Jaws: From tooth to total – Determining the size of sharks from small tooth measurements
I remember the first time that I saw the 3 sets of shark jaws in the office. Ralph was showing me around on my very first day and a couple of mere moments after I stepped foot in his office, I saw it for the first time. It was right by the door. Recognized it on the spot. A great white, a massive one too. A sawed off yet gorgeous functional row of teeth with those unmistakable serrations on either side of each tooth. I couldn’t believe my eyes!
I had seen a great white before, 12 actually, coincidentally as a client on Slashfin, on the 9th of May 2015. The single greatest day of my life! I was but an impressionable kid, all of 15, with no idea what the future held for me. I always think to myself that, that day was the turning point. I came back 7 years later as a scientific intern and I couldn’t wait to get my hands dirty again.
And there I was standing, completely awestruck, in front of a full jaw of a beast belonging to a species that’s roamed this planet for more than 400 million years. That moment, that one moment is when I knew that I had come back to the right place. I was so uber-focussed on the jaw of the great whites that I had completely missed the jaws of the 2 shortfin makos that were also in the office. But little did I know that I was going to do something so unbelievably cool with the jaws of the adult shortfin mako.
Fast forward to my second month and I had just walked into the office to continue with my project dealing with Catshark population dynamics. I set my bag down, opened my laptop and had just begun to work when Ralph asked me if I wanted to do something different today. I immediately said yes! He wanted me to do estimate the total body size of the adult shortfin mako shark using only its jaw.
Figure 1 – The Shortfin Mako jaw (Left), great white jaw (Right) and the Lowry et. al. paper open on my laptop.
Now I didn’t know squat about any correlation between a shark’s jaw size and its total body length, so I started researching about it. Found this fascinating paper by Lowry et. al. titled ‘Determining shark size from forensic analysis of bite damage’. So, what they did in the research study was that they calculated the interdental distance (distance between two adjacent teeth) and the bite circumference from different species of sharks and created this entire collated formula sheet that mathematically correlated their total body length to their interdental distance. I thought to myself, “Well this is absolutely brilliant, I’ve struck a goldmine!” I grabbed a pair of vernier callipers and got to work.
To calculate the average interdental distance, I located the symphyseal teeth. These are the centremost teeth, closest to the symphysis, or the ridge in the middle of the mandible. Taking the vernier callipers, I calculated the symphyseal interdental distance of the maxilla and the mandible (the upper and lower jaws of the shark), from the tip of one tooth to the tip of the adjacent tooth. Now, since the symphyseal teeth generally had a larger interdental distance than the adjacent teeth, I had to take a couple of more measurements. I calculated the interdental distance between 5 teeth on either side of the symphysis, on both the upper and lower jaws, which generated 11 measurements for each jaw. These measurements were averaged out including and excluding the symphyseal interdental distance due to the aforementioned significant disparity between the numbers. This provided a margin of error for our calculations with lower and upper intervals.
Using these simple measurements, and the formulae gathered through the Lowry et. al. paper, I estimated the total length of the adult shortfin mako to be between 2.69 to 2.8 meters. I cross-referenced the known maximum total lengths of the mako shark, just to make sure that this figure was within the known limits. The largest shortfin mako ever recorded in history was a 3.8-meter beast of a specimen, which meant that the estimations fell well within the plausible size limits. Sadly, the fact that the mako was between 2.69 and 2.8 meters means that the individual was most likely a sexually mature breeding adult, and the removal of this age demographic usually has the most devastating impact on the species’ population replacement rates.
Box 1 – Estimating the Total Length of the shark from the Interdental distance (IDD)
Having gathered all this info on the specimen that was now mounted in all its glory in the office, I couldn’t stop there. I had this burning desire inside me to uncover as much information about the shark as I physically could. I’d always wanted to have an opportunity to do some research with the big enigmatic shark species around the world, and now a prime opportunity had presented itself to do something really cool and interesting with the cousin of the great white. I wasn’t going to let this opportunity pass by without getting as much out of it as possible.
I scoured the internet looking for more calculations and estimations I could make about the shark with the raw data at my disposal, and found a research paper by Ribot-Carballal et. al. titled “Age and growth of the shortfin mako shark, Isurus oxyrinchus, from the western coast of Baja California Sur, Mexico”. This study correlated the growth rate of the shortfin makos with their age, and although it was based off of specimens sampled solely in the Baja California region, I wanted to use the findings from this research study to give us an idea on how old the shark might have been before its unfortunate demise. Using the growth rate formulae collated in the study, I estimated the age of the shark to be between 16.6 to 18.2 years, the range of ages being due to the margin of error introduced through the interdental distance calculations. The estimated age of the shark also added further credence to the hypothesis that this set of jaws belonged to a sexually mature breeding adult, since male makos reach sexual maturity at age 7 whereas females reach sexual maturity at age 15.
Box 2 – Estimating the age of the shark using its total length (TL)
For a few final estimations, I hopped on to the South African Marine Linefish species profile website by SAAMBR (South African Association for Marine Biological Research), to make some approximations about the fork length and weight of the shark. Using the estimated total length, I estimated the fork length (length from the tip of the snout to the fork in the tail) to be between 2.48 to 2.58 meters. Finally, using the fork length, through a simple calculation, the weight of the shark was estimated to be between 174 to 197 kilos. An incredible specimen.
Box 3 & 4 – Estimating the Total Length of the shark from the Interdental distance (IDD)
You might wonder why fork length is used more widely to compare the sizes of sharks. It’s all to do with the structure of the tail. Sharks have 2 lobes in their tail, an upper and a lower lobe. The varied tail structures are determined by the very difference in the sizes of these upper and lower lobes. Makos have a homocercal tail, with a perfectly even and balanced upper and lower lobe. None bigger than the other, perfectly built to cut through water at lightning-fast speeds. Compare this with a pelagic Thresher shark, the upper lobes of which are much, much longer than the lower lobes. This is known as a heterocercal tail. While species with a heterocercal tail like the threshers aren’t quite built for speed, they look extremely graceful when gliding through the water, akin to an ice skater performing a ribbon dance. Now, while comparing and contrasting the threshers and makos, and finding any correlation between their body sizes, the fork length would be a much better estimate than the total body length, due to the simple fact that a thresher’s upper lobe can almost grow to the size as the rest of its body!
To sum up the research project very plainly, I started off with a set of vernier callipers and the jaw of a shortfin mako. I ended up garnering data on it’s estimated total body length, age, fork length and weight. Shocking, the amount of data you can gather from an iota of raw data. To make a tiny adjustment to Dr. Ian Malcom’s classic catchphrase from the greatest movie of all time ‘Jurassic Park’, life science finds a way!
Figure 3 – A fun edited image of a scene from Jurassic Park