Have you ever wondered why there are no land animals on Earth today that are comparable to the size of the largest dinosaurs? The Argentinosaurus, the biggest known dinosaur, weighed around 77 tons (70 metric tons), yet the largest land animal alive today, the African elephant, weighs less than 7 tons (6 metric tons).
The answer is complex, but it comes down to a few key reasons. Following the cataclysmic event that wiped out the dinosaurs 66 million years ago, a diverse group of mammals flourished in their place. Unlike dinosaurs, mammals are endothermic, meaning they regulate their own body temperature through their metabolism. Mammals can’t become as large as the biggest dinosaurs because they have much faster metabolisms, requiring around 10 times as much food as similarly-sized reptiles to keep their bodies warm.
Furthermore, sufficient resource-rich land is needed to support populations of very large creatures – and with humans dominating the planet, there just isn't enough space or resources for them to thrive. In fact, many scientists consider humans to be partly responsible for the rapid decline of Pleistocene megafauna such as the cave bear and the wooly mammoth.
To explain why there's a physical limit to how large a land animal can get, Felisa Smith, a professor of paleoecology at the University of New Mexico, cites the “square-cube law.” This principle states that as an animal increases in size, its volume will grow faster than its surface area – so larger animals would need much bigger limbs to support their weight. If it were to exceed roughly 120 tons (109 metric tons), an animal would need such wide legs to support its body that it couldn’t walk. Marine creatures don't have the same limitations, as the buoyancy of water can support much larger creatures, which explains how the blue whale can weigh in at 165 tons (150 metric tons).
Bigger and bigger:
- The square-cube law is a mathematical principle first described by Galileo. It states that "the ratio of two volumes is greater than the ratio of their surfaces." In other words, as an animal increases in size, its volume will grow faster than its surface area.
- Large mammals such as elephants, giraffes, bears, and buffalo have lengthy gestation periods that require an immense expenditure of resources. For these species, expectant mothers typically carry a single fetus for a long time, limiting their mobility and increasing their vulnerability to predators.
- Unlike mammals, dinosaurs laid eggs, typically six to 10 at a time, resulting in regular additions of baby dinosaurs in an ecosystem that was ideal for their growth. With abundant plant life, it’s believed that dinosaurs evolved in size at tremendously fast rates.