is that it helps to drain the sinuses, which are unusually large in warthogs.

But why are they so large in this species? Or, to answer our original question: why have sinuses in the first place?

There is no clear definitive answer to this question. Several different possibilities have been raised down the years and they aren't all mutually exclusive, since sinuses could have more than one function. In some species, for instance, they might help increase the resonance of the animal's calls, but that's unlikely to be the case for the majority of mammals. Other suggestions include the fact that they might provide thermal insulation, that they might secrete extra mucus to keep the interior of the noise moist, or that, by increasing the area covered by the nasal lining, they might improve our sense of smell.

It has been shown that, at least in humans, the sinuses produce a small quantity of nitric oxide, which would help fight off microbial infection. In a similar vein, it's also possible that the lining might be able to produce a more active immune response, boosting the protection of this potentially vulnerable opening to the body. Even if neither of these were the original purpose of sinuses, it may be that they are at least a reason for retaining them despite the inconvenient (if usually non-lethal) consequences when they get overwhelmed and sinusitis results. In light of which, it may be worth noting that the extra passage into the sinuses in warthogs happens to start next to the adenoids, which are also important for immune protection.

Perhaps the most common theory, though, is that they're there just to lighten the skull. That would be particularly beneficial for pigs, which have proportionately large heads for their body and need to be able to move them rapidly and easily when they are fighting one another tusk-to-tusk.

In fact, in the case of warthogs, that brings up another possibility that would likely be of more use to them than it is to humans; they could function as shock absorbers. This is almost certainly the case in species that fight by whacking each other about the head with horns, and the frontal (forehead) sinuses can be particularly large in, for example, bison. Pigs don't have horns, of course, but they do wrestle violently with their tusks, and the main force would be directed against the sides of the frontal bones and, to a lesser extent the sphenoid - both of which turn out to have unusually large sinuses in warthogs. 

Indeed, we know that these sinuses help protect the brain from sudden trauma, because you can't stun a domestic pig in an abbatoir by the usual methods - the skull just buffers the blow.

Clearly, humans don't have sinuses to make headbutting each other easier. Perhaps, in our case, lightening the head is more important, or that boost to immune function really does help. Or they could just be there because they aren't enough of a disadvantage to have been evolved away once some function that our distant ancestors needed was no longer required.

[Photo by Charles J Sharp, animation by Anatomography, from Wikimedia Commons.]