Lecture 27

Listen to part of a university lecture by a professor of Paleontology.

Professor: For more than a hundred years after the first dinosaur fossils were dug up, paleontologists just assumed that dinosaurs were cold-blooded animals, like modern lizards and snakes are. Dinosaurs were big, and they were slow, and they were stupid- and none of these characteristics are consistent with small, active, intelligent, warm-blooded animals like birds and mammals.

But in the late 1960s, two youngYale scientists, Robert Bakker and John Ostrum, came out with the hypothesis that dinosaurs were in fact faster, smarter and more energetic organisms than science previously thought. This radical idea caused an immediate stir, and the controversy about dinosaur physiology is still continuing today.

First, here are some of the arguments put forth for warm-bloodness, or endothermism, in dinosaurs. ‘Endothermism’ means ‘inside heat’, and an endothermic organism is one that generates heat with its own body in order to counteract a colder environment to keep itself at a constant internal temperature. The arguments for endothermic dinosaurs include:

One- many dinosaurs actually moved pretty quickly- as those of you who have seen Jurassic Park already know- and that would require a high metabolic rate and an internal heater.

Two- the dinosaurs evolved alongside mammals and competed with them for 170 million years, so they must have been competitive with other warm-blooded species.

Three- some dinosaurs had very large bodies and very long necks, so they would need a four-chambered heart and a lot of energy to move their blood those distances.

And four- the structure of dinosaur bone is more similar to bird and mammal bone than it is to typical reptile bone, so they were probably more closely related to the endothermic groups.

Those are some of the arguments that are used to support the idea that dinosaurs were warm-blooded. Of course, none of them prove that these animals were endotherms- they’re just theories- but scientists are continuing their research in order to come up with some more solid data. For instance, Doctor Herman Pontzer and his team of British researchers have recently determined that the energy cost of animal movement, of locomotion, is proportional to the length of their legs and to their leg muscle volume. Pontzer first calculated this with 98 percent accuracy in a wide range of living land animals, and then he applied it to dinosaur measurements. He discovered that a dinosaur’s energy requirements were simply too high for cold-blooded animals to be able to produce.

But overall, there’s still no incontestable evidence for endothermy, and other researchers offer strong arguments for the opposite view- for the old idea of ectothermy in dinosaurs. ‘Ectothermy’, as you should be able to guess now, is the opposite of ‘endothermy’. ‘Ectothermy’ means ‘outside heat’, and it’s used for organisms like reptiles and insects that rely on their environment or on their behaviour to regulate their body heat. They cannot produce heat themselves; they’re cold-blooded, but they can lie in the sun to get warmer or hide in shade to cool down.

As evidence that dinosaurs were ectothermic, more traditional paleontologists suggest such ideas as these-

One- dinosaurs were so big that they didn’t need to be endotherms; they could’ve been what’s called ‘inertial homeotherms’. And now, ‘homeothermy’ should be easy for you to figure out- it means ‘similar heat’. Homeotherms can maintain a relatively constant body temperature with or without producing internal heat. A very large animal, because it has a massive body volume, warms up and cools down very slowly, so that its behaviour can keep it at a relatively constant temperature all day and night.

Two- or there’s a simpler probability: the climate in the Mesozoic Era, the dinosaur era, was much warmer worldwide than now, so cold temperatures were just not a problem.

Three- another clue is that some dinosaur bones show lines of arrested growth, or LAGs, indicating that they grew seasonally, like trees- as many ectotherms, but not endotherms, do.

And four- all warm-blooded animals today have what are called ‘respiratory turbinates’, which are folded bones in the nose area used to minimize water loss when they breathe out warm air. But dinosaur fossils don’t show these respiratory turbinates.

Well, they are all thoughtful ideas, and so the controversy of warm-blooded dinosaurs versus cold-blooded dinosaurs goes on, with strong supporters on both sides, and with no final solution in sight. It’s one of the most interesting problems of evolution, actually. And in fact, the real answer may be something else- some sort of intermediate physiology. Remember that dinosaurs were not reptiles or any other kind of animals that are alive today, so all we have to rely on are for evidence are fossils in stone. This problem may never be solved. Or maybe one day, one of you will help solve this mystery of dinosaur physiology.