Gharials are weird and so are their pathologies
I'm happy to announce the publication of my lab's latest study. This one is on the strange nasal anatomy of an equally strange group of animals called gharials.
Bourke, J.M. Fontenot, N., Holliday, C. 2021. Septal deviation in the nose of the longest faced crocodylian: A description of nasal anatomy and airflow in the Indian gharial (Gavialis gangeticus) with comments on acoustics. Anat. Rec. 1–21.
Gharials—also known as gavials (genus name: Gavialis)—are an enigmatic group of crocodylians from India that are renowned for their extremely long and thin snouts, bulbous eyes and highly aquatic lifetyles (adults are incapable of standing or walking on land). Most of the attention on gharials has focused on their placement in the tree of life (e.g., Harshman et al. 2003; Brochu 2003; Sookias 2020) and conservation efforts (e.g., Nair et al. 2012; Stevenson 2014; Sharma et al. 2021). Very little analysis has been performed on the natural history and functional morphology of these animals.
The beginning
The story of my lab's experience with Gavialis gangeticus all started with a simple e-mail and photo from the Holliday Lab.
Casey Holliday and his team were doing a separate study on gharials and were given the opportunity to CT scan a deceased, large adult female from the Fort Worth Zoo (TNHC 110000). The specimen was to undergo a diceCT procedure (infusing Lugol's iodine into the tissues to make them more radio opaque for CT scanning). In the process, Dr. Holliday uncovered this strangely wavy nasal septum. Sending an image to me showing the septum, Dr. Holliday asked about potential acoustic functions and whether this was pathological or a trait of gharials in general. So, down the rabbit hole we went.
What was up with Louise?
The first step was to examine the data on TNHC 110000. At the time, an accession number had yet to be given to the animal, so my lab initially referred to this specimen as the Lugol's gharial. Later on in the segmentation process, my lab student and co-author, Nicole Fontenot, nicknamed her Louise. Louise, the Lugol's gharial. It turned out that this simple naming scheme we used in-house would come in handy later on in the paper, as TNHC 110000 does not easily roll off the tongue.
Our initial observations of the nasal passages in Louise indicated that her nose was strongly deformed by this wavy septum. Such deformation was reminiscent of the nasal septal deviations observed in humans and other animals, but unlike other septal deviations, this one was wavy. Usually, if a person or other animal suffers from a septal deviation, the nasal septum has collapsed or bowed into one side of the nose only. This reduces airflow in one nasal passage and requires more airflow to move through the more open side of the nose. In Louise, both sides of the nose were alternating between heavily compressed and wide open.
More specimens reveal a pattern
Still, if we were going to say that Louise had a pathological nasal septum, we needed to have a control group. In other words, we needed to look at more gharials. Unfortunately, CT and even general anatomical data on gharials are extremely rare. Thanks to the efforts of Kent Vliet and the St. Augustine Alligator Farm, we were able to obtain CT data for one other specimen. This was an adult male gharial named Taj. The neat thing about Taj was that he was alive when scanned, giving us a better idea of what the nasal passages in a healthy animal would look like. It turned out that healthy Taj here also had a deviated nasal septum. However, his nasal septum deviation was minuscule compared to Louise. So, a wavy nasal septum is both pathological and a common trait for gharials.
This led us to our next logical question: why do gharials have such a strange nasal septum? Well, we suspect it has to do with those crazy thin snouts of theirs.
Like all crocodylians, the nasal passages reside inside the elongated snout of gharials. So too, do the paranasal sinuses, blood vessels, nerves, and replacement teeth. If you are an alligator or crocodile, there is ample room in those wide, flattened snouts to fit all of these structures. But, if you are a gharial, you have to sacrifice a lot of real estate to make that snout thin. Something needs to give. Gharials have a lot of teeth, and need to maintain their upkeep over their lifetime if they hope to keep eating. So, replacement teeth, nerves and blood vessels need to stay put. Paranasal sinuses are less important and do seem to be reduced compared to other crocodylians (more on that in a bit). Gharials need to breathe air, so the nasal passages can't really be compressed (and compared to other crocodylians, they even seem to be more expanded). What else is left to reduce but the nasal septum. Our results found that, compared to other crocodylians, the nasal septum in gharials is very reduced in width. The middle—most prominent part—of the nasal septum made up <20% of the available airway in gharials. In contrast, the same area of the nasal septum in alligators made up ~40% of the airway. The only crocodylian that appeared similar to gharials here were Crocodylus johnstoni (freshwater crocodile), which is another crocodylian that has evolved an elongate and thin snout (though nowhere near to the same degree).
These findings led us to conclude that the way in which crocodylians elongate their snouts places their nasal septum in danger of becoming deviated. Which means that extinct crocodyliformes that also elongated their snouts, such as the pholidosaurids, likely also suffered from various degrees of septal deviation. We can extend that to other, unrelated taxa like champsosaurs as well.
All in all, these were interesting findings with broader implications for analogous extinct taxa, and for gharial conservation management. For instance, Louise's long life (she died at 50) was likely due in part to good management from her zookeepers. Wild gharials may not be so lucky, making this a potential area of concern for farms that are raising captive specimens for future release.
Gharial acoustics (noses that make sounds)
There was another area of our study that has unfortunately been overlooked in most of the news articles. That would be the strange acoustic properties of gharial noses.
Aside from using this opportunity to simply describe the weird anatomy and physiology of gharial noses, we also wanted to delve into the acoustics of gharials. Unlike other crocodylians, gharials are considered to be a fairly quiet group of crocodylians (Dinets 2013). This may be in part due to the reliance on jaw claps and jaw slaps in other crocodylian species (a behaviour that benefits from having a strong, wide snout). That said, recent data from Jeffrey Lang and colleagues has indicated that gharials may be more vocal than we had once assumed. Gharials certainly stand out from other crocodylians in being the only species to have nasal structures that are believed to function solely in acoustics.
One of these structures is the eponymous ghara seen in males. This is an enlarged, bulbous structure residing around the nostril of mature (and dominant) male gharials. I say nostril, because one of the findings of our study was that gharials only have a single nostril. The nasal passages split into two about 1 cm below the confluent nostril (deeper if the animal has a well-developed ghara). This nostril confluence may be important for sound production in the species. In dominant males, forceful expulsion of air from the nose causes vibration of the ghara, producing a unique buzzing sound that seems to attract the ladies.
On the opposite end of the nose is another structure believed to function in acoustics. This is an enlargement of a set of paranasal sinuses that have infiltrated the pterygoid bones. These sinuses create a large, egg-shaped structure referred to as the pterygoid bulla. Other crocodylians show pterygoid sinuses to varying degrees, but only gharials form this unique bulla.
Prior work on gharial pterygoid bulla only gave a passing reference to this structure as a possible acoustic resonator. We went deeper in our analysis, running our CFD code through the nose, we could see how this structure likely does function as a resonator, at least in males (Louise, our female, also had pterygoid bullae, but they were not as well defined). We discovered a distinct constriction between the anterior and posterior chambers of the bullae in males. This "pterygoid waist" created a Bernoulli effect, alternately compressing and decompressing air that moved through it during a forced expiration. Much like popping a champagne cork, this sudden release of pressure, forced air to jet into the pterygoid bulla and would have likely made a "pop" sound. Such jaw "pops" have been recorded in gharials before. It has previously been compared to the jaw claps of other crocodylians, but was noted for being produced without having to move the jaws. We suspect that this is how gharials are able to perform this unique feat.
We're not done yet!
Which brings us to the end of our first study of gharials. I say first, because they are such a fascinating group of critters that I don't see my lab veering away from them just yet. They represent one of the few instances we no of where an animal is using their nose to not just accentuate sound, but to actually produce it. Needless to say, this has direct implications for other alleged nasal resonators. Namely, these guys:
Till next time.
References
Brochu, C. 2003. Phylogenetic Aproaches Toward Crocodylian History. Ann. Rev. Earth. Plan. Sci. 31:357–397.
Dinets, V. 2013. Long-distance signaling in Crocodylia. Copeia, 3:517–526.
Harshman, J., Huddleston, C.J., Bollback, J.P., Parsons, T.J., Braun, M.J. 2003. True or False Gharials: A Nuclear Gene Phylogeny of Crocodylia. Syst. Biol. 52(3):386–402.
Nair, T., Thorbjarnarson, J.B., Aust, P., Krishnaswamy, J. 2012. Rigorous Gharial Population Estimation in the Chambal: Implications for Conservation and Management of a Globally Threatened Crocodilian. J. Appl. Ecol. 49(5):1046–1054.
Sharma, S.P., Ghazi, M.G., Katdare, S., Dasgupta, N., Mondol, S., Gupta, S.K., Hussain, S.A. 2021. Microsatellite Analysis Reveals Low Genetic Diversity in Managed Populations of the Critically Endangered Gharial (Gavialis gangeticus) in India. Sci. Rep. 11(5627).
Sookias, R.B. 2020. Exploring the Effects of Character Construction and Choice, Outgroups and Analytical Method on Phyogenetic Inference from Discrete Characters in Extant Crocodilians. Zool. J. Linne. Soc. 189(2):670–699.
Stevenson, C.J. 2014. Conservation of the Indian Gharial Gavialis gangeticus: Successes and Failures. Int. Zoo. Year. 49(1):150–161.