I’m procrastinating on a couple of rather grim posts, and I was looking for something chill to write about on this cool and rainy day, when I came across something rather astonishing. I want to say that while I do think sharks are really cool, they’ve never really been a special interest of mine. That’s why it strikes me as odd that I am now writing a third post about sharks and temperature, none of which have had anything to do with global warming. It started with the recent eruption of the Sharkcano, and we learned about sharks that make their living in and around the natural hot tub of an active volcanic crater. Then, there was the discovery that hammerhead sharks hold their breath to dive, to avoid being chilled by the cold of deeper waters.

And now, we have the novel discovery that basking sharks are warm-blooded, after a fashion. Basking sharks are filter feeders, like whale sharks, megamouths, and baleen whales. They’re big – second only to the whale shark – fairly slow-moving, and they just cruise around, inhaling copepods. They are also, we now know, one of the vanishingly small minority of fish that internally regulate their own body temperature, and it was discovered right in my back yard!

The image is a color-adjusted photo of a basking shark, and frankly it looks like something from a cartoon. The shark is swimming from right to left, and its head is closer to the camera than its tail. This slightly exaggerates the size of its head, but not by much. It looks as though someone took a relatively ordinary shark, and gave it a giant muppet-like shark hat to wear. It has a large, bulbous nose, with two small eyes behind it. Below is a massive, gaping cavern of a mouth, that acts as a trawling net as the rest of the shark pushes the mouth around. This is less of a shark, and more of a mouth with a shark attached to it. It’s near the surface, so the sunlight is forming a pretty dappled pattern on its gray-brown skin. This image was uploaded to Wikimedia Commons by Greenfire Productions.

Approximately 99.9% of fish and shark species are “cold-blooded”, meaning their body tissues generally match the temperature of the water they swim in – but researchers have just discovered the mighty basking shark is a one-in-a-thousand exception.

Instead, these sharks keep the core regions of their bodies warmer than the water like the most athletic swimmers in the sea such as great white sharks, mako sharks and tuna.

The latter examples are so-called “regional endotherms” and are all fast swimming, apex predators at the top of the food chain. Scientists have long reasoned that their ability to keep warm helped with this athletic predatory lifestyle, and that evolution had shaped their physiology to match their requirements.

However, an international team of researchers led by those from Trinity College Dublin, has now shown that gentle, plankton-feeding basking sharks are also regional endotherms despite having very different lifestyles to white sharks and tunas.

This surprising discovery has implications for conservation, as well as raising a plethora of ecological and evolutionary questions.

Haley Dolton, PhD Candidate in Trinity’s School of Natural Sciences, was lead author of the study that has just been published in international journal, Endangered Species Research. She said:

“The basking shark is a shining example of how little we know about shark species in general. That we still have lots to uncover about the second biggest fish in the world – such a huge, charismatic animal that most people would recognise it – just highlights the challenge facing researchers to gather what they can about species to aid in effective conservation strategies.”

Basking sharks gained legal protection in Irish waters just last year, with the species having undergone significant population declines throughout the NE Atlantic in the last century. But they still face many challenges in the future.

Haley Dolton added: “Regional endotherms are thought to use more energy, and possibly respond differently to ocean warming than other fish species. So lots more work will need to be done to work out how these new findings regarding an endangered species might change previous assumptions about their metabolism or potential distribution shifts during our climate crisis, which is something marine biologists are focusing on as our planet and its seas continue to warm. 

“Hopefully this kind of research will continue the momentum needed to effectively protect these incredible animals in Irish waters and further afield.”

Well, ok, it is a little about global warming, because everything is during a catastrophic warming event, but the sharks were doing this before we started mucking up the atmosphere. It seems as though a lot of the heat is generated by their primary “drive shaft” – the cruising muscles that keep them constantly moving forward, but I hadn’t realized that the ability to heat your own body is reflected in how your heart’s put together:

To make the discovery, the research team (including scientists from University of Pretoria, Marine Biological Association, Queen’s University Belfast, Zoological Society of London, University of Southampton, and Manx Basking Shark Watch) first undertook dissections of dead basking sharks that washed up in Ireland and the UK.

They found that the sharks have cruise-swimming muscles located deep inside their bodies as seen in white sharks and tunas; in most fish this “red” muscle is instead found toward the outside of the animals.

They also discovered basking sharks have strong muscular hearts that probably help generate high blood pressures and flows. Most fish species have relatively “spongy” hearts, whereas basking shark hearts are more typical of the regional endotherm species.

Next, the team designed a new low-impact tagging method to record body temperature of free-swimming basking sharks off the coast of Co Cork, Ireland. Researchers were able get close enough to 8 m basking sharks to safely deploy the tags, which recorded muscle temperature just under the skin for up to 12 hours before they automatically detached from the animals and were collected by the researchers.

These tags revealed that basking shark muscles are consistently elevated above water temperatures, and to almost exactly the same extent as their regionally-endothermic predatory cousins.

Nicholas Payne, Assistant Professor in Trinity’s School of Natural Sciences, was senior author of the study. He said:

“These results cast an interesting new light on our perception of form versus function in fishes because until now we thought regional endothermy was only found in apex predatory species living at high positions in the marine food web. 

“Now we have found a species that grazes on tiny plankton but also shares those rather uncommon regional endotherm features, so we might have to adjust our assumptions about the advantages of such physiological innovations for these animals.

“It’s a bit like suddenly finding that cows have wings.”

As Dolton said, it’s a good reminder of how little we know; not just about sharks, but about oceanic life in general. For all the problems in the world, I’m glad to live in a time when I get to learn new things like this.