Passage IV

NATURAL SCIENCE: This passage is adapted from “Were Dinosaurs Cold Blooded?” by Brendan J. Koerner (©1997 by U.S. News and World Report Inc.). Warm-blooded creatures maintain a nearly constant body temperature under most conditions.

The claim that dinosaurs were warm blooded has divided the paleontological world into rival camps. For some, endothermy, the scientific name for warm bloodedness, is the only way to explain the dinosaurs' evolutionary success. Without the ability to keep their bodies at optimum temperatures regardless of their surroundings, they argue, dinosaurs could never have dominated the globe for 160 million years. Skeptics counter that ectothermy, the proper label for cold bloodedness, was the logical strategy for dinosaurs living in the Mesozoic Era's generally sweltering heat—and, this group claims, the only option that is supported by physiological, rather than circumstantial, evidence.

The revisionist view that has so captured the public imagination has long been led by Robert Bakker, a crusader against slow-moving, dimwitted, crocodilian dinosaurs. He painted a picture of dinosaurs that were every bit as endothermic as humans. Instead of spending their days lazily basking in the sun and occasionally trudging along at a torpid pace, Bakker's dinosaurs moved at constant speeds, their postures fully erect in the manner of birds and mammals. “Meat-eạting dinosaurs related to Tyrannosaurus rex cruised at 3 to 4 miles an hour,” claims Bakker, who bases his conclusion on fossilized footprints. “No turtle anywhere cruises at 3 to 4 miles an hour.”

Bakker and his acolytes also point to dinosaurs' relatively fast growth as evidence of endothermy. Mammals and birds, which develop quickly compared with ectothermic reptiles, have bones characterized by microscopic channels that appear complex and crystal-like under the microscope. These elegant patterns form when growing bone meets and meshes with connective tissue, capturing blood vessels in dense, woven structures called Haversian canals. Armand de Ricqlès, a University of Paris anatomist, found that dinosaur bones exhibited those same intricate channels rather than the simpler, less dense structures common to rep-tiles. “We see the same well-vascularized bone in mammals but not in turtles and crocodiles,” says Kevin Padian, a paleontologist at the University of California-Berkeley. “The way the bones grew, dinosaurs seem to have been active all the time.” That pace of activity, argue Bakker and his cohorts, is the telltale sign of warm bloodedness.

Many within the scientific community remain wary of Bakker's claims. Since measurements show that edotherms require up to 20 times more food than ectotherms, some question how the gigantic dinosaurs could possibly have eaten enough if they were warmblooded.

The evidence based on bone-structures has come under fire, too. Tomasz Owerkowicz, a Harvard University researcher, has asserted that the dense canals that de Ricqlès detected could have resulted from physical exertion rather than endothermy. In an ingenious experiment, Owerkowicz gave cold-blooded monitor lizards regular treadmill workouts and then compared their bones with those of nonaerobicized contemporaries. The well-exercised group showed the same kind of complex channels characteristic of mammals, birds, and de Ricqlès's dinosaurs, suggesting that Haversian canals are causally linked to an active lifestyle rather than warm bloodedness. South African histologist Anusuya Chinsamy has also countered some of the bone structure argument, contending that dinosaur bones exhibit bands called lines of arrested growth. These are characteristic of modern-day ectotherms, whose growth rate speeds up and slows down according to seasonal temperature fluctuations. Chinsamy concluded that dinosaurs grew at a more reptilian pace than envisioned by the Bakkerites.

The ectothermic side has sought to boost its case with hard physiological evidence. Turbinates, tiny whisps of bone or cartilage deep inside the nasal cavities of mammals and birds, make warm bloodedness possible by limiting water loss. Turbinates have never been found in living ectotherms—nor in dinosaurs.

Many on the cold-blooded side now use the term “gigantothermy” to describe the unique energetics of large dinosaurs. Being huge is one way to maintain a relatively constant body temperature despite cold bloodedness: Large things—which have a lot of bulk in relation to their skin area—lose heat to the outside world much more slowly than do small things. If dinosaurs were indeed cold blooded, the slow heat loss associated with gigantothermy would allow them to stay relatively warm—and avoid a reptilian torpor—when confronted by the night or an overcast day.