The largest shark alive today, reaching up to 20 meters long, is the whale shark, a sedate filter feeder. As recently as 4 million years ago, however, sharks of that scale likely included the fast-moving predator megalodon, famous for its utterly enormous jaws and correspondingly huge teeth.

Because of incomplete fossil data, we're not entirely sure how large megalodon was and can only make inferences based on some of its living relatives, like the great white and mako sharks. But thanks to some new research on its fossilized teeth, we're now fairly confident that it shared something else with these relatives: it wasn't entirely cold-blooded and apparently kept its body temperature above that of the surrounding ocean.

Taking a temperature

Most sharks, like most fish, are ectothermic, meaning that their body temperatures match those of the surrounding water. But a handful of species, part of a group termed mackerel sharks, have a specialized pattern of blood circulation that helps retain some of the heat their muscles produce. This enables them to keep some body parts at a higher temperature than their surroundings. A species called the salmon shark can maintain a body temperature that's 20° C warmer than the sub-Arctic waters that it occupies.

Megalodon is also a mackerel shark, and some scientists have suggested that it, too, must have been at least partially endothermic to have maintained its growth rates in the varied environments that it inhabited. But, as we mentioned, the megalodon remains we have aren't even sufficient to let us know how large the animal was, much less whether it had the sort of specialized circulatory structure needed for shark endothermy.

So, a team of researchers decided to directly test whether there were signs it regulated its body temperature using things we actually do have: its teeth.

The work relies 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 within a material. But as things cool down, heavier isotopes tend to pool together, forming clumps within a material. We now have equipment that can track the distribution of isotopes within a material at high resolution, allowing a direct measure of its clumpiness. That, in turn, can be used to generate an estimate of the temperature at which the material formed.

(Scientists have used this technique to estimate ancient temperatures to track our changing climate.)

The new work relied on fossil beds that contained at least three distinct types of fossils. One was obviously megalodon teeth. But the others were needed to provide some degree of outside reference for the estimates obtained from the sharks. These include the bones of known cold-blooded fish, which provided a baseline for the environmental temperatures. They also obtained samples of the ear bones of whales to have a known warm-blooded control. Critically, they obtained these samples from widely distributed sites in the Atlantic and Pacific Oceans, ensuring that any differences weren't simply a matter of local environmental conditions.

Heat up, move fast

The samples of ectotherms showed the sorts of regional variations you'd expect from seawater temperatures, with estimates ranging from a low of 17° C in California to a high of 23° C in the Mediterranean. The megalodon samples, in contrast, were consistently warmer, with an average temperature difference of about 7° C compared to the cold-blooded samples.

This isn't as warm as the whale samples. But, as the researchers point out, the whale samples came from their inner ears, which are fairly removed from the environment, and so likely to reflect the animal's internal temperature. In sharks, in contrast, the teeth are relatively exposed to the environment and so may be intermediate between the typical body temperature and that of the outside world. The temperature of mackerel sharks also tends to vary across different body parts.

So why might an elevated body temperature have been selected for in megalodon? There are two potential reasons. One is, as noted above, that the temperatures might have been essential to maintain the growth rates needed to allow something as big as megalodon to develop in non-tropic environments. The second is speed. Warm muscles could be necessary to power the animal through the water quickly enough to be an effective predator. The mako shark, for example, is the fastest shark and partly endothermic.

Megalodon's large body size might have also made heat retention somewhat easier, as it increases the ratio of body volume to surface area, meaning there's less surface to lose heat compared to the amount of muscle generating it.

The authors of the new paper, however, suggest that might also have left megalodon vulnerable to climate change. The high metabolic demands involved in maintaining its endothermy could have made megalodon sensitive to changes in the ecosystem. And, near the time of its extinction, the Earth generally got cooler, causing sea levels to fall, which would have disrupted coastal ecosystems. And megalodon seems to have relied on coastal nurseries during its early years.

PNAS, 2023. DOI: 10.1073/pnas.2218153120  (About DOIs).