By Jonny Lupsha, Wondrium Staff Writer
Were the dinosaurs warm-blooded like birds or cold-blooded like reptiles? Unfortunately, the answer isn’t simple. Each possibility has its merits. Some species may have run warm, while others ran cold.
Dinosaurs were a specific type of reptile, and they came from an ancestry of reptiles that are cold-blooded. However, scientists have since learned that birds are also descendants of dinosaurs, and, of course, birds are warm-blooded. So, did dinosaur blood run warm or cold? Even in 2022, the jury is still out, as a recent study pointed to conflicting evidence—and even the possibility that it varied from species to species.
Thermoregulation and metabolism are at the heart of the issue of warm-blooded and cold-blooded animals. In her video series Rediscovering the Age of Dinosaurs, Dr. Kristy Curry Rogers, Professor of Biology and Geology at Macalester College, examines the evidence for both cases.
What’s the Difference Between Warm-Blooded and Cold-Blooded?
When categorizing modern and extinct animals, we think of warm-blooded animals as having higher body temperatures than their respective ambient temperatures and cold-blooded animals as having body temperatures that are much more closely linked to their surroundings. However, it’s much more helpful to think of both in terms of thermoregulation and metabolism.
“Thermoregulation relates to the constancy of body temperature,” Dr. Rogers said. “There are two options. You might be an organism that maintains a relatively constant body temperature, called a homeotherm, or you might be an organism whose body temperature is more variable, in which case you are a poikilotherm.”
Metabolism also influences body temperature. Some animals’ metabolic processes generate enough heat energy to optimize their internal chemical reactions. These animals are called endotherms. Animals whose metabolic processes don’t generate enough heat energy, and they need help from the Sun to warm up, are called ectotherms. Most mammals and birds fall into the former category, while reptiles, amphibians, and fish fall into the latter category.
“There are animals that cross the boundaries of these categories,” Dr. Rogers said. “Some scientists have argued that dinosaurs may have been similar, using terms like mesothermy, gigantothermy, or mass homeothermy to describe the possible process for dinosaurs. This is an interesting idea, but one that is hard to apply to dinosaurs as a group.”
Why Did Some Dinosaurs Have Feathers?
According to Dr. Rogers, one of the best indicators of dinosaur metabolism is taken from “bone histological data.” Histology is the study of the microscopic structure of various tissues. Dr. Rogers said that she and her colleagues used bone histology in dinosaurs to determine that dinosaurs grew faster than reptiles.
“Maintenance of these high rates of growth for years of life history points towards homeothermic endothermy, in the style of birds and mammals,” she said.
Another characteristic that points toward homeothermic endothermy in some dinosaurs is that some dinosaurs had insulation.
“Feathers first evolved in dinosaurs and, at the start, they are more akin to downy insulator feathers than to the flight feathers we commonly associate with modern birds,” Dr. Rogers said. “If we link this to modern animals, the only ones that have insulation—whether it’s hair, feathers, or fat—are warm-blooded.”
Cold-blooded organisms not having insulation is partly because they want to ensure the rapid transfer of heat from the Sun to their bodies when they’re cold, and insulation would slow that down. On the other end of the spectrum, if cold-blooded organisms are too hot, insulation like feathers would only slow down their heat dissipation.
So, while dinosaurs are often associated with cold-blooded reptiles, ample evidence suggests that at least some were warm-blooded, as well.
Rediscovering the Age of Dinosaurs is now available to stream on Wondrium.