View articles by date | ON SCIENCE Home Page | View articles by topic

The isotopes in their bones suggest that dinosaurs were warmblooded

Few things in science are more fun than a good argument. Scientists are a scrappy lot, by and large, and thoroughly enjoy the give-and-take of contesting ideas.

One of the fiercer debates among palentologists (scientists who study fossils) has focused on wether or not dinosaurs were “warm blooded.” Humans like you and I are said to be warm blooded because we maintain a relatively constant body temperature higher than the surrounding air. A lizard, by contrast, adopts the temperature of its surroundings and is said to be cold blooded.

There are other more formal terms that professional biologists use to talk about body temperature, like homeotherm, endotherm, and pokliotherm, but the basic sense of the issue is clear: warm blooded animals like mammals keep their body temperature constant, while cold blooded ones like reptiles don’t.

Because dinosaurs are classified as reptiles, they have been traditionally viewed as cold blooded. This view began to be seriously challenged four decades ago by John Ostrom, an influential professor of paleontololgy at Yale University, and his vocal student Robert Baaker.

Ostrom and Baaker marshalled a battery of indirect but attractive arguments to support the contention that dinosaurs were in fact warm blooded. Their argument about growth rate will provide some of the flavor of the case they made.

Warm blooded animals grow quickly, while cold blooded animals do not. A lion grows to 200 pounds in two years, while it takes an alligator 20 years to grow to 200 pounds.

How fast did dinosaurs grow? Very fast indeed. Studies of tiny dinosaur baby bones in hadrosaur nests by Jack Horner indicate that some of the young reached considerable size while still in the nest, direct evidence that baby dinosaurs grew quite fast.

Additional evidence for rapid dinosaur growth can be found in the microscopic structure of the bones. Dinosaur bones are rich in the passage ways called Halversian canals that transport nutrient-laden blood to the cells that lay down new bone. The fast-growing bones of warm blooded mammals have this same high density of Halversian canals, while the slow-growing bones of cold blooded reptiles have far fewer.

For many years, indirect arguments like these were batted back and forth, sustaining an intense and scientifically rich dispute. Of course, the only way to truly settle the matter would be to somehow take the temperature of a dinosaur, but as they are extinct, this didn’t seem an option.

In 1992, two scientists from North Carolina State, Reese Barrick and Bill Showers, found a way.

They started with the observation that the extremities of a cold blooded animal like an alligator or other reptile are colder than its torso -- the tip of the tail is colder than the interior of the chest cavity because the tip looses heat more readily to the surrounding air. By contrast, a warm blooded mammal like a cow or deer constantly circulates warming blood to its extremities, keeping its tail at the same temperature as the body core.

So Barrick and Showers set out to determine if the extremities of a dinosaur were maintained at the same temperature as its body core (warm blooded), or not (cold blooded).

How could one possibly make such a determination? The researchers were very clever. Bone is composed of the mineral calcium phosphate (CaPO4). The oxygen atoms in calcium phosphate exist in nature in the form of two isotopes, 16O and 18O. Because the bonds formed by 16O atoms break a little easier than ones formed by 18O atoms (physitists call this the “isotope effect”), the oxygen atoms taken up into newly-formed bone tend to be preferentially more 16O at higher temperatures.

The ratio of 16O to 18O can be accurately measured. However it may vary from one animal to another, it should be the same at the extremities and the core of one animal, if that animal is warm blooded. That is just what Barrick and Showers found: In both deer and cow, the 16O/18O ratios are similar in tail and rib bones. In cold blooded animals, by contrast, the 16O/18O ratio was much more different, comparing tail to rib.

So what about dinosaurs? In six different Cretaceous dinosaurs, there was little or no difference between tail and rib in 16O/18O ratios. Comapring the unmineralized interior of T rex bones to rule out possible artifacts, the same result was obtained. Fossil reptiles found in the same deposits with T rex, by contrast, exhibited the difference characteristic of cold blooded animals.

Could this result be just a matter of large body size retaining heat? No. Small dog-sized dinosaurs show the same constancy of 16O/18O ratio.

It looks like Cretaceous dinosaurs were indeed warm blooded.

© Txtwriter Inc.





Txtwriter Home Page | BACKGROUNDERS | ON SCIENCE | Text Books | Book Reviews
This website is the copyright ©1998-2013 of Txtwriter Inc. All rights reserved.
Google