Why Did Dinosaurs Get So Big?

People who visit the dinosaur Museums are known for their posture as their heads are tucked back. Because there is a giant in the museum, a brachiosaurus more than 13 meters high that lived about 150 million years ago. Weighing possibly nearly 50 tons, it was much larger than any land animal that populated the earth after the dinosaurs. But how did brachiosaurus and other long-necked herbivores called sauropods get so big?

There is a simple evolutionary driving force behind this: Bigger is better, that applies to all animals. Because large bodies offer many advantages. They protect against being eaten by predators and offer more space for digestive organs, which means that more food can be used. Besides, large males are more attractive to females, which means their genes are more likely to reproduce. Still, there must have been other reasons why dinosaurs dwarfed all land animals before and after them as they grew.

Researchers have pondered this for a long time and put forward various hypotheses. One of them is based on the assumption that external causes such as high oxygen content in the air or richer vegetation favored the gigantism of the sauropods at that time. However, the scientists rule this possibility out as the nutritional value of the vegetation was the same as it is today. The oxygen content of the air fluctuated at the time of the dinosaurs, but the size of the sauropods did not.

The Secret Of The Dino Lung

Therefore, for Sander and his colleagues, the physique of the giant animals came into focus as a possible solution to the riddle. What they found was that Gigantism comes about through a constellation of primitive features and evolutionary innovations. The primitive features include the fact that dinosaurs did not chew their food and lay eggs. The innovations were the long neck, rapid growth, and lungs similar to those of birds.

The bird-like lungs in particular are a key factor behind the enormous size of the sauropods. Because it offers several advantages. Regardless of whether the animal inhales or exhales, the lungs constantly take in oxygen. The lungs of mammals, on the other hand, work more like bellows in which the air either goes in or out.

A second advantage of the bird-like lungs is their large volume. If the lungs were smaller, as in mammals, the dinosaurs would have suffocated because of their long necks. The third advantage is that the bird lung has the ability that parts of it can migrate into the bones. Evidence of such an air sac system as that of birds has been found in the bones of dinosaurs. The air sacs make the bones very light which was a necessary prerequisite for the sauropod’s long neck.

And it is precisely this incredibly long neck which was a novelty among land animals of the time, which was an important factor in the giant growth. The giant herbivores hardly had to move to graze huge amounts of plant food from low plants to leaves on tall trees. As a large animal with a long neck, you can absorb a lot of energy with little expenditure of energy.

Small Head With No Chewing Tools

For the neck to be so long, the head must not be too heavy either. But this requires certain conditions. An animal can only have a small head if it does not chew. Because then it saves weight because heavy teeth are eliminated, as are the jaw muscles, which increase massively with the size of the animal. The habit of chewing is therefore probably also what inevitably limits the body size of mammals.

Another biological advantage for the sauropods in terms of size growth was the fact that they laid eggs and probably very many. Because of their enormous size, the population density of these animals was probably rather low, which is a disadvantage when external events such as catastrophes decimate the grown giants, at least if they don’t have many offspring. Just like the rhinos who only give birth to one young at a time. If they are hunted by poachers, the population threatens to collapse quickly because it cannot recover. In contrast, the offspring of the egg-laying sauropods were probably much more numerous, which kept the population stable even under difficult circumstances.

And above all, the young animals had to set a decent growth rate to reach the weight of an adult animal in its lifetime. The ability to grow quickly was probably another condition for the gigantism of the dinosaurs. After all, a young animal weighing around ten kilograms only had 20 to 30 years to increase its weight by 5000 times.

Freeloader T-Rex

But not only the herbivorous sauropods such as Brachiosaurus and Diplodocus became gigantic, but also carnivores of this time such as the predatory dinosaur Allosaurus were very large. This was later surpassed by the famous Tyrannosaurus Rex, which could weigh up to 14 tons. Predatory dinosaurs were freeloaders of the giant herbivore growth. Because as sauropods took more energy from the plant world, more was also available for the carnivores. However, their size is limited by another principle that is only a tenth of the energy reaches each step of the food chain. The predators like T-Rex could only reach a fraction of the weight of their prey.

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Why, after the dinosaurs died out, their descendants, the birds, no longer reached gigantic proportions is still an unanswered question. Until a few thousand years ago, large birds weighing almost a tonne lived on islands such as Madagascar or New Zealand. Presumably, however, competition from mammals prevented birds from reaching the dimensions of their dinosaur relatives.