Hot-Blooded or Cold-Blooded??

Back in 1968, Robert T. Bakker, a young aspiring paleontologist finishing an undergraduate education at Yale, wrote a brief article, "The Superiority of Dinosaurs," in which he contended that dinosaurs were "fast, agile, energetic creatures." According to Bakker, their advanced physiology was so incredible that even the 10-ton ceratopsians could outrun a human. A year later, Dr. John H. Ostrom (also at Yale) described an unusual new theropod dinosaur, Deinonychus antirrhopus, which he envisioned as "an active and very agile predator." These men were not the first to propose an active nature for the dinosaurs; many others had before; but their work can be cited as the beginning of the ongoing debate on dinosaur physiology that rages even today, making the news with every new hypothesis or controversy.

We'll take a quick review of what has transpired in the last few decades. Form your own opinions on what dinosaurs were like. Were they sluggish and stupid as the old conventional wisdom said, or were they mammal or bird-like in their high activity as in Jurassic Park? Or can we even be sure what they were like? A few preliminary warnings:

  1. Dinosaurs like T. rex and Diplodocus were not mammals. They were not birds. There are significant differences between these groups. Since we have no dinosaurs alive today (except for birds, which have about 145 million years of evolutionary separation from non-avian dinosaurs), it is hard to compare them with anything living whose metabolism we understand.
  2. We are only now beginning to understand living mammal and bird metabolism. Understanding dinosaur metabolism is complicated by our lack of knowledge of modern animal metabolism.

  3. The issue is a tangled, complex one. There are not just two sides to the issue; there are numerous competing hypotheses. If you're looking for a major controversy in science, look no further!

What is "hot-blooded"? What is "cold-blooded"? Actually, there is no such thing scientifically. The popular term "hot-blooded" basically means having an average body temperature higher than that of the surroundings. "Cold-blooded" is obviously the opposite. Scientists have more technical terms that they use:

  • Endothermic: Generating internal heat to moderate body temperature, e.g., modern birds and mammals.
  • Ectothermic: Relying on the environment and behavior to regulate body temperature, e.g., typical reptiles.

  • Homeothermic: Maintaining a constant internal body temperature, e.g., modern mammals, birds, and some others.

  • Poikilothermic: Having a fluctuating internal body temperature depending on the local environmental conditions, e.g., typical reptiles and actinopterygiian fish.

Lines of evidence

The evidence for endothermic dinosaurs
The evidence for ectothermic dinosaurs
Summary of current hypotheses

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