For more than 100 years, researchers assumed that the dinosaurs they were like giant lizards: sluggish reptiles that spent most of the day basking in the sun. This image changed when we began to realize that dinosaurs were much more similar to birds than modern lizards. Today, researchers agree with this birds are technically dinosaurs — the only survivors of the mass extinction 66 million years ago. However, if this is true, why aren’t birds cold-blooded like most modern-day reptiles?
The answer is straightforward: most dinosaurs were probably also warm-blooded.
Birds are descended from a diverse group of two-legged dinosaurs called theropods, which included giant meat-eating predators like. Tyrannosaurus rex, as well as smaller 3 feet tall (1 meter) Mononycus.
Like mammals, birds are warm-blooded, or endothermic, meaning they regulate their body temperature internally. Endothermic animals have a higher metabolism, which allows for more demanding physical activities – such as flying – but requires more calories to maintain.
“Warm-blooded animals are usually more active,” Holly Woodward, a professor of anatomy and paleontology at Oklahoma State University, told Live Science. “They can be active at night. And so it’s an evolutionary strategy in that you can forage when other animals can’t because they’re too cold and too slow.”
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Birds generally have one higher metabolism than mammals of similar size and keep their body temperature up – between 106 and 109 degrees Fahrenheit (41 to 43 degrees Celsius). Hummingbirds flapping their wings 720 to 5400 times per minuteshould consume about half their body weight each day, or eat every 10 to 15 minutes.
In contrast, cold-blooded animals, or ectotherms—like most modern reptiles and fish—rely on their environment to change their body temperature. Because they don’t expend as much energy heating themselves, they don’t need to eat as regularly; alligators, for example, can go more than a year without food.
For years, researchers assumed that because most modern reptiles are ectothermic, ancient reptiles must have been too.
“So often, if you look at living animals and make an assumption about the ancestral condition based on the current condition, it will lead you wrong.” Jingmai O’Connorassociate curator of fossil reptiles at the Field Museum in Chicago, told Live Science.
Views began to change around the end of the 1960s, with the discovery of a specimen similar to the named bird Deinonic. Since then, researchers have found physical characteristics that indicate that many dinosaurs, including ancient birds, were warm-blooded. The presence of feathers is one such indicator – feathers help animals conserve body heat, which is not necessary in ectothermy.
In her lab, Woodward has looked at another representative: the microstructure of bone tissue. She has found that endotherms have very different bones than ectotherms, mainly because ectotherms usually grow more slowly. This growth rate is reflected in the mineral component of bone, which she described as “small fibers.”
“I think of them as Pick-up sticks: If you’re growing really slowly, those fibers tend to be oriented parallel to each other, and so they become flat,” Woodward said. “But if you’re growing faster, the fibers are just kind of braided,” which is the kind of structure she tends to see in warm-blooded bones.
Her observations have shown that the bone structures of dinosaurs are more similar to birds and mammals than to crocodiles.
It is unclear exactly when hot blood first appeared. All dinosaurs (including birds) and crocodiles share a common reptilian ancestor, and Woodward and O’Connor said there is good evidence that this ancestor was warm-blooded — meaning endothermy arose before the dinosaurs. Cold-blooded dinosaurs would have appeared later.
But it is possible that endothermy appeared even earlier. If both mammals and most reptiles were endothermic, maybe Theirs the 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 may challenge these ideas. “We make so many assumptions,” O’Connor said, “and then the data proves us wrong.”