While the majority of fish are cold-blooded and rely on the temperature outside of their bodies to regulate their internal temperatures, less than one percent of sharks are actually warm-blooded. The extinct but mighty megalodon and the living great white shark generate heat with their muscles the way many mammals do. However, they are not the only sharks with this warm quirk. A study published November 7 in the journal Biology Letters found that there are more warm blooded sharks than scientists initially believed.
[Related: Megalodons were likely warm-blooded, despite being stone-cold killers.]
Warmer muscles might help these giant carnivores be more powerful and athletic, by using that heat to generate more energy. Regional endothermy in fish has been seen in apex predators like the great white or giant tuna, but there has been debate on when this warm bloodedness evolved in sharks and if the megalodon was warm blooded. A previous study from June 2023 found that the megalodon was warm blooded and that the amount of energy it used to stay warm may have contributed to its extinction about 3.6 million years ago.
The new study looked at the results of autopsies from some unexpected shark strandings in Ireland and southern England earlier in 2023. The sharks belonged to a rarely seen species called the smalltooth sand tiger shark. These sharks are found around the world in temperate and tropical seas and in deep waters (32 to 1,700 feet deep). They have a short and pointed snout, small eyes, protruding teeth, and small dorsal and anal fins and can reach about 15 feet long. Smalltooth sand tiger sharks are considered a “vulnerable” species by the International Union for the Conservation of Nature. While they are not targeted by commercial fisheries, the sharks may be mistakenly caught in nets and may face threats from pollution.
Smalltooth sand tiger sharks are believed to have diverged from the megalodon at least 20 million years ago. The autopsies from this year’s stranded sharks unexpectedly served as a timeline that took marine biologists from institutions in Ireland, South Africa, and the United States back millions of years.
The team found that these rare sharks have physical features that suggest they also have regional endothermy like the megalodon, great white, and some filter-feeding basking sharks. This new addition means that there are likely more warm-blooded sharks than scientists thought and that warm bloodedness evolved quite a long time ago.
“We think this is an important finding, because if sand tiger sharks have regional endothermy then it’s likely there are several other sharks out there that are also warm-bodied,” study co-author and marine biologist Nicholas Payne said in a statement. “We used to think regional endothermy was confined to apex predators like the great white and extinct megalodon, but now we have evidence that deep water ‘bottom dwelling’ sand tigers, and plankton-eating basking sharks also are warm bodied. This raises plenty of new questions as to why regional endothermy evolved, but it might also have important conservation implications.” Payne is affiliated with Trinity College in Dublin, Ireland.
[Related: Were dinosaurs warm-blooded or cold-blooded? Maybe both.]
Scientists believe that the megalodon’s warmer body allowed it to move faster, tolerate colder water, and spread all over the world’ oceans. However, this evolutionary advantage could have contributed to its downfall. The megalodon lived during the Pliocene Epoch (5.33 million years to 2.58 million years ago) when the world cooled and sea levels changed. These ecosystem changes and competition with newcomers in the marine environment like great whites may have led to its extinction.
Understanding how extinct sharks met their end could help scientists gauge how today’s warm-blooded sharks could fare due to warmer ocean temperatures from human-caused climate change. It has potential conservation implications and could explain some shifting patterns of where sharks are foraging.
“We believe changing environments in the deep past was a major contributor to the megalodon’s extinction, as we think it could no longer meet the energetic demands of being a large regional endotherm,” study co-author and Trinity College marine biologist Haley Dolton said in a statement. “We know the seas are warming at alarming rates again now and the smalltooth tiger that washed up in Ireland was the first one seen in these waters. That implies its range has shifted, potentially due to warming waters, so a few alarm bells are ringing.”