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 Dinosaurs, for eons Earth's dominant land animals until being
wiped out by an asteroid 65 million years ago, were in fact
somewhere in between.
Scientists said on Thursday they evaluated the metabolism of
numerous dinosaurs using a formula based on their body mass as
revealed by the bulk of their thigh bones and their growth rates as
shown by growth rings in fossil bones akin to those in trees.
The study, published in the journal Science, assessed 21 species of
dinosaurs including super predators Tyrannosaurus and Allosaurus,
long-necked Apatosaurus, duckbilled Tenontosaurus and bird-like
Troodon as well as a range of mammals, birds, bony fish, sharks,
lizards, snakes and crocodiles.
"Our results showed that dinosaurs had growth and metabolic rates
that were actually not characteristic of warm-blooded or even
cold-blooded organisms. They did not act like mammals or birds nor
did they act like reptiles or fish," said University of Arizona
evolutionary biologist and ecologist Brian Enquist.
 "Instead, they had growth rates and metabolisms intermediate to
warm-blooded and cold-blooded organisms of today. In short, they had
physiologies that are not common in today's world."
There has been a long-standing debate about whether dinosaurs were
slow, lumbering cold-blooded animals - as scientists first proposed
in the 19th century - or had a uniquely advanced, more warm-blooded
physiology.
As scientists unearthed remains of more and more fast-looking
dinosaurs like Velociraptor, some championed the idea dinosaurs were
as active and warm–blooded as mammals and birds. The realization
that birds arose from small feathered dinosaurs seemed to support
that view.
University of New Mexico biologist John Grady said the idea that
creatures must be either warm-blooded or cold-blooded is too
simplistic when looking over the vast expanse of time. Like
dinosaurs, some animals alive today like the great white shark,
leatherback sea turtle and tuna do not fit easily into either
category, Grady added.
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"A better answer would be 'in the middle.' By examining animal
growth and rates of energy use, we were able to reconstruct a
metabolic continuum, and place dinosaurs along that continuum.
Somewhat surprisingly, dinosaurs fell right in the middle," Grady
said.
The researchers called creatures with this medium-powered metabolism
mesotherms, as contrasted to ectotherms (cold–blooded animals with
low metabolic rates that do not produce much heat and bask in the
sun to warm up) and endotherms (warm–blooded animals that use heat
from metabolic reactions to maintain a high, stable body
temperature).
Grady said an intermediate metabolism may have allowed dinosaurs to
get much bigger than any mammal ever could. Warm–blooded animals
need to eat a lot so they are frequently hunting or munching on
plants. "It is doubtful that a lion the size of T. rex could eat
enough to survive," Grady said.
(Reporting by Will Dunham; Editing by Marguerita Choy)
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