Dinosaur Metabolism & Thermoregulation

Modern birds, such as penguins, are endotherms ("warm-blooded")

Turtles, shown here basking for warmth, are ectotherms ("cold-blooded")

Since dinosaurs were first discovered in the mid-19th century, there has been debate about their energy levels & metabolism - were they warm-blooded, like mammals, or cold-blooded, like reptiles? The current consensus is that most dinosaurs were more on the "warm-blooded" side.
The terms "warm-blooded" and "cold-blooded" are generally considered to be antiquated and potentially misleading (the blood of "warm-blooded" animals might not actually be warmer than that of reptiles), so throughout this page we will use the following terms instead:

Endotherms are animals that regulate their own temperature internally ("warm-blooded").
Ectotherms are animals that rely on external sources of heat, such as lizards which bask in the sunlight.

Endothermy comes at a significant energy cost - endotherms generally need to eat much more food than ectotherms of the same size. However, endothermy does have several advantages. Endotherms can live in colder climates, where finding an external source of heat may be difficult or impossible. They can also maintain high activity levels without needing frequent breaks, and can be active during the night or early morning before the environment has warmed up.

Below we will discuss some of the main pieces of evidence on dinosaur metabolism & thermoregulation. It is important to note that no one of these points conclusively proves endothermy or ectothermy, and also that not all modern animals (and likely not all dinosaurs) fit neatly into one of these 2 groups. Animals that are intermediate between ectotherms and endotherms are called "mesotherms". Additionally, remember that dinosaurs roamed the earth for over 150 million years and were incredibly diverse in shape, size, diet, etc. - so they may well have been diverse in terms of metabolism and methods of thermoregulation.

Stance

We now know that dinosaurs had an erect stance, with their limbs directly underneath their body, rather than a sprawling stance like lizards. An erect stance is advantageous for more active animals, because the bones in the limbs can support the body's weight without putting as much strain on the muscles (think of how difficult it would be to maintain a push-up position with your arms sprawled out to the side). A sprawling stance makes more sense for ectotherms like lizards, which spend most of their time lying on their belly. In the animal kingdom today, there is a strong correlation between an erect stance and ectothermy.

It is important to note, however, that this is just a correlation and not a rule - there are modern examples of endotherms with a more sprawling stance (e.g. echidnas) and of ectotherms with a more erect stance (e.g. chameleons).

Growth Rate

Bone histology, which means cutting a thin slice of bone to look at under a microscope, also provides some evidence for endothermy. Dinosaur bones often contain lines of arrested growth (LAGs), which can be used to estimate growth rates. This, along with other methods, suggests a fairly rapid growth rate in dinosaurs. A rapid growth rate would require the dinosaurs to eat a lot of food, and convert it into body weight quickly, suggesting a high metabolic rate. Again though, this is not conclusive on its own as there are difficulties in estimating growth rates, and some ectotherms can grow relatively quickly.

Isotope Analysis

Isotope analysis of dinosaur bones & teeth can also give us some information about their body temperatures. Isotopes are atoms of the same element but with different numbers of neutrons. In this case, the 2 isotopes we are interested in are 18O and 16O (oxygen-18 and oxygen-16). The ratio of these two isotopes in bones or teeth is dependent on the temperature when they formed. Additionally, because these isotopes are stable (meaning they do not radioactively decay), they will be preserved unless altered by the process of fossilization.

In 1999, isotope analysis was carried out on bones of Tyrannosaurus Rex and Giganotosaurus, and showed that both dinosaurs maintained almost constant body temperatures. A further study in 2006 on dinosaurs from the cretaceous period yielded similar results. Unfortunately, it is not possible to say with certainty that these ratios were not altered by the process of fossilization - however both papers provided reasoning to suggest that the original ratio had likely been preserved. In the 2006 study the researchers preferentially analyzed tooth enamel, which was thought to be one of the substances most likely to preserve the original ratio.

Gigantothermy

Alamosaurus, a large sauropod, would have benefited from gigantothermy

It has been suggested that large dinosaurs - such as large sauropods (long-necked herbivores) - may have regulated their body temperature through inertial homeothermy, also known as gigantothermy. Basically, this means that their massive size helped them to maintain a constant temperature.

As any animal - or any shape in general - gets bigger, its volume increases more quickly than its surface area. For example, if you take a cube and make it twice as big (e.g. twice as long/wide/high) then its surface area will be 4 times as big - but its volume will be 8 times as big. This is known as the square-cube law. Because of this fact, larger dinosaurs had less surface area (relative to their size) exposed to the outside environment than smaller dinosaurs - and therefore would have gained or lost heat more slowly. While some dinosaurs were quite small, many reached truly enormous sizes so this would almost certainly have been significant.

Biomolecule Analysis

In 2022, a group of researchers analyzed dinosaur bones looking for metabolic "waste" products, produced as an animal breathes, which could give an indication of their metabolic rate. Their findings suggested that some dinosaurs, like Tyrannosaurus Rex and Allosaurus, had metabolic rates comparable to that of modern birds (i.e. endothermic/"warm-blooded"). However, they found that over time the metabolic rate of ornithischian dinosaurs seemed to decrease - with Triceratops and Stegosaurus, for example, having a more ectotherm-like metabolic rate.

Summing Up

Overall, while dinosaurs displayed a range of metabolic strategies, the preponderance of evidence indicates that most were closer to modern endotherms than ectotherms. Rather than fitting neatly into today's "warm‑blooded" or "cold‑blooded" categories, there is some evidence of mesothermy - maintaining higher metabolic rates than reptiles but not quite matching the constant heat production of mammals and birds - and gigantothermy likely helped to stabilize body temperature in the largest dinosaurs. This spectrum of thermoregulatory adaptations reflects the extraordinary diversity of dinosaur size, shape, and ecology.

References & Attributions

Image: Alamosaurus - Alamotitan, CC BY-SA 4.0, via Wikimedia Commons
Image: Penguins - Ian Duffy from UK, CC BY 2.0, via Wikimedia Commons
Image: Turtles basking - Oxlamb, Public domain, via Wikimedia Commons