Recent unexpected shark strandings and surprising autopsies have, paradoxically, transported marine biologists millions of years back in time, as they confront a pressing concern for the future.

The new findings, which contribute additional chapters to the evolutionary narrative of the infamous megalodon shark (the “Meg”), suggest that there may be more warm-blooded sharks than previously recognised. The research implies that these species, much like the Meg, could be at significant risk due to warming seas.

Traditionally, only about 1% of shark species, including notable examples such as the great white shark and the extinct megalodon, were considered warm-blooded or “regional endotherms.” It was long believed that regional endothermy was exclusive to apex predators like the great white shark or giant tuna. However, debates have existed about the timing of the evolution of regional endothermy and whether extinct species like the megalodon were warm-blooded.

A new study led by researchers at Trinity College Dublin has identified anatomical features in the smalltooth sand tiger shark—an ancient species thought to have diverged from the Meg at least 20 million years ago—that suggest it is a regional endotherm. This discovery follows a similar revelation that slow-moving, filter-feeding basking sharks are also regional endotherms. The researchers now propose that the phenomenon of warm-bloodedness in sharks may be more widespread and older than previously thought.

Dr Nicholas Payne, a senior author from Trinity’s School of Natural Sciences, commented:

"This finding is significant because if sand tiger sharks exhibit regional endothermy, it is likely that other shark species might also be warm-blooded. We previously believed that regional endothermy was limited to apex predators like the great white shark and the extinct megalodon, but now we have evidence that it extends to deep-sea ‘bottom-dwelling’ sand tigers and plankton-eating basking sharks. This raises new questions about the evolution of regional endothermy and could have important conservation implications."

The research team, which includes scientists from institutions such as the University of Pretoria, ZSL, University of Zurich, Swansea University, Smithsonian Tropical Research Institute, and University College Dublin’s College of Agriculture, Food Science and Veterinary Medicine, conducted dissections of dead smalltooth sand tiger sharks that washed up in Ireland and the UK.

Dr Haley Dolton, lead author of the study and also from Trinity, added:

"Our scientific understanding continues to evolve, and it is becoming evident that regional endothermy, once thought to have evolved at specific points in history, is now found in an increasing number of shark species with varying lifestyles. When we first identified regional endothermy traits in smalltooth sand tigers, I was initially surprised, but subsequent discoveries in other species may be less shocking.

Above: Drs Nicholas Payne and Jenny Bortoluzzi measure the shark on a rocky Wexford coast. Image credit: Jenny Bortoluzzi and Kevin Purves.

"This finding is intriguing from a marine biology perspective and has significant conservation implications. We believe that environmental changes in the past were a major factor in the extinction of the megalodon, as it struggled to meet the energetic demands of being a large regional endotherm. With the seas warming at alarming rates today, the appearance of a smalltooth sand tiger in Ireland—a shift likely due to warming waters—raises concern about the potential impacts on these species."

Dr Dolton’s research is funded by the Irish Research Council, while Dr Payne’s work is supported by Science Foundation Ireland.