When an animal bites, the muscles in the head pull on the jaw (also called the mandible) to close it. How hard an animal can bite comes down to a few different things, but arguably the most important is the size of the muscles that close the jaw. In this activity we’re going to figure out how much force each group of muscles in the jaw produces, and from that roughly estimate how powerfully the jaws of an animal can be closed, using Tyrannosaurus rex as an example.
How this works:
A few different muscles all act together to help close the jaw for a big bite, and we can figure out how strong they are in a few different ways. An easy way is to cut straight through the middle of the muscle, which gives us a cross-section. If we measure the cross-section and figure out its area, that gives us a pretty good idea of how big that muscle is.
Muscles are made up of lots of little strands called muscle fibres, so if we cut perpendicularly across those fibres, and then measure the area, we get something called the “physiological cross-sectional area” or PCSA. Using the PCSA we can do a little bit of maths to figure out how much force that muscle produces by multiplying it by a number that represents how much force a muscle can produce per millimetre of area (this is called the specific tension). In this case we’ll use 30cm–².
In the case of Tyrannosaurus rex we don’t have the actual muscles anymore, but we can estimate the PCSA of its muscles by looking at its fossilised skull. By looking at subtle marks on the bones where muscles attached and pulled when the dinosaur was still alive, scientists can estimate how big each of these muscles were.
In this case, we’ve grouped those individual muscles into three big groups that work together to help close the jaw, and produce a really big bite force. Those groups are: the pterygoid muscles (where they pull the jaw from and to is indicated by the purple arrow in the picture below), the adductor mandibulae externus (also called MAME, here shown by the red arrow), and the adductor mandibulae posterior (also called MAMP, shown by the blue arrow). As those groups pull on the jaw towards the skull, the jaw rotates around its joint (green circle) and pulls the jaw closed (indicated by the dashed black arrow).
So how powerfully could T. rex actually close its jaws? Well the estimated PCSA data we have for this huge dino is in the table below. If we take each of these numbers and then multiply it by 30cm–² (that specific tension number again) we come away with something that looks like this:
|Muscle Group Name||Physiological Cross-Sectional Area (PCSA)||Total Force Produced in Newtons|
|MAME (Red)||450cm²||13,500 Newtons|
|MAMP (Blue)||100cm²||3,000 Newtons|
|Pterygoid (Purple)||580cm²||17,400 Newtons|
If we add all that force together, and double it to give us the total force for both sides of the jaw, we can see that the muscles in the jaw of this T. rex could produce somewhere around 67,800 Newtons of force!
A lot of other factors play a role in exactly how much of that force can be put into a bite, however (the distance from the joint of your jaw to the teeth you’re biting with; how big you are, etc.), and muscles aren’t always producing 100% of their force with every use. Even so, we currently estimate that T. rex had a bite force of around 34,000-63,500 Newtons! To put that in context, the force of gravity acting on you right now (your weight) is (9.8 times your mass in kilograms) Newtons, an adult human can produce a maximum bite force of just around 400-600 Newtons, and even an alligator can “only” manage around 4,500 Newtons. The range of bite forces shown by animals is incredibly broad and is often (but not always) related to their body size, with small animals such as the domesticated cat producing bites of around 200 Newtons, while their much larger cousins, lions, have a mighty chomp at 1760 Newtons. Knowing what an animal eats doesn’t always help predict how powerful an animal can bite either, with the primarily herbivorous giant panda exhibiting one of the strongest bite forces in all bears, at around 1600 Newtons.
Hopefully this has given you a little bit of an idea as to how scientists figure out how strong muscles of dinosaurs can be, and has shown you just how powerful the jaws of T. rex really were!
Data for Tyrannosaurus rex PSCA is modified from: Gignac, P.M. and Erickson, G.M. The Biomechanics Behind Extreme Osteophagy in Tyrannosaurus rex. Sci Rep 7, 2012 (2017). https://doi.org/10.1038/s41598-017-02161-w
For other perspectives (with bite forces that might be twice as large!), see:
Bates, K. T., & Falkingham, P. L. (2018). The importance of muscle architecture in biomechanical reconstructions of extinct animals: a case study using Tyrannosaurus rex. Journal of Anatomy, 233(5), 625-635. https://onlinelibrary.wiley.com/doi/full/10.1111/joa.12874
Cost, I. N., Middleton, K. M., Sellers, K. C., Echols, M. S., Witmer, L. M., Davis, J. L., & Holliday, C. M. (2020). Palatal biomechanics and its significance for cranial kinesis in Tyrannosaurus rex. The Anatomical Record, 303(4), 999-1017. https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.24219