For over a hundred years, researchers have assumed this dinosaurs were like giant lizards: sluggish reptiles that basked in the sun most of the day. This view changed when we started to realize that dinosaurs were much more similar birds than for modern lizards. Today, researchers agree birds are technically dinosaurs – the only ones to survive the mass extinction 66 million years ago. But if that’s true, why aren’t birds cold-blooded like most modern-day reptiles?
The answer is simple: most dinosaurs were probably warm-blooded too.
Birds are descended from a diverse group of two-legged dinosaurs called theropods, which include giant, carnivorous predators such as Tyrannosaurus rex, as well as the smaller 3 foot long (1 meter) Mononykus.
Like mammals, birds are warm-blooded or endothermic, meaning they regulate their own body temperature internally. Endothermic animals have a higher metabolic rate, which allows for more physically demanding activities, such as flying, but requires more calories to sustain them.
‘Warm-blooded animals are generally more active’ Holly Woodwarda professor of anatomy and paleontology at Oklahoma State University told LiveScience. “They can be active well into the night. And so it’s an evolutionary strategy in the sense that you can forage for food while other animals can’t because they’re too cold and too slow.”
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Birds generally have a higher metabolic rate than mammals of similar size and maintain their body temperature — between 106 and 109 degrees Fahrenheit (41 to 43 degrees Celsius). Hummingbirds flapping their wings 720 to 5,400 times per minuteshould consume about half their body weight every day, or eat every 10 to 15 minutes.
In contrast, cold-blooded animals, or ectotherms, such as most modern reptiles and fish, rely on their environment to change their body temperature. Because they do not use much energy on heating themselves, they do not need to eat as regularly; alligators can go for example over a year without food.
For years, researchers assumed that because most modern reptiles are ectothermic, ancient reptiles must have been too.
“When you look at living animals and make an assumption about the state of their ancestors based on their current state, you so often end up on the wrong foot.” Jingmai O’Connorassociate curator of fossil reptiles at the Field Museum in Chicago, told LiveScience.
Attitudes began to change around the late 1960s discovery of a bird-like specimen called Deinonychus. Since then, researchers have found physical features that indicate many dinosaurs, including ancient birds, were warm-blooded. The presence of feathers is one such indication: feathers help animals retain body heat, which is not necessary in ectotherms.
In her lab, Woodward has looked at another proxy: the microstructure of bone tissue. She has discovered that endotherms have many different bones than ectotherms, mainly because ectotherms tend to grow more slowly. This growth rate is reflected in the mineral component of the bones, which she described as “small fibers.”
“I imagine them a bit like pick-up sticks: When you grow very slowly, those fibers tend to orient themselves parallel to each other, and so they flatten out,” Woodward said. “But as you grow faster, the fibers are kind of a jumbled mess,” which is the type of structure she often sees in warm-blooded bones.
Her observations have shown that the bone structures of dinosaurs are more similar to those of birds and mammals than to those of crocodiles.
It is unclear exactly when warm-bloodedness first appeared. All dinosaurs (including birds) and crocodiles share a common reptilian ancestor, and both Woodward and O’Connor said there is good evidence that this ancestor was warm-blooded – meaning endothermy arose before the dinosaurs. Cold-blooded dinosaurs are said to have appeared later.
But it’s possible that endothermy showed up even earlier. If both mammals and most reptiles were endothermic, maybe their common ancestor, which lived about 310 million years ago, was also endothermic. However, endothermy most likely evolved independently in mammals, O’Connor said.
However, future research could challenge these ideas. “We make so many assumptions,” O’Connor said, “and then the data proves us wrong.”