33.4: Movement and thermoregulation - Get that blood flowing!

Figure \(\PageIndex{1}\): Leaping Laelaps, an 1897 painting by Charles R. Knight, might be the first depiction of active dinosaurs.

How dinosaurs moved and how dinosaurs regulated their internal temperature has been debated and controversial since their discovery. What may be less obvious is how much these ideas are interlinked. Dinosaurs’ closest living relatives, crocodiles, are ectothermic (commonly known as cold blooded). This means they rely on their environment for warmth. This has advantages and disadvantages: less resources are needed, but they are less adaptable to cold climates and have limitations on endurance. Crocodiles, by the way, have several advantages over their ectothermic kin. They have a 4-chambered heart for better blood exchange efficiency, a secondary palate to allow breathing while eating, and the ability to walk upright (known as a high walk) for more efficiency in movement. These are adaptations that mammals share, and most scientists think dinosaurs had these adaptations as well.

Figure \(\PageIndex{2}\): An Emperor Penguin jumping out of icy water in Antartica. (Photo by Christopher Michael.)

Birds, the direct descendants of dinosaurs, are mostly endothermic (commonly known as warm blooded). That means birds (and the other endotherms, mammals) can live in many more climates across Earth and have a higher metabolism. This allows for faster movement but also requires more resources. It should be noted that there are also mesotherms, animals that are not quite warm blooded or cold blooded, but in between. Another complexity is homeothermy, the ability to maintain body temperature so that metabolic processes can be maintained. This is related to endothermy, but is distinct, because some endotherms do not have homeothermy. The question remains: where do dinosaurs fit into this spectrum? And how do we find out?

Early dinosaur movement

Figure \(\PageIndex{3}\): Outdated Megalosaurus model from Crystal Palace in London. (Photo by C.G.P. Grey.)

When dinosaurs were first described, they were thought to be related to modern reptiles, and were imagined as just larger versions of them. They were slow and lumbering in the minds of scientists at the time and were assumed to be ectothermic as well. They were mounted with sprawled legs and dragging tails. Evidence started to build that dinosaurs were related but distinct from living reptiles on Earth today. For example, dinosaurs have their legs positioned under their bodies. This is not just an aesthetic choice; a sprawled posture is much more energy intensive, so dinosaurs were more efficient than their reptile brethren. Also, sprawled posture interferes with breathing by flexing the lungs (called Carrier’s Constraint), causing most sprawled animals to hold their breath while moving. Pontzer et al. (2009) argued that the posture of dinosaurs links them to endothermy alone. Even in 1869, Thomas Henry Huxley had concluded that dinosaurs were more closely related to birds as part of his defense of the newly-proposed theory of evolution by Charles Darwin. These ideas did not help the image of dinosaurs in the minds of the public, however. Even into the mid 20th Century, dinosaurs were nothing but big cold-blooded lizards.

Dinosaur Renaissance

Figure \(\PageIndex{4}\): Deinonychus skeleton from the Field Museum in Chicago. (Photo by Johnathan Chen.)

Starting in 1964, John Ostrom, his student Robert Bakker, and others discovered and described the animal Deinonychus. With its small, light frame and bird-like posture, it was a great example that went against the slow and sluggish stereotype dinosaurs had in public opinion. This started what is now known as the Dinosaur Renaissance. Ideas that changed during this time include: a closer evolutionary connection to birds, a more nimble posture (without dragging tails), stronger ideas about herd behavior, and a more open mind on endothermic thermoregulation. The current idea about the extinction of the non-avian dinosaurs by a meteor impact also was developed during this time.

Figure \(\PageIndex{5}\): Bone microstructure of polar hypsilophodontid. Arrows indicate annuli, places where growth would have stopped in the winter. Scale bars, 500 µm. (A) is femur, (B) is zoomed in. (Photo by Holly N. Woodward, Thomas H. Rich, Anusuya Chinsamy, and Patricia Vickers-Rich.)

This revolution in the thinking about dinosaurs has basically continued to this day, but the debate over dinosaur thermoregulation is still ongoing, mostly because it is harder to decipher than some of the other debates. One practice that has increased our understanding of physiology of dinosaurs is histology: the microscopic study of tissues, namely bone. Several studies have found that dinosaurs had fast growth rates. They also had fibrolamellar bone and Haversian canals, both of which are common in endothermic animals (but not exclusive to them).

Figure \(\PageIndex{6}\): Graph of blood flow verses body mass, showing dinosaurs are closer to mammals than reptiles. (By Cervical67 via Wikimedia.)

Bone structures can also estimate blood flow, with high blood flow being attributed to endothermy. By looking at the small holes throughout bones, called nutrient foramina, and comparing the size of the holes to the size of the animal, you can estimate whether the animal is endothermic or ectothermic. As you can see from the figure, dinosaurs match much more closely with mammals, and may have been highly active, and possible endotherms as a result.

Figure \(\PageIndex{7}\): Image showing parallels between bird air sacs and proposed dinosaur air sacs. (By Zina Deretsky, National Science Foundation)

Another clue comes from breathing. Some scientists think dinosaurs had an air sac system similar to birds. The sacs are found mostly in their skulls and vertebrae, and would have allowed more efficient exchange of oxygen. They have also been pointed to as a weight-saving system in larger species. Of course, these features are almost impossible to fossilize. While there is no doubt that dinosaur descendants (birds) have them, where they developed in dinosaur’s evolutionary history is very much up for debate. Theropods, the branch of the dinosaur tree that turned into birds, have the best evidence for possible air sacs in non-avian dinosaurs.

Endotherm without Endothermy?

Figure \(\PageIndex{8}\): Life sized model of Argentinosaurus at Clore Garden of Science in Weizmann Institute of Science in Israel. (Picture by MathKnight and Zachi Evenor via Wikimedia.)

The paragraphs above were just a partial list of evidence that could be used to explain if dinosaurs were endothermic or ectothermic, there is another factor to consider: dinosaurs could have been functionally endotherms without being actual endotherms. How? By keeping in their heat. This could have happened in two main ways. First, feathers (or similar proto-feather structures) may have evolved as a warmth structure first, then became used for flight later. Secondly is an idea that is called inertial endothermy, bulk endothermy, or gigantothermy. This suggests the large size of dinosaurs, with selective blood pumping taking advantage of surface area to volume ratio, could have had high blood temperatures without the food costs of endothermy. Of course, smaller dinosaurs and the young of larger dinosaurs would not have had this advantage, but feathers on the smaller animals or young could have helped.

Dino Debate – Did I Get It?

Exercise \(\PageIndex{1}\)

Which of these evidences for dinosaur social behavior is hardest to find?

a. Toothmarks on bones

b. Individuals interacting with each other

c. Nest sites

d. Many individuals found in close proximity

e. Footprints and trackways

Answer

b. Individuals interacting with each other

Exercise \(\PageIndex{2}\)

Which of these dinosaurs is most likely not a valid species?

a. Apatosaurus

b. Brontosaurus

c. Triceratops

d. Torosaurus

e. Stygimoloch

Answer

e. Stygimoloch

Exercise \(\PageIndex{3}\)

The first feathers developed in dinosaurs are most likely used for what?

a. Warmth

b. Climbing

c. Sexual display

d. Floating

Answer

a. Warmth

Exercise \(\PageIndex{4}\)

The dinosaur renaissance most changed the way we think about:

a. dinosaur size

b. dinosaur art styles

c. dinosaur parental behavior

d. dinosaur food consumption

e. dinosaur movement

Answer

e. dinosaur movement