Warm-blooded? Cold-blooded? Dinosaurs were apparently 'just right'

Thursday, June 12, 2014, 5:30 PM - One of the biggest debates in the science community over the years has been about whether dinosaurs were cold-blooded, like lizards, or warm-blooded like mammals. A new study is shedding light on why it's been so difficult to conclusively pin this down, as it finds that they took the middle road between the two.

Since dinosaurs only left behind fossil remains for scientists to examine, it's been difficult to know exactly what they were like when they were alive. When these fossils were first discovered, scientists saw enough in common with lizard skeletons that they assumed dinosaurs must have been bigger lizards, and thus cold-blooded (or 'ectothermic'). That means that their body temperature would have been mainly determined by the temperature of the air or water around them. However, over the years, enough evidence has been found to suggest that at least some dinosaurs were warm blooded (or 'endothermic'), generating their own heat from their metabolism to maintain a constant body temperature, but also requiring more energy from their diet than cold-blooded animals. 

Along with this difference in the energy requirements, growth rate differed as well, with warm-blooded animals growing faster than cold-blooded animals. This is where a team of researchers from the University of New Mexico looked in their search for a definitive answer on dinosaur metabolism. They gathered information on the metabolism and growth rate of many species of animal, and compared this database to the growth rate of dinosaurs - which can be seen from their bones.

Credit: John Grady/University of New Mexico

They found that dinosaur growth rates weren't as fast as those of mammals, but they were definitely faster than those of reptiles. Relating this to metabolism, this meant that, rather than skewing to one end of the 'spectrum' or the other, dinosaurs look like they were in the middle - closer to what sea turtles, tuna, and great white sharks are like these days, which are 'mesothermic'. For example, while most sharks are cold-blooded, great white sharks have a strip of red muscle at their core, which generates heat as they swim, keeping their core temperature roughly 14 degrees C warmer than the temperature of the water around them. So, their body temperature does go up and down, like a cold-blooded creature's does, but they can still take advantage of some of the benefits of warm-bloodedness.

"This higher energy use probably increased speed and performance,” John Grady, a graduate student at UNM who was involved in the study, said in a statement. “Mesothermic dinsoaurs were likely faster predators or better able to flee from danger than the large reptiles found earlier in during the Mesozoic."

This middle-of-the-road metabolism - 'tepid-blooded'? - is apparently one of the reasons why dinosaurs become very successful, especially compared to their cold-blooded lizard cousins. They grew faster, and were able to grow very large, while still keeping their energy requirements manageable, and this allowed them to climb to the top of Earth's ecosystems and remain their for around 135 million years.

The study, titled Evidence for mesothermy in dinosaurs, was published today in the journal Science.