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.

Warm and cold are not the only possibilities, and some researchers have argued that many dinosaurs sat in between. A study published in 2014 compared growth rates across hundreds of living and extinct animals and placed dinosaurs in a middle group its authors called mesotherms, alongside tuna, some sharks and the leatherback turtle: animals that make a good deal of their own heat but do not hold their temperature as steady as mammals do. Critics disputed the growth figures and the statistics behind the claim, and the argument has not been fully settled.

The leatherback shows how such a middle way can work. It is a reptile, yet it swims in water cold enough to slow most reptiles to a standstill and keeps its body well above the temperature of the sea. It does this through sheer size, a thick layer of fat, heat exchangers in its flippers and the warmth produced by its working swimming muscles. For a large dinosaur, a similar mix of bulk, insulation and a moderately raised metabolism could have kept it warm while needing less food than a mammal of the same weight.

The modern argument has a turning point you can name. In 1964 the palaeontologist John Ostrom and his team found the remains of a small predatory dinosaur in Montana, which he described in 1969 as Deinonychus, meaning terrible claw. It had a large sickle-shaped claw on each foot and a slender, stiffened tail that seemed built for balance during fast running and leaping. To Ostrom it looked like an agile, active hunter rather than a sluggish reptile, and he pointed out many similarities between its skeleton and that of the early bird Archaeopteryx, reviving the old idea that birds descend from dinosaurs.

His student Robert Bakker pushed the case further and more provocatively, arguing in articles and later in a popular book that dinosaurs were fast, warm-blooded animals. Not all of his evidence held up, and some of his arguments were later criticised, but the period of rethinking that followed is often called the dinosaur renaissance. Museum mounts were rebuilt with tails held off the ground, artists began drawing dinosaurs running, and the discovery in the 1990s of feathered dinosaurs in China confirmed the bird link that Ostrom had argued for. The bone and chemical evidence came later, testing ideas this generation had put forward.

Apart from birds, the closest living relatives of the dinosaurs are the crocodiles and alligators, which belong with them in the group called archosaurs. Crocodilians are cold-blooded today, yet they carry a feature that looks out of place in a reptile: a heart fully divided into four chambers, like those of birds and mammals, which keeps oxygen-rich and oxygen-poor blood apart and supports a high-pressure circulation. Some early members of the crocodile line also have the blood-rich, fast-growing bone seen in dinosaurs. Together these clues suggest that the common ancestor of both lines may already have had a raised metabolism.

If that is right, crocodiles are the reverse of the birds' story: a lineage that once ran warm and slowed down. Their way of life offers a reason. An ambush hunter that lies still in water for hours, can stay submerged for long periods and may go months between large meals gains a great deal from a cheap, slow metabolism, and crocodiles even have a special valve and an extra main artery that let them divert blood away from their lungs while diving. The idea is still debated, but it fits the pattern seen in the ornithischians, where warm-bloodedness was lost as well as gained.

In 1923 an American expedition to Mongolia found a toothless, beaked dinosaur lying near a nest of eggs thought to belong to the plant-eater Protoceratops. The animal was named Oviraptor, egg thief, on the assumption that it had died robbing the nest. In the 1990s new finds turned the story round. An embryo inside one of these eggs proved to be an oviraptorid, and a close relative, Citipati, was found sitting on a ring of its own eggs with its arms spread over them, in the posture of a brooding bird. The supposed thief had been a parent, apparently sitting on its clutch as birds do.

Brooding fits the picture of active, bird-like animals, but the eggs themselves add a twist. Embryonic teeth, like adult ones, lay down daily growth lines, and in 2017 researchers counted them in fossil embryos close to hatching. They estimated that Protoceratops eggs took about three months to hatch and those of the duck-billed Hypacrosaurus about six, far longer than the eggs of birds and closer to the slow incubation of reptiles. Both are ornithischians, the group that seems to have slowed its metabolism, but the result is also a reminder that bird-like features did not arrive all at once. Dinosaur biology was a mosaic of the old and the new.

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.