Were dinosaurs warm-blooded?

Some chemical reactions in the body, such as the burning of sugar to produce energy while releasing carbon dioxide and water, are referred to as metabolism. These reactions run most efficiently at a particular optimum temperature. In mammals and birds, the optimum temperature is higher than the normal outside temperature and is regulated internally. So these animals maintain generally higher body temperatures and are called "warm-blooded" or endothermic, for their internal temperature control.

Other animals, such as turtles, crocodiles, and lizards, have more variable body temperatures based on their level of activity, and they regulate their body temperatures through external sources, such as warming up in the sun or cooling off in the shade. They are often called "cold-blooded," or more appropriately, ectothermic, for their external temperature control.

Were dinosaurs ectothermic or endothermic? Since birds are a group of living dinosaurs, we know that at least some dinosaurs are endothermic. But further evidence comes from cutting open dinosaur bones. By counting the rings on dinosaur bones, paleontologists have determined the yearly growth rates for many species. While growth rates vary within dinosaurs, no dinosaurs grew as slowly as modern reptiles. Small dinosaurs grew approximately as fast as modern marsupial mammals (which grow more slowly than placental mammals like humans, but faster than reptiles), and larger dinosaurs grew even faster, allowing them to attain extremely large body sizes within the span of two decades. However, relatively speaking, birds are the fastest-growing modern vertebrates—they don’t put on very much mass but can reach adult size in weeks. No dinosaurs grew as quickly as modern birds. Baby dinosaurs also have the woven bone texture typical of fast-growing baby mammals.

The size and density of blood vessels within dinosaur bones provides more strong evidence for endothermy. Dinosaur bones are marked by extensive Haversian and Volkmann’s canals, which are spaces for blood vessels crisscrossing the bone tissue to provide it with nutrients and oxygen. Ectotherms also have these structures, but because their metabolic rates are so low, they have few of them; only very large reptiles develop a large quantity of Haversian and Volkmann’s canals. Endotherms of all sizes have extensive canal networks within their bones, and dinosaurs do, too.

Also, the size of the openings where blood vessels enter the bones are proportionally larger in dinosaurs than they are in any living mammal, indicating that dinosaurs required an even greater blood supply to sustain their metabolism. So, their metabolic rates may have been even higher than those found in modern mammals.