Animals

Dinosaur Blood Ran Just Right: Not Warm, Not Cold

Exploiting the middle ground as a strategy may have helped dinosaurs rule the Earth for more than 100 million years.

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.

PHOTOS: Dinosaur Claws Evolved from Basic to Badass

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."

Pinocchio Rex: T's Tough Cousin (No Lie)

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."

This article originally appeared on ABC Science Online.

As paleontologists increasingly unearth evidence of feathers in prehistoric fossils, our conceptions of what dinosaurs looked like when they roamed the earth has gradually evolved.

Instead of the reptilian appearance we all recognize from childhood toys and films like Jurassic Park, many dinosaurs in fact more closely resembled birds, kind of like this recently discovered little guy, Eosinopteryx brevipenna, a flightless theropod dinosaur that roamed China during the Middle/Late Jurassic period.

Archaeopteryx, also known as Urvogel, the German word for "original bird" or "first bird," was first discovered in 1860 and later fossils of this species presented some of the earliest evidence of flight in these prehistoric animals.

An intermediate creature that was not quite dinosaur but not exactly a bird either when it lived 150 million years ago, Archaeopteryx had teeth, a long tail, and wings capable of flight with claws at the end for grabbing prey.

A century after the discovery of Archaeopteryx, paleobiologists increasingly found anatomical connections between birds and dinosaurs. In the 1970s, artists began to portray dinosaurs with feathers based on accumulating evidence.

Megapnosaurus, shown here, was another species with whom researchers began early to identify with feathers. A lightweight animal that could reach up to 10 feet in length and roamed Jurassic Zimbabwe, Megapnosaurus, also known as Syntarsus, traveled in packs and preyed on small reptiles and fish.

Many of the fossils unearthed that provided evidence of feathers had deteriotated over the eons they remained buried. It wasn't until 2010 that researchers identified color pigments in feathers from dinosaurs and early birds.

Sinosauropteryx, illustrated here, was a theropod dinosaur that had "simple bristles -- precursors of feathers -- in alternate orange and white rings down its tail," according to a description of the study's findings.

Given that so many feathered dinosaurs were in fact flightless, the purpose of the feathers has been a subject of debate. Some dinosaurs may have evolved feathers for social signaling; others had plumage to provide insulation.

In the cases of some dinosaurs, such as the two oviraptors, herbivores related to T. rex that lived during the Cretaceous period, researchers believe the feathers were used for mating displays, similar to modern-day peacocks and turkeys.

You might be fooled into thinking the animals in this illustration are something between a murder of crows and a band of blue jays. In fact they are Microraptors that lives more than 130 million years ago.

These four-winged, plumed dinosaurs were no larger than modern-day pigeons and sported iridescent tail feathers.

Researchers believe the shimmering plumage was likely used in mating and other social interactions.

Like the diversity among birds today, not all feathered dinosaurs were lightweight, agile animals. A massive tyrannosaur that lived in China until about 65 million years ago, Yutyrannus huali, meaning "beautiful feathered tyrant," grew up to 30 feet long and could weigh more than 3,000 pounds.

This titanic tyrannosaur, as it was described, significantly increases the size range for feathered dinosaurs.

In a stunning find published in the journal Science in 2011, paleontologists uncovered dinosaur feather preserved in amber that dated back some 79 million years ago.

This discovery provided scientists a new window into the evolution of feathers in terms of structure in the evolutionary timeline from dinosaurs to birds. Even shades of color remained well preserved in the amber.