How Did Dinosaurs Become So Big?

The average dinosaur was bear sized, while the average mammal was dog sized. Dinosaurs got even larger than this, though, dwarfing large land animals, like the elephant. What lead to the gigantism of dinosaurs and what allowed them to get so big? There are varying theories in explaining what kind of animals lead to gigantism: terramegathermy and gigantothermy. In addition, the large head and how they reproduced assisted them in becoming so large.

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Terramegathermy

This theory proposed that only tachyenergetic animals could grow to enormous sizes. Tachyenergetic animals tend to have high resting metabolic rates and high aerobic exercise capacity, allowing them to sustain moderate physical activities for prolonged periods of time. Having high metabolic rates, tachyenergetic animals are endothermic and produce heat internally, leading to more stable body temperatures. Sauropods have been found in polar regions which tells us that the sauropods were more tachyenergetic and endothermic than crocodiles to survive in colder climates. Tachyenergetic animals are able to have enough energy to find and eat large amounts which then powers them to gather even more food, leading to more growth. Crocodiles who are bradyenergetic, meaning they have lower resting metabolic rates and lower aerobic exercise capacity, could not grow to be that big because their circulatory system didn’t produce enough pressure to push blood throughout the whole organism. Looking at the height of some sauropods, we can infer that their hearts probably could push blood many meters up against gravity by generating a lot of pressure. Giraffes need 200 mm Hg of pressure to oxygenate their brains, but some sauropods needed two to three times higher amount of pressure to keep themselves functioning. It may be difficul to have super tall animals over 20 meters because it would require immense amounts of pressure to pump blood to the brain. All in all, this idea, terramegathermy, centers itself upon the notion that gigantic dinosaurs were tachyenergetic, rather than bradyenergetic.

Gigantothermy

This theory believed that metabolic systems of reptiles converged with the systems of mammals which led to energy efficiency in giant animals. According to this theory, the giant animals relied on their mass, instead of internal heat production, to obtain thermal stability. However, this is a misunderstood concept concerning power systems of animals. Just because an animal has a consistently high body temperature does not imply that the animal will be able to sustain physical activity for long periods of time. If a large reptile were to have a high body temperature, it would not be able to be physically active for very long.

Feeding and Reproduction

The heads of sauropods were basically all mouth and interestingly, their heads weren’t all that small. Some of the largest sauropods could fit the head of a giraffe. The largest sauropod’s heads probably weighed as much as a human body itself and probably had mouths of half a meter wide. A sauropod that weighed 50 tons would need to consume over a half of ton of food to sustain itself. If it fed for fourteen ours every day and took a bite every minute, then it would need to bite only half a kilogram worth of food each time which would be feasible. In addition, to its head size and diet, their mode of reproduction plays a role in making dinosaurs so big. There are two different type of reproductive type strategies: K and R. Large mammals are slow K-strategists, meaning they reproduce less amount of offspring and usually occupy more stable environments. Lower amount of children leads to a higher investment in each individual’s survival. For example, there are about as many breeding elephants as there are juvenile elephants in a herd because juveniles are dependent on their parents for survival. In this ecosystem, the number of adult elephants is restricted by the carrying capacity because the habitat has limited food resources. If there were to be too many, the population would die off. Hence, this carrying capacity limits slow-reproducing mammalian herbivore from getting past 10-20 tons. On the other hand, giant dinosaurs were fast-breeding r-strategists, meaning they produced a lot of offspring that were independent. Their offspring did not need the care of adults to survive. If all the adults were to die, the children would survive. Since dinosaurs could survive with a small amount of adult dinosaurs around, there was less stress on the ecosystem’s resources. This allowed some dinosaurs to grow to 20-100 tons.

By Nobu Tamura (http://spinops.blogspot.com) (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

Works Cited
“K and R Reproductive Strategies.” K and R Reproductive Strategies. N.p., n.d. Web. 08 Feb. 2016.
Paul, Gregory S. The Princeton Field Guide to Dinosaurs. Princeton, NJ: Princeton UP, 2010. Print.

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