Largest Dinosaurs Ate Often, Exercised Little

July 6, 2009 -- Some dinosaurs were more than eight times the size of today's largest land animals due to an abundance of resources coupled with low energy expenditures, according to a new study published in the latest Proceedings of the National Academy of Sciences.

Prior research focused on bone structure, growth rates and other factors to explain dinosaur gigantism, but the new study is one of the first to take into account what, and how, the biggest dinosaurs were eating.

"The large herbivorous dinosaurs undoubtedly spent much of their day feeding," study author Brian McNab told Discovery News. "One should notice that the heads of dinosaurs related to the size of the bodies were very small, which means that the dinosaurs spent little time chewing the food, so most processing occurred in the gut. Therefore, the process of eating was probably inexpensive."

"This is very different from the behavior of most herbivorous mammals, which have large heads that house many teeth and spend much time chewing," added McNab, a professor of zoology at the University of Florida.

Since estimates of dinosaur size and weight are possible, and energy expenditure for many modern animals is known, McNab was able to create a model that describes the energy expenditures for all vertebrates. It accounts for any such organism's mass and eating habits.

The model demonstrates a balance between the amount of energy used for body maintenance and growth.

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"If we have two animals with the same total energy intake and expenditure, the one that uses more energy for maintenance will be smaller than the one that uses less energy for maintenance and uses the remaining energy input to facilitate growth," he explained.

Having eight times more edible resources than what's available for land animals today could help to explain dinosaur heft, but grasses -- the principle food for today's herbivores -- were not even around during the Mesozoic.

While herbivorous dinosaurs had apparent easy access to other vegetation, McNab believes plant-eating dinosaur metabolism and resulting body temperature control also helped to support their impressive size and girth.

"A large body size implies a large thermal inertia and a small surface-to-volume ratio, both of which would permit large animals to have rather constant body temperatures, and this temperature constancy would not require high rates of metabolism," he said.

McNab asserts that dinosaur rates of metabolism were probably "intermediate between those of living mammals and probably well above those of typical lizards."