By Dr Chris Wacker, UNE Discovery

With great anticipation, I await the sequel to 2018’s The Meg, Meg 2: The Trench, which came out on the 3^rd of August in Australia. I have a confession: Monster movies are a guilty pleasure of mine, going right back to a childhood passion for Jaws. As a Zoologist, this can be hard to admit, as perpetuating animal myths is somewhat of an academic sin. However, sometimes it’s nice to abandon the need for realism and just enjoy a good monster movie. As with most of these movies, there is some truth to them, and movie makers take a ‘creative licence’. So why is this article in the Discovery (science) newsletter? Because very, very recently, scientists have discovered that the big fish, Megalodon was ‘warm-blooded’, and that may have been the cause of its demise (not Jason Statham).

Megalodon

Megalodon (Otodus megalodon) was a shark that went extinct approximately 3.6 million years ago. It could grow to be at least 15 m long; while massive for a shark, it was still only about half the size of a blue whale. For comparison, great white sharks can grow up to 6 m long, and the whale shark can grow up to 12 m. There has been a lot of discussion about how a big apex predator like Megalodon could become extinct. Could it be related to its body temperature? In a recently published paper (linked at the bottom of this article), researchers found that Megalodon had a higher body temperature than other sharks at the time, which may imply that it could produce enough body heat to maintain this higher temperature.

Temperature affects everything

Life depends on temperature. All of our internal processes are temperature-dependent, so animals can’t get too hot or too cold. Birds and mammals, including humans, are endotherms. This means that we produce enough heat via our metabolism to keep us warm, independent of the temperature of our environment. Endotherms also need a good layer of insulation in the form of fat, feathers, or fur to keep in the heat we produce; otherwise, it would be lost to the environment. Being endothermic means our bodies can function in most environments, and birds and mammals can explore and thrive across much of the Earth.

Ectotherms and endotherms

Ectotherms, such as reptiles, fish and amphibians, on the other hand, have a lower metabolism and not as much insulation, so they lose heat to the environment. This means that they can’t be as active in colder environments, and therefore you don’t see snakes in Antarctica. Thermal biology is important because it dictates where an ectothermic animal can go, how long it can be there and how active it can be. Endothermy can be considered an advantage over ectothermy because endothermic animals aren’t as dependent on the temperature of their environment, but the disadvantage is that it costs more energy (i.e., food) to run an endothermic body. Ectothermy, on the other hand, is energetically cheaper. This is why it is important not to consider endotherms as being more advanced than ectotherms and appreciate that they are just two different ways of life, and ectotherms are well-suited to their environments.

The complicated world of thermal biology – it’s not just about ectotherms and endotherms

As with many things in biology, thermal biology can get a little complicated. You will see the terms cold-blooded and warm-blooded used interchangeably with ectotherm and endotherm. These terms are pretty awful for thermal biologists to hear. Why? Well, when a snake or lizard is basking in the sun, it is still an ectotherm, but it is hardly cold-blooded, and when a pygmy possum is hibernating, and its body temperature is a low 5 degrees Celsius, it could hardly be called warm-blooded.

There are also different types of endothermy, but we will just focus on the one relevant to this great Megalodon discovery:  regional endothermy. Regional endothermy is a type of endothermy where fish, for example, can maintain some of the heat produced from their metabolism using some pretty nifty physiological mechanisms, such as the organisation of their blood vessels. Some types of fish can also maintain certain parts of their body at higher temperatures than the surrounding water because of how large they are. The movement of muscles as they swim produces heat, and because the fish are large, the heat isn’t lost to the surrounding water as fast, and they can maintain an elevated body temperature. This is common in species of Tuna.

Endothermic Megalodon

Getting back to Megalodon: the question the researchers have asked is whether or not the higher energy requirements of being endothermic may have been too much for Megalodon and made it vulnerable; needing more food means more energy needed to hunt and more time put into hunting and therefore more risk. But what about whale sharks? The whale shark is big enough to maintain any heat produced so it can cope with deep dives, but it doesn’t produce extra heat, so it doesn’t have higher energy requirements.

Thermal biology and fossils

It is difficult to determine any physiological properties of extinct animals because organs don’t fossilise. So how can these scientists determine anything about the thermal biology of Megalodon? Oxygen can exist in different forms called stable isotopes: oxygen-16 and the rarer oxygen-18. The ratio of these two isotopes in a fossil depends on where the animal lived and its body temperature. By comparing the oxygen in the bones and teeth, they can find clues to body temperatures.

Conclusion:

I, for one, was pretty excited to read about this Megalodon discovery. I suspect, though, that as with most things related to thermal biology, the outcome of this discovery will lead to more wonderful and surprising findings and give us more clues about the thermal biology of extinct animals and, in turn, more clues as to how they lived, and how they died.

References:

Dickson, K.A. and Graham, J.B. (2004). Evolution and consequences of endothermy in fishes. Physiological and Biochemical Zoology. 77 (6): https://doi.org/10.1086/423743.

Grady, J.M., Enquist, B.J., Dettweiler-Robinson, E., Wright, N.A. and Smith, F.A. (2014). Evidence for mesothermy in dinosaurs. Science. 344 (6189): 1268 – 1272.

Griffiths, M.L., Eagle, R.A., Kim, S.L., Flores, R.J., Becker, M.A., Maisch, H.M., Trayler, R.B., Chan, R.L., McCormack, J., Akhtar, A.A., Tripati, A.K. and Shimada, K. (2023). Endothermic physiology of extinct megatooth sharks. PNAS, 120 (27) e2218153120 https://doi.org/10.1073/pnas.2218153120.

https://www.pnas.org/doi/10.1073/pnas.2218153120

McNab, B. (1983). Energetics, body size, and the limits to endothermy. Journal of Zoology. 199 (1): 1-29.

Rey, K., Amiot, R., Fourel, F., Abdala, F., Fluteau, F., Jalil, E., Liu, J., Rubidge, B.S., Smith, R.M.H., Steyer, J.S., Viglietti, P.A., Wang, X. and Lécuyer, C. (2017). Oxygen isotopes suggest elevated thermometabolism within multiple Permo-Triassic therapsid clades. eLife. 6: 10.7554/eLife.28589.