hot question of whether dinosaurs were warm-blooded like
birds and mammals or cold-blooded like reptiles, fish and
amphibians finally has a good answer.
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 say 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
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
"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
"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
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.
"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.