Were Dinosaurs Warm-Blooded? What Bones, Feathers and Growth Rings Suggest

Warm-blooded, or endothermic, animals generate their own heat by burning food fast and keep a steady body temperature regardless of the weather; mammals and birds do this, and it lets them stay active at night and in cold climates, at the cost of eating about ten times as much as a reptile of the same size. Cold-blooded, or ectothermic, animals take their temperature from their surroundings, warming in the sun and slowing in the cold, which is cheap but limiting. The two are ends of a spectrum rather than a switch: tuna and some sharks keep parts of their bodies warm, and a few mammals let their temperature drop to save energy.

Bone grows differently at different rates. Fast-growing bone, typical of birds and mammals, is riddled with blood vessels and laid down in a woven, disordered texture; slow-growing reptile bone is dense, orderly and poorly supplied with blood. When palaeontologists cut thin sections of dinosaur bone and looked at them under a microscope, from the 1970s onward, they found the fast-growing kind in almost every dinosaur examined, including the giant sauropods. A Tyrannosaurus put on around two kilograms a day in its teens and reached adult size in about twenty years, a rate no cold-blooded animal approaches.

The bones also show growth rings, like a tree's, laid down each year when growth slowed in the lean season. Those exist in mammals too, so they do not prove cold-bloodedness, but they allow ages to be counted, and the ages confirm the fast growth.

Several other lines of evidence point the same way:

• •Insulation: many small and medium dinosaurs had feathers or fuzz, which makes sense only for an animal with internal heat worth keeping in
• •Posture: dinosaurs stood with their legs beneath them and walked and ran like mammals and birds, an active lifestyle that cold-blooded animals rarely sustain
• •Polar dinosaurs: fossils from Alaska and Australia, which lay inside the polar circles in the Cretaceous, show dinosaurs living through months of darkness and near-freezing winters
• •Breathing: the air sacs that fill birds' bones and give them an efficient one-way lung are found in theropod and sauropod skeletons
• •Predator-prey ratios: dinosaur communities have far fewer predators relative to prey than reptile communities do, which fits predators that needed to eat like mammals

The most direct test came in 2022. Burning food fast leaves chemical traces, the by-products of oxygen reacting with proteins and fats, and those traces survive in fossil bone. A team measured them across dozens of species and found high metabolic rates in most dinosaur groups, including the meat-eating theropods, the long-necked sauropods and the flying pterosaurs, at levels comparable to modern birds. Earlier work on the oxygen isotopes in dinosaur teeth, which record body temperature, had put large dinosaurs at around 36 to 38 degrees, in the mammal range.

The twist is that the same 2022 study found low metabolic rates in the ornithischians, the group that includes Triceratops, Stegosaurus and the duck-billed dinosaurs. Their ancestors had apparently been warm-blooded, and they seem to have lost the trait, evolving back towards a slower, cheaper metabolism as large plant-eaters. Warm-bloodedness was not something dinosaurs achieved once; it came and went.

The argument was slow to settle partly because the word dinosaur covers 165 million years and thousands of species, and the answer differs between them. It was also slow because of the giants. A forty-tonne sauropod with a mammal's metabolism would seem to need an implausible amount of food and to risk cooking in its own heat, and one proposal is that the biggest dinosaurs were warm mainly because they were big, a body that size losing heat so slowly that it stays warm regardless. That is probably part of the story, but the bone growth rates say the sauropods were generating heat as well as retaining it. The dinosaurs that survived, the birds, are the most thoroughly warm-blooded animals alive, and it now looks as if they inherited the trait rather than inventing it.

Most dinosaurs were warm-blooded or close to it. Their bones show the fast, blood-rich growth of birds and mammals, many were feathered, some lived through polar winters, and chemical traces of a high metabolism have been measured directly in theropods, sauropods and pterosaurs. The exception is the ornithischians, such as Triceratops and Stegosaurus, which appear to have evolved back towards a slower metabolism.