Hot new finding: Some sharks are warmer than you’d expect

New research arising from a collaboration between scientists at the University of Pretoria (UP) and Trinity College Dublin has shown there are likely more warm-bodied sharks than previously thought.

While most sharks have body temperatures equal to that of the water they’re swimming in, some retain heat and maintain body temperatures several degrees above that of the water.

These are warm-bodied sharks, or what scientists refer to as regional endotherms – “regional” because they only keep certain tissues warm, typically the red muscle used to power routine swimming.

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Some of the most famous sharks are warm-bodied, like the great white shark.

Keeping warm in cold water is difficult because all the blood pumped from the heart goes directly to the gills, where it comes into very close contact with the cold surrounding water, resulting in rapid heat loss. But warm-bodied sharks have a trick up their sleeve (or under their pectoral fin).

Dr Ned Snelling, an experimental physiologist from UP’s Faculty of Veterinary Science, explains: “Warm-bodied sharks have red muscle positioned toward the core of the body, which helps keep it warm by insulating it farther from the cold water.

“Perhaps more importantly, warm-bodied sharks also have a complex network of blood vessels that act as a counter-current heat exchanger, where cool blood arriving from the gills comes in close contact with warm blood draining from the muscle.

“As these two sources of blood run alongside each other, heat is exchanged across the vessel walls, so by the time the cool blood reaches the red muscle, it is nearly as warm as the muscle itself.

“It’s a remarkable system because it allows heat to be retained internally at the site of production – the red muscle.

“Warm muscle functions better than cold muscle and so it allows these sharks to remain active and mobile in low-temperature waters.”

Not just the macropredators

New research, involving Dr Snelling and shark scientists Drs Nicholas Payne and Haley Dolton from Trinity College Dublin, was published in the Royal Society’s Biology Letters. It shows that at least two additional sharks are likely to have warm bodies.

What makes the finding surprising is that these sharks are not top-level macropredators. Instead, the smalltooth sand tiger shark is a rather shy and elusive deep-water species found on the ocean bottom, where it feeds on smaller fish and invertebrates.

And the basking shark is a slow-moving, filter-feeding, gentle giant of the ocean. Payne said this finding was important because it means “it’s likely there are several other sharks out there that are also warm-bodied.”

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