Ancient sharks really put the “mega” in Odontus megalodon, with their ~18-centimeter (7-inch) toothy remains indicating they were likely at least 15 meters (50 feet) long (though we still don’t really know what they looked like). New research in Historical Biology posits a theory as to what made megatooth sharks grow so large, suggesting that colder waters may have been a driver for supersizing.

While cold ancient oceans might seem like a trying environment for thriving, the association with bigger body size is actually because animals need more mass to be able to survive the frosty conditions.

“Larger body size increases the ability for animals to retain body heat more efficiently by minimizing heat loss,” Kenshu Shimada, a paleobiology professor at DePaul University in Chicago, told IFLScience, “so those animals will be able to remain metabolically active in colder waters.”

Megalodon’s enormous body size may have instead been dictated by its geography, suggests Shimada and colleagues, in following an ecological pattern known as “Bergmann’s rule”. Introduced by German biologist Carl Bergmann in the mid-1800s, it outlines the theory that large size is an adaptation to cooler climates in boosting an animal’s ability to stay warm.

If true of O. megalodon, it would suggest that there may have been smaller megatooths in warmer waters while the supersized specimens were limited to the ocean’s cooler climes.

“The main conclusion of this study is that not all geographically different Megalodon individuals grew to gigantic sizes equally,” said Shimada in a statement. “The common notion that the species reached 18–20 m TL should be applied primarily to populations that inhabited cooler environments.”

Megalodon nursery sites have previously been suggested as explanations for collections of smaller than average teeth, but the study authors identified that these spots are associated with the warm waters of the equator.

“It is still possible that O. megalodon could have utilized nursery areas to raise young sharks,” said coauthor Harry Maisch, a faculty member at Bergen Community College and Fairleigh Dickinson University in New Jersey.

“But our study shows that fossil localities consisting of smaller Megalodon teeth may instead be a product of individual sharks attaining smaller overall body sizes simply as a result of warmer water.”

As well as floating the adorable notion of mini-me megalodons, the findings could inform the future of modern-day (and comparatively already smaller) sharks, and how they might respond to an ever-warming world.

Next, Shimada hopes to learn more about the life and times of megalodon by taking a much closer look at their teeth.

“In collaboration with a number of geochemists, I am currently investigating the biology and lifestyle of O. megalodon and other extinct sharks, such as their diet and metabolism, based on chemical signature preserved in their fossilized teeth,“ he said.