A prehistoric Popeye effect? For the dinosaur, food meant bulk

July 7, 2009

GAINESVILLE, Fla. — A University of Florida biologist thinks he knows how dinosaurs got so big. And it turns out, Popeye and Pachycephalosaurus may have a thing or two in common.

In a paper appearing this week in the online edition of the Proceedings of the National Academy of Sciences, UF biology professor Brian McNab concludes that contrary to common belief, dinosaurs didn’t attain their colossal body sizes because they had more food to eat. Instead, McNab says, like Popeye with his spinach-induced bulging muscles, dinosaurs simply converted more of the energy in their food to body mass.

“Dinosaurs used energy in a different way than mammals use it. Mammals use much of their energy for body maintenance, temperature regulation and activity, and less of it for growth,” McNab says. “Dinosaurs used more energy for growth and less for maintenance.”

McNab argues that the conifers, ferns and other prehistoric plants that proliferated during the Mesozoic era ending 65 million years ago largely constituted “garbage food” that required a lot of energy to digest and provided little energy in return. Despite this limitation, dinosaurs were able to grow as much as eight times bigger than modern mammals, with the largest herbivores reaching weights of 40 to 80 tons or more – six to eight times an elephant’s weight of about 7 tons. (A large rhinoceros relative that lived in the Miocene weighed 11 to 15 tons and was the largest terrestrial mammal.)

Most of today’s herbivorous mammals can eat grasses, which were not present in the Mesozoic, and are a high-energy, more readily digestible forage, McNab noted. At the other end of the scale, plankton have a high energy content and are easy to digest, which is why the largest whales can reach weights of 160 tons, eclipsing the sizes of even the largest dinosaurs, he said.

McNab says dinosaurs attained their gigantic proportions because their biology apparently combined characteristics of modern warm-blooded and cold-blooded animals. On the one hand, they could regulate their own body temperature because of their large size and small relative surface area, he concludes. On the other, their metabolic rates were only about a quarter of those of mammals, but four times those of most reptiles.

Among modern animals, McNab says, those whose metabolisms most closely match those of dinosaurs are the varanid lizards, which include the Komodo dragon and other Monitor lizards.

“If you extrapolate varanid lizards’ metabolic rate up the metabolic rates of elephants, you get an animal that weighs 60 to 80 tons,” he says. “That’s the size of the biggest dinosaurs, the sauropods.”