The largest shark alive today, up to 20 meters in length, is the whale shark, a placid filter feeder. But only 4 million years ago there was probably a shark species of that size, including the fast-moving predator megalodon, famous for its huge jaws and correspondingly huge teeth.
Due to incomplete fossil records, we’re not entirely sure how big megalodon was and can only draw conclusions based on some of its living relatives, such as the great white sharks and mako sharks. But thanks to some new research on his fossilized teeth, we’re now pretty sure he shared something else with these relatives: He wasn’t quite cold-blooded and apparently kept his body temperature above that of the surrounding ocean.
Record temperature
Most sharks, like most fish, are ectothermic, meaning their body temperature matches that of the surrounding water. But a handful of species, part of a group called mako sharks, have a specialized pattern of blood circulation that helps retain some of the heat their muscles produce. This allows them to keep some body parts at a higher temperature than their surroundings. A species called the salmon shark can maintain a body temperature 20°C warmer than the subarctic waters it inhabits.
Megalodon is also a mako shark, and some scientists have suggested that it too must have been at least partially endothermic to maintain its growth rate in the varied environments it inhabited. But, as we mentioned, the megalodon remains we have aren’t even enough to let us know how big the animal was, let alone whether it had the sort of specialized circulatory structure needed for shark endothermy.
So a team of researchers decided to test directly for signs that it was regulating its body temperature using things we actually have: its teeth.
The work is based on a phenomenon known as isotope clumping. If an environment is warm enough, the small weight differences between atomic isotopes don’t matter, as the heat is warm enough to thoroughly mix isotopes in a material. But as things cool, heavier isotopes tend to come together and form clumps in a material. We now have equipment that can monitor the distribution of isotopes in a material at high resolution, allowing direct measurement of the lumpiness. That in turn can be used to estimate the temperature at which the material formed.
(Scientists have used this technique to estimate ancient temperatures to track our changing climate.)
The new work was based on fossil beds containing at least three different types of fossils. One was clearly megalodon teeth. But the others were necessary to provide some degree of external reference for the estimates obtained from the sharks. These included the bones of known cold-blooded fish, which provided a basis for environmental temperatures. They also obtained samples from whale ear bones to have a known warm-blooded control. Crucially, they obtained these samples from widely dispersed locations in the Atlantic and Pacific Oceans, to ensure that any differences were not just a matter of local environmental conditions.
Warm up, move fast
The ectotherm samples showed the kind of regional variations you’d expect from sea surface temperatures, with estimates ranging from a low of 17°C in California to a high of 23°C in the Mediterranean. The megalodon samples, on the other hand, were consistently warmer, with an average temperature difference of about 7°C compared to the cold-blooded samples.
This is not as warm as the whale monsters. But, as the researchers point out, the whale samples came from their inner ears, which are quite far from the environment, so likely reflecting the animal’s internal temperature. In sharks, on the other hand, the teeth are relatively exposed to the environment and so can be between typical body temperature and that of the outside world. The temperature of mako sharks also varies between different body parts.
So why would megalodon have chosen to have an elevated body temperature? There are two possible reasons. One, as noted above, is that the temperatures may have been essential to maintaining the growth rates necessary for something as large as megalodon to develop in non-tropical environments. The second is speed. Warm muscles may be required to propel the animal through the water fast enough to be an effective predator. The mako shark, for example, is the fastest shark and partly endothermic.
Megalodon’s large body size may also have made heat retention a little easier, as it increases its body volume to surface area ratio, meaning less surface area to lose heat compared to the amount of muscle it generates.
However, the authors of the new paper suggest that megalodon may also have become vulnerable to climate change. The high metabolic demands required to maintain endothermy may have made megalodon sensitive to changes in the ecosystem. And by the time of its extinction, the Earth was generally getting cooler, causing sea levels to fall, which would have disrupted coastal ecosystems. And megalodon seems to have relied on coastal nurseries in its early years.
PNAS, 2023. DOI: 10.1073/pnas.2218153120 (About DOIs).
