Dinosaurs not warm or cold-blooded
Just right Dinosaurs were neither warm-blooded like mammals, nor cold-blooded like reptiles, instead they were somewhere in between, suggests a new study.
Exploiting the middle ground as a strategy may have helped dinosaurs rule the Earth for more than 100 million years, scientists report today in the journal Science .
The question of whether dinosaurs were lumbering cold-blooded or active warm-blooded animals has been debated for decades, but finding a definitive answer has proven difficult.
Now, biologist John Grady from the University of New Mexico and colleagues, have developed a new method for analysing dinosaurs' metabolic rates.
Building on previous work by palaeontologists and physiologists, they created a large database on growth and energy in both living and extinct groups of vertebrates including 21 species of dinosaurs.
They then used statistical analyses and energetic models to determine the relationship between growth rate and energy use.
Annual growth rings in fossils were used to determine growth rates, while metabolic rates were estimated by using changes in body size as an animal grows from birth to adult (known as ontogenetic growth).
"We found that growth rate is a good indicator of energy use in living animals. Warm-blooded (endothermic) mammals grow 10 times faster than cold-blooded (ectothermic) reptiles, and metabolise 10 times faster; in general doubling one's metabolic rate leads to a doubling in growth rate," Grady explains.
However, when they examined the growth rates of dinosaurs, although there was some variation in the rate they grew, they had neither the high metabolic rate of mammals and birds, nor the low metabolic rate of reptiles.
"Surprisingly we found that, instead, they occupied the middle energetic ground."
Today, mesothermic animals are uncommon, but living species come from across the evolutionary spectrum, and include leatherback turtles, tuna, great white sharks and the echidna.
These animals at times rely on internally-generated metabolic heat to maintain body temperatures, while being subject to external temperatures in others.
"They generate enough heat to warm their blood above ambient temperature, but don't do anything to maintain it, such as shivering which humans do when they are cold," says Grady.
"Meanwhile, echidna body temperatures can fluctuate by up to 10 degrees when they are active."
Dinosaurs evolved around 200 million years ago, and competed for resources with ectothermic animals like lizards.
Their higher metabolic rate meant they could move faster making them a more dangerous predator, or more elusive prey, says Grady.
"A higher metabolic rate gave them other competitive advantages as well: they could grow faster and reproduce faster.
"But being completely warm-blooded like a mammal limits the maximum size an animal can reach — it is doubtful that a lion the size of T. rex would be able to eat enough wildebeasts (or elephants) without starving to death.
"With their lower food demands, however, the real T. rex was able to get really big while still maintaining their advantage over their competition."
As well as helping us understand how warm-blooded animals evolved, understanding dinosaurs' energy use challenges our understanding of how life operates, Grady explains.
"They were ecologically dominant for more than 100 million years, and understanding how they lived and what contributed to their dominance helps us understand why some animals win over others.
"Dinosaurs' intermediate lifestyle may have been the key to their evolutionary success. Against today's polarised landscape, dinosaurs stand out as a successful middle way."