What Made Dinosaurs So Gigantic, and Why Don’t Such Enormous Animals Roam the Earth Today?

Untorne Nislav:

Before we jump into this, it's crucial to understand that while dinosaurs were indeed massive, they weren't quite *that* massive. You’ve probably heard the statistics: the largest land mammals reach a length of about 6-8 meters (19-26 feet), but the biggest dinosaurs were... was it really 40 meters (131 feet)?

Wow, that’s an impressive figure!

But, numbers can be *deceptively* misleading. Take a look at the second-largest land mammal of all time, *Indricotherium*, and one of the biggest dinosaurs, *Brachiosaurus*, here.

The gap between them seems impossible to compare…

*But…*

These two creatures have entirely different body structures: most of the Brachiosaurus's size comes from its towering neck and lengthy tail. To make it clearer, use your two thumbs: place one on the dinosaur’s neck and the other on its tail (and, hopefully, you’re not reading this on a touchscreen).

Suddenly, 'enormous' becomes a bit more... 'normous.' Sure, *Brachiosaurus* is still larger than *Indricotherium*, but it’s not four times larger as the numbers might suggest. The real, more comparable difference between them is closer to the gap between an elephant and a hippo:

Lesson to be learned: don’t be fooled by body shape.

So, here's the answer to the 'so large' part of your question: because they weren’t.

That said, there is still a genuine size difference to consider. And at least two factors might explain it:

1) Different feeding habits of herbivores.

During the age of mammals, the most successful herbivorous strategy was grazing.

Grasslands are exceptionally effective. The two most productive ecosystems dominated by mammals were savannas and the now-extinct mammoth steppes: both could support vast populations of large mammals. With grass growing at astonishing rates everywhere, no other food source on Earth could sustain such high mammalian biomasses.

Lesson to take away: if you want to grow big and strong, eat grasses.

But this wasn’t always the case. Back in the time of the dinosaurs, grasses didn’t exist. So, the largest creatures of the era had to turn to the second-best herbivorous approach: browsing.

Tree foliage doesn’t grow the same way grasses do, but there’s still usually a substantial amount per unit area, as it often overlaps vertically multiple times.

Dinosaurs that fed on canopy foliage could afford to grow to enormous sizes: for thermoregulation or protection from predators—these are the usual explanations.

But…

Any animal that grows too large eventually faces challenges with obtaining enough food. Today, any herbivore that grew too big would likely switch to grazing on grasses. But dinosaurs couldn’t do that. So, their only solution was to develop even longer necks to reach more foliage. But with a longer neck, they also needed a larger tail (for balance). Then came the need for wider, thicker bones to support all those muscles, stronger legs to bear the extra weight, and so on and so on.

In essence, it created a perpetual cycle: dinosaurs grew large, which led them to develop longer necks to meet the increasing demand for food, which, in turn, made them even larger, which then escalated their need for food. Browsing herbivory was likely the key factor behind the size of sauropods, and in the end, the only real limit was probably the height of the tallest canopy.

2) Reproductive limitations

This doesn’t really address the "why were sauropods so large?" question, but it does help answer the "why aren’t mammals that large?" question.

A typical sauropod was, in essence, a reproductive frog. It laid dozens, if not hundreds, of small eggs that hatched into tiny babies, completely separate from the adults: they lived in different environments and ate different foods. For sauropods, this solved two major problems: first, it made pregnancy simple and unobtrusive (a key factor when you weighed 60 metric tons), and second, it removed competition for food between adults and babies.

In other words, sauropods could afford to grow as large as needed without being overly concerned about the effects on their pregnancy and reproduction.

On the other hand, being a 60-ton (66-ton) pregnant mammal is a real and deadly nightmare.

All (placental) mammals give birth to relatively large offspring. However, if you weighed 60 tonnes, that would mean … what, 2 tonnes (4400 pounds) of offspring? Carrying an extra 10 kilograms (22 pounds) of weight at the peak of pregnancy is tough enough for humans, but carrying an additional 10 tonnes (22,000 pounds) is simply impossible—unless you're a whale and you can swim.

Not to mention, it would be a very long pregnancy.

Let’s not forget that pregnant females also require even more food.

And let's not forget that the young must also be fed, only to grow up and compete with you for the same food later on.

Moral of the story: children are costly … unless you're a frog or a sauropod.

Q: What about live-bearing smaller babies?

There are two main issues with this. First, it just doesn’t occur. While some placental mammals give birth to relatively small newborns, there's nothing comparable to the size difference seen between adult sauropods and their babies.

Second, if babies are too small, they become isolated from social interactions: in fact, it’s better for them to stay away from their parents right away to avoid being trampled. Social behavior and learning are the core of mammalian success. It's not worth eliminating that.

In the end, dinosaurs that weren’t ridiculously large were large because they reproduced like frogs and their food supply was … somewhat underdeveloped.

This article was first published on Quora. Click here to read it.