In suggesting that dinosaurs had a mammalian-style metabolism, Jurassic Park was, surprisingly for a HolIywood blockbuster, up to date. However, state-of-the-art theory has just done another flip. A researcher at Harvard has been putting iguanas to work on a treadmill, and the results have brought ideas about temperature tumbling down again.
So what, you may say. We know that dinosaurs ruled the world millions of years ago and that some of them were huge and some of them were fierce. What's the big deal about the temperature of their long-vanished blood? Well, for a start, while trying to answer the question, researchers have made intriguing discoveries about the differences between modern mammals and reptiles.
The very word "dinosaur", which means "terrible lizard", puts them in the camp of cold-blooded reptiles - dim, sluggish and oversized creatures which didn't have to worry about their slow metabolism because they lived in tropical swamps. But some years ago, a few researchers began looking at fossil finds which suggested that dinosaurs were rather mammal-like in some ways, such as looking after their young, and being built for speed.
So the idea of warm-blooded dinosaurs began to win supporters, crucially from Dr Armand de Ricqles of the University of Paris. He claimed that mammals, with their higher metabolism, had faster-growing bones than reptiles, and that the blood vessels in them showed up under the microscope as a denser, more intricate structure. Examining dinosaur bones, he declared them to have a warm-blooded structure.
Now this claim is being challenged by the savanna lizards running on treadmills in the laboratory of Tomasz Owerkowicz at Harvard. He has been comparing the structure of exercised reptiles and mammals - squirrels and hedgehogs - with comparable couch potatoes. What he has found tells us something new - that the denser, more complex structures are found in the animals which work out, whether reptile or mammal. What Ricqles had been doing was comparing sedentary dinosaurs with active mammals.
And there is another new piece of evidence for cold-bloodedness, located in the nose. In the Sixties, researchers discovered that the kangaroo rat had coils of cartilage or bone in the nasal passages. At first, they thought this was a unique feature that helped the animal survive in an arid environment by condensing moisture out of the air as it breathed out. But gradually it became clear that virtually all mammals have these coils, known as turbinates, and that we need them because we use up so much oxygen. Reptiles, which have a slower metabolism, do not need them.
So here was a cold-blooded/warm-blooded distinguishing feature which seemed reliable, and could be detected in fossils. John Ruben, of Oregon State University, first looked at prehistoric mammals that lived 160 and 250 million years ago, in the dinosaur period, and found evidence of turbinates. Then he scanned four dinosaur skulls and found - no sign of them.
Some of that, say the believers in cold-bloodedness, is not a problem. Reptiles such as pythons, for instance, are good parents, and leatherback turtles can keep their internal body temperature high as they swim for thousands of miles through the chilly waters of the Atlantic. Some of the smaller carnivorous dinosaurs could well have hunted for only two hours a day - about the same as a lion - even if they did have a slower metabolism.
However, velociraptors, which appear to have been high-speed running machines, do still pose a problem. One idea is that they may have been using a system similar to that found in tuna, which use movement of dark muscle to keep their internal temperature up. "I believe that they were reptiles," says Owerkowicz, "but not exactly as we know them today".