The baffling world of bird noses
I'm happy to announce the publication of our latest paper.
Bourke, J.M., Witmer, L.M. 2016. Nasal Conchae Function as Aerodynamic Baffles: Experimental Computational Fluid Dynamic Analysis in a Turkey Nose (Aves: Galliformes). Resp Physiol Neurobiol. 234:32–46. (freely available to 1st November 2016)
Working with my advisor, Larry Witmer, at Ohio University, we tackled the question of conchae function in birds. What are conchae and how do they function? Read on to find out.
Birds are an interesting group of animals that have always sparked the curiosity of humans. Besides their ability to fly through the air, another fascinating aspect of birds is just how much they converged on mammals. This includes their fuzzy covering, extensive parental care and automatic endothermy, among other things.
One interesting point of convergence between mammals and birds is that both groups have what we might call "complicated" noses. The nasal cavities of almost all mammals (essentially anything that's not a whale or dolphin) are filled in thin scrolls of bone known as turbinates. On top of these bones are well vascularized mucosal structures called conchae. Because conchae fill such a large portion of the nose in mammals, they create a huge surface area to volume ratio. This results ina very efficient transfer of heat and moisture from the body to the inspired (inhaled) air, as well as the reverse during expiration (exhalation). Birds, too, are known to house multiple conchae inside their noses. Unlike mammals, though, the conchae are rarely as complicated and often rest on cartilaginous (rather than bony) turbinates. Nonetheless, avian conchae still perform the same physical function of increasing the surface area to volume ratio inside the nose, thus allowing for efficient heat and moisture transfer during respiration.
At least that has been the go to story. If one peruses the respiratory physiology literature, one will find ample examples of effective heat exchange inside the nasal passages of mammals. However, a similar perusal of the literature for birds finds more mixed results. Yes, birds have conchae and yes they offer a similar heat exchange function, but the effectiveness of this function is highly variable. Some studies, such as Geist (2000) found impressive heat transfer within the nasal passages of multiple bird species. However, other studies such as Tieleman et al. (2001) found negligible effects on heat exchange when the nasal passage was bypassed. Further confusing matters are the variable degrees of conchae development in bird species, with some species like cormorants and pelicans, removing their conchae entirely. Others, such as pigeons—which Geist found to have very efficient heat transfer—have almost no conchae at all. So despite having similar nasal structures as mammals, the importance of these structures in birds has been hard to pin down.
Part of the problem was that we have usually thought of conchae as performing a single function (increasing surface area for heat and moisture exchange), but conchae can also do something else: they can direct air to specific parts of the nose. Conchae can function as baffles (i.e., structures that redirect fluids).
Just by being in the way, conchae will affect where air can and cannot go. This is a function that has been hypothesized before for some conchae in birds (Bang 1971), and occasionally mentioned for some conchae in mammals (e.g., Craven et al. 2010), but it had yet to be really tested.
With this in mind, We looked at testing this "baffling" function in birds by using computational fluid dynamics to simulate air movement through a bird nose. In this case, we used a turkey as they were readily available and had nasal structures that are fairly typical for birds.
The nice thing about using digital models is that they allowed us to go in and chop out the different conchae that we were interested in testing, without having to worry about harming the animal (not as important in our case, since it was already dead before we worked on it), or destroying the specimen.
We were able to test the effects of removal of each of the four conchae found in turkeys (atrial, rostral, middle, and caudal), as well as the combined effects of multiple conchae removal. Our results found strong support for conchae acting as aerodynamic baffles, but we were surprised to find that the extent of their effect was fairly limited. For instance, the atrial and rostral conchae of a turkey reside in the nasal vestibule of the nose (front part, see figure below), and their ability to modify the respired air field only extends to that portion of the nose. This limited areal extent means that getting air to reach specific portions of the nose requires either a long enough concha, or multiple conchae working in succession. Birds employ the latter.
Another surprising thing we found was that even a rather simple shaped concha can have a dramatic effect on where air will go. For instance, the caudal concha of turkeys (where odorant molecules are analyzed allowing birds to smell things) is a simplistic little hill-shaped structure. Yet, this little hill was enough to keep an appreciable portion of the olfactory recess (the chamber where the caudal concha resides) separate from the rest of the air field, allowing for air to travel slower in this region, giving time for odorant molecules to bind to receptors, and allowing the birds to smell better.
In the end we found that bird conchae do offer a seemingly important air redirecting function, that may have played an important role in bird evolution (birds did weird things to their skulls as they became more and more aerodynamic). This study offers up a new take on how and why respiratory conchae evolved. That's not to say that birds don't use their conchae for heat and moisture exchange (it's going to happen regardless, so you might as well take advantage of it), only that this other, less publicized function of conchae, means that we need to be cautious about what drove / drives their evolution.
So even though both mammals and birds have superficially similar structures, that doesn't necessarily mean that they evolved for the same reason.
Bang, B.G. 1971. Functional Anatomy of the Olfactory System in 23 Orders of Birds. Acta Anat. 79 (58), 1–76.
Geist. N.R. 2000. Nasal Respiratory Turbinate Function in Birds. Physiol Biochem Zool. 73(5):581–589. Tieleman, B.I., Williams, J.B., Michaeli, G., Pinshow, B. 1999. The Role of Nasal Passages in the Water Economy of Crested Larks and Desert Larks. Physiol Biochem Zool. 72(2):219–226.