Sci-Day 9: Dinosaur Metabolism

Happy Sci-Day, fans! Today, I'd like to touch upon a topic that is rather hard to directly investigate... The metabolism of non-avian dinosaurs.

Dinosaur metabolism is very difficult to study or to determine with a great degree of certainty for obvious reasons. Unlike with modern animals, it is impossible to directly measure respiration rates (since doing so requires a live specimen). This means that paleontologists must use other techniques to indirectly determine these aspects of dinosaur biology. Unfortunately, informed inferences are the closest we can get to understanding dinosaur metabolism unless someone manages to create a time machine.

While currently it is agreed that dinosaurs in general had faster metabolic rates than extant reptiles such as lizards and crocodilians, the situation is not black and white. While some of the smaller species may have been truly endothermic, it has been hypothesized that some of the larger species of dinosaur were intertial homeotherms (Paladino et al., 1990), or that many dinosaurs were somewhere in between the metabolic rates of reptiles and birds (Barrick et al., 1996).

One reason it has been considered unlikely for fully endothermic metabolism to be present in all dinosaur species is the fact that the largest dinosaurs grew up to 5 times as massive as the largest endothermic terrestrial animals known. Since maximum size is limited by metabolic rate (due to the fact that the amount of energy required is dependent on both), the significantly larger sizes of dinosaurs when compared to true endotherms is considered evidence of a slower field energy expenditure. In fact, the field energy expenditure curve based on varanid lizards between 2.2 and 45.2kg (FEE = 1.07g^0.735, g = mass of animal in grams) works quite well for dinosaurs, and can be used to explain the larger maximum sizes of the largest species. Using this equation, the field energy expenditure of 59-ton sauropod would be equivalent to that of a 7.5 ton African elephant, and an 83-ton sauropod would have the same energy requirements as an 11 ton Paraceratherium, the largest known terrestrial mammalian herbivore (McNab, 2009). While some might find this surprising given that many people think of lizards as slow, sluggish animals, varanids actually have a metabolic rate that is 3 times higher than that of other lizards, and many live a rather active life, roaming around in their search for food. Additionally, the larger species of varanids can grow VERY quickly (a phenomenon also seen in dinosaurs) given the right conditions (ie allowing for constant optimum body temperature, abundant food - in captivity essentially) - my Nile monitor grew from a 25g hatchling to a 5', 7kg animal in merely 18 months!

Understanding the energetics and energy requirements of dinosaurs is essential for Dinosaur Battlegrounds, as we want to represent the biology of these animals as accurately as possible given current evidence. Using this equation, we can figure out how much energy the creature would need to get from its food in order to survive.

However, there is another piece of the puzzle that needs to be considered - the energy yield per unit mass of the food they consumed (ie units of kJ/kg dry mass). This requires thorough research, as it is critical to creating the mechanic for food. There actually is a paper that gives metabolizable energy amounts for various different plant groups found in the Morrison, most of which were also present in Hell Creek (Hummel et al., 2008). However, there was a massive diversity of angiosperm taxa present in Hell Creek, and as such those were not included in the analysis (Angiosperms were not present in the Morrison) - currently, no data exists for the metabolizable energy/digestibility of the various angiosperm taxa from Hell Creek [at least as far as I'm aware - if I am wrong, please do let me know and give a source, I would greatly appreciate it!], which is a problem, since angiosperms were extremely abundant and likely were a considerable component of herbivorous dinosaurs' diets.

This gap provides the opportunity for a research project - one could replicate the procedure as detailed in the Hummel et al. paper, but using samples of the closest modern relatives of extinct Hell Creek angiosperm taxa. This would need to be done not only with leaves, but also with bark, fruit, and flowers (where applicable). Such a project, while important to Dinosaur Battlegrounds, would also provide very useful data to any scientists working on nutrition/energetics of both modern and extinct taxa, as they could use the published numbers in their analyses. This opportunity for formal scientific research is part of what Dinosaur Battlegrounds is about - we are not just a game, we want to use the game and the process of creating it to further the knowledge of a world long since lost to us by the eroding sands of time.

Well, I hope this has given you a bit of a better idea on dinosaur metabolism, and overall energetics!

Acknowledgements:
Paladino, F. V; O'Connor, M. P.; Spotila, J. R. 1990. Metabolism of leatherback turtles, gigantothermy, and thermoregulation of dinosaurs. Nature 344 (6269): 858-860.
Barrick, R. E.; Showers, W. J.; Fischer, A. G. 1996. Comparison of Thermoregulation of Four Ornithischian Dinosaurs and a Varanid Lizard from the Cretaceous Two Medicine Formation: Evidence from Oxygen Isotopes. PALAIOS 11 (4): 295-305.
McNab, Brian K. 2009. Resources and energetics determined dinosaur maximal size. PNAS 106 (29): 12184-12188.
Hummel, Jürgen; Gee, Carol T.; Südekum, Karl-Heinz; Sander, P. Martin; Nogge, Gunther; Clauss, Marcus. 2008. In vitro digestibility of fern and gymnosperm foliage: implications for sauropod feeding ecology and diet selection. Proceedings of the Royal Society 275: 1015-1021.