Dinosaurs: Cold-Blooded Or Warm-Blooded?

Author: Ashley Strack
Since their initial discovery, dinosaurs have had many mysteries surrounding their existence. How did they get so large? Why did they die out? And for many, a question arose surrounding their activity and mobility. Namely, were dinosaurs warm-blooded, like active birds and mammals, or were they cold-blooded, like modern reptiles?

When they were first introduced to the scientific community in the mid to late nineteenth century, the general consensus at the time was that dinosaurs were cold-blooded, sluggish animals, similar in lifestyle to modern reptiles, but scaled up. Were this the case, it would be hard to imagine any of these creatures living in any location with heavy seasonal variance, or engaging in any strenuous activity for long periods of time. But as further discoveries were made, the less it made sense to think of dinosaurs as lethargic and inactive. They stood with upright postures, grew to enormous sizes over short periods of time, and are thought to have engaged in lifestyles unsuited for purely cold-blooded animals.

The debate over dinosaur thermoregulation and metabolism still occurs to this day. Dinosaurs existed over a span of over 186 million years, and in that time many metabolic and thermoregulatory forms may have emerged. The dinosaurs’ living representatives, the birds, are all warm blooded, with a 4 chambered heart maintaining a constant internal body temperature, on average higher than mammalian internal temperatures. The immediate ancestors of dinosaurs, basal ornithodirans archosaurs, are less clear. Gauging the metabolic rates of fossil species is difficult, as the bodily functions more obviously correlated to metabolism are no longer observable. Crocodilians, the only other living archosaurs outside of birds, have higher metabolic rates than most other reptiles, and can control their internal body temperature in a limited fashion, are still considered ectotherms, organisms that cannot control their internal temperature, and rely on external environmental factors to maintain homeostasis.

Endothermy, Ectothermy, Or Something Inbetween: What Are The Facts?

Currently, scientists are split on the question of dinosaur metabolism. The major competing hypotheses are endothermy similar to birds and mammals, poikilothermy like modern reptiles, or various forms of “mesothermy”, unique standalone adaptations that could have enabled internal temperature control without biologically committing to full endothermy.

One of the counterpoints to endothermic hypotheses is the question of energy expenditure. From an energy standpoint, pure endothermy is very expensive. This energy expenditure increases dramatically the larger an animal is, which would mean that, if they were endothermic, the largest dinosaurs would be pouring an astronomical amount of calories towards maintaining homeothermy.

For animals, the major way heat is lost is by ambient loss in a colder environment. When an animal has a higher mass but lower surface area, that means that there is less area for heat to be lost to the environment. This temperature-maintenance strategy which relies on pure size is called Gigantothermy. Gigantothermy is in theory advantageous to large animals because it means they are spending far less energy actively maintaining homeothermy, and don’t need to eat as much as a result. By having so much more internal mass relative to surface area exposed to the cooler air, an animal like a large dinosaur can maintain its internal body temperature for a much longer period of time. For a modern example, a Lion is less than half the size of a saltwater crocodile, but the crocodile doesn’t need to eat as much as the Lion to maintain a healthy lifestyle.

Studies on dinosaur growth rates seem to support this idea. Dinosaurs grew relatively quickly, with large dinosaurs growing from egg-size to several tons over rapid periods of only a few years. Dinosaur eggs had a range of sizes, but, in accordance with current evidence, none were much larger than the largest bird eggs known today, such as ratite (ostriches and emus) eggs. For a dinosaur to grow to its huge adult size over so short a period of time (for example, a Tyrannosaurus reached adulthood at around 18-20 years of age, and weighed about 8 tons) it would have needed to digest and metabolize its food very quickly, consistent with endothermic animals. Indeed dinosaurs, especially larger dinosaurs, have been measured to have growth rates similar to extant mammals. Large sauropods have a similar rate to whales, while smaller dinosaurs grew at a similar rate to marsupials. In a study on dinosaur temperature in relation to growth rate, large dinosaurs were found to have higher body temperatures than smaller dinosaurs, and this was seen as evidence for “inertial homeothermy”, the underlying mechanism behind gigantothermy.

This would work well for the enormous sauropod dinosaurs, as well as other large dinosaurs, but it fails to account for infant and juvenile dinosaurs, as well as for dinosaurs that maintained a small body size for their entire lives. It also fails to account for the activity that these creatures would have engaged in in their lives. In a study on the energy expenditure and active ability of large ectotherms vs endotherms of comparable size, a 200 kilogram saltwater crocodile was only able to exert 14% of the energy of a comparably sized endothermic mammal, and could not maintain activity over the same lengths of time. This is perfectly fine for an aquatic ambush predator that maintains a low activity level for most of its day to day life, but would not account for prolific and widespread dinosaur populations that lived in a broad variety of ecological niches.

One piece of evidence that pushes towards the endothermic hypothesis is the presence of feathers for insulation. Feathers are known in some dinosaurs (mainly theropods, but the origin point of feathers in the dinosaur family tree is hotly contested), and there have been several theories posited to their origins. Obviously today, feathers are known on every extant bird species and serve a variety of functions, including insulation, sexual display, and perhaps most notably, powered flight. One theory proposed for how this adaptation proliferated is that feathers quickly were adapted from their original purpose into a way of insulating the body. Normally, this would be used to keep heat inside the body, which would be a sure indicator of endothermy. In warmer climates as well, depending on the morphology and coloration of the feathers and proto-feathers, they could also be used to keep heat out of the body, like some modern ratite birds do. Additionally, feathers and their precursors are highly derived and complex biological structures, and would have consumed a lot of energy to create, something that would have been less taxing for an endothermic organism. Feathers are, however, only known in certain lineages of theropod dinosaurs, with tentative evidence for other lineages.

Histological evidence is often used to point to at least some species of dinosaurs being endothermic. “Haversian canals”, a structure in bones associated with fast growth and warm blooded animals. Fibrolamellar bone has also been found in fossils, also associated with fast growing, endothermic animals. This could be taken as evidence that dinosaurs were endothermic, however other osteological evidence runs counter to this claim. “Lines of Arrested Growth”, or “LAG”s, appear periodically on some dinosaur bones. In some cases, these can even be used to determine the age of individuals. These LAGs are most often associated with periods of low metabolic activity and slow growth, and are commonly found in ectothermic animals. In the theropod genus Timimus, found in Australia, these LAGs have been compared to contemporaries to propose that this species may have hibernated over colder periods. In addition, while not common in reptiles, haversian canals and fibrolamellar bone have been found in turtles and crocodilians, the latter of which are dinosaurs’ closest living non-avian relatives.

Bones can also tell us about blood flow in dinosaurs. By examining the femora of dinosaur bones, we can make inferences about blood pressure and probable nutrient flow in and out of bones. High relative blood pressure and high nutrient flow are both traits that are highly correlated with endothermic animals. The relative blood flow index in some dinosaurs has been measured to be equivalent to or even exceeding similarly sized mammals, suggesting a similar metabolic rate.

Crocodilians present problems to endothermic hypotheses from certain angles. As the only other extant group of archosaurs, crocodilians being cold-blooded suggests that the ancestral quality of archosaurs is ectothermy. If this were true, then it would be difficult to place where exactly on the dinosaur family tree endothermy was developed. Crocodilians maintain internal body temperatures that are higher than most other reptiles, around 30-33 degrees celsius, much closer to endothermic animals, however they use the heat of their ambient environment to do so. Evidence exists for endothermy or endothermy-adjacent adaptations in highly disparate dinosaur families separated by millions of years of evolution. Today, the only living archosaurs are the endothermic avian dinosaurs (birds), and the crocodilians, which are ectothermic. However, the semi-aquatic, sessile ambush predators that represent modern crocodilians are only one branch of a larger crocodilian family tree, which once included more active, terrestrial members. These members have many of the macroscopic characteristics we associate with endothermy or at least mesothermy in dinosaurs. These include an upright walking gait, and slim, terrestrial body plans, both of which appear very early in the archosaur family tree. In addition, modern crocodilians, despite being ectotherms, have a suite of adaptations mainly found in endotherms, which they retain despite having seemingly little use for an ectotherm. These include their four-chambered heart, something only found in birds and mammals otherwise, a secondary palate allowing them to breathe and eat at the same time, a diaphragm allowing them to locomote and breathe at the same time, and a hepatic piston, an adaptation to pump the lungs, which may have an analogue (or possibly homologue) in some dinosaurs. This could mean that endothermy is the ancestral quality of all archosaurs, and that crocodilians made a reversion when they adapted into slower moving semi-aquatic ambush predators. This appears to be in line with fossil evidence of extinct crocodile relatives from the early Triassic, as well as with the expected timeline for the evolution of endothermy in the archosaurs, making it comparable to the evolution of endothermy in mammals.

One of the most direct pieces of evidence toward endothermic dinosaur evolution is the presence of dinosaurs in regions that were thought to be colder than the habitable range of known ectotherms. Dinosaur remains have been found in Alaska, Antarctica, and Southeastern Australia, dating to periods of time when those regions would have been drifting into or been located in the polar regions of the Earth. Temperature in the Mesozoic era was generally higher across the board, with a cooling trend going into the late Cretaceous period. Nevertheless, geological evidence suggests that these regions during the Cretaceous period would still have experienced colder temperatures, especially during the long polar nights caused by the Earth’s axial tilt. These include large genera, such as the large tyrannosaur Nanuqsaurus, the ceratopsian Pachyrhinosaurus, and the hadrosaur Edmontosaurus in Alaska, as well as smaller and mid-sized dinosaurs like Orodromeus and Alaskacephale. In Australia and Antarctica, many small to mid-sized theropods and hypsilophodont-like herbivorous dinosaurs have been discovered, albeit mainly by scant remains. The remains of the theropod Timimus, discussed earlier, have been shown to contain Lines of Arrested Growth, or LAGs, which indicate periods in the animal’s life where it would have slowed its growth. In a polar region like Southeastern Australia, this may be indicative of hibernation, or a similar behavior. The contemporaneous dinosaur Leallynasaura does not show these LAGs, but lived in the same region, which could be an indication that it maintained an active lifestyle throughout the year, something that likely would have been impossible for an ectotherm.

While not a settled debate by any means, there seems to be much more evidence in the corner of endothermy for the vast majority of dinosaurs, and possibly an ancestral quality of archosaurs as a whole. If any dinosaurs were ectothermic, it seems much more likely that they were basal to the dinosaur family tree, or reverted to an ectothermic state because of some kind of evolutionary pressure. Considering that mammals, which were decidedly endothermic from a fairly early evolutionary stage, evolved contemporaneously with the dinosaurs, yet did not compete with them in the major ecological niches, it seems quite likely that dinosaurs were metabolically competitive with these early mammals.

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