Why were dinosaurs so large in size?

Before we start, what you need to realise is that dinosaurs were definitely large, but not so large. You probably know the numbers: the largest land mammals ever are around 6–8 meters long, while the largest dinosaurs were… is it 40 meters?

Damn, what a number!

However, numbers can be veeeery misleading. Below are the second-largest land mammal ever, Indricotherium, and one of the largest dinosaurs, Brachiosaurus:

The difference seems to be incomparable…


Those are two entirely different body shapes: most of the brachiosaur’s length is used up by its enormous neck and tail. To make it fair, I want you to use your two thumbs: place one over the dinosaur’s neck, and the other over the tail (hopefully, you are not reading this from a touchscreen).

And suddenly, enormous becomes quite… normous. Obviously, Brachiosaurus is still larger than Indricotherium, but it’s not four times larger like the numbers would suggest. The real, fair difference between the two is roughly the same as the difference between an elephant and a hippo:

Moral of the story: don’t let the body shape mislead you.

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

However, there is still some “true” difference in size to account for. And at least two factors could’ve contributed to it:

1) Different rules of herbivory

In the age of mammals, the most effective strategy of herbivory is grazing.

Grasslands are super-effective. The two most productive mammal-dominated ecosystems ever are savannas and (now gone) mammoth steppes: both can feed enormous numbers of huge mammals. With grasses growing at insane rates everywhere, no other foodsource on Earth can provide for such high mammalian biomasses.

Moral of the story: if you want to grow up big and full, eat grasses.

However, it wasn’t always so. In times of dinosaurs, grasses didn’t exist. So, largest animals then were forced to resort to the second-best herbivory strategy: browsing.

Tree foliage doesn’t grow like grasses, yet still there’s usually a considerable amount of it per area unit, because it overlaps vertically many times.

Dinosaurs that fed from canopies could afford to grow large: for thermoregulation or defense from predators — usual reasons.


Any animal that grows too big inevitably experiences difficulties with food. At present, any herbivore that became too large would likely just move onto grasses. But dinosaurs couldn’t. Hence, the only solution that they had was to grow necks even longer to get even more foliage. But if you grow a larger neck, you also need a larger tail (for balance). Then, you also need broader and thicker bones for all those muscles to attach, stronger legs to support the extra tonnes of weight, and so on and so on.

Effectively, it was a dead loop: dinosaurs became large, then they grew longer necks to support the growing need for food, which in turn made them become even larger, which in turn further increased their need for food. Browsing herbivory was likely the driving force of sauropod size, and in the end, the only limiting factor was probably the height of the highest canopy.

2) Reproductive limitations

This one doesn’t really answer the “why sauropods were large?”, but the “why mammals aren’t that large?

A typical sauropod was, effectively, a reproductive frog. It laid dozens if not hundreds of small eggs that hatched into very small babies that had little to do with adults: they occupied very different niches and fed on different food. For sauropods, it killed two problems: firstly, it made pregnancies easy and unnoticeable — which is a factor when you weighed 60 tonnes —, and secondly, it removed competition for food between adults and babies.

In other words, sauropods could afford to become as large as necessary without worrying much about how it would affect their pregnancy and reproduction.

On the contrary, being a pregnant 60-tonne mammal is a nightmare — of a real and deadly kind.

All (placental) mammals bear relatively large offspring. However, if you weighed 60 tonnes, that would be… what, 2 tonnes heavy offspring? Carrying extra 10 kg of weight at the peak of pregnancy is difficult enough for humans, but having to carry 10 extra tonnes is just impossible, unless you are a whale and swim.

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

Not to mention that pregnant females require even more food.

Not to mention that the young must be fed, only to grow up to compete with you for the same food later.

Moral of the story: children are expensive… unless you are a frog or a sauropod.

Q: How about livebearing smaller babies?

There are two problems with this. Firstly, it just doesn’t happen. There are relatively small newborns in some placental mammals, but nothing like the difference between sauropod adults and babies.

Secondly, if babies are too small, then they become unavailable for social interactions: in fact, they are better to stay away from parents immediately to avoid being stomped on. Social behaviour and learning are the backbone of mammalian success. Trying to get rid of it just isn’t worth it.

So in the end, dinosaurs that weren’t so large were large because they bred like frogs and because their kitchen was… a little underrepresented.

This information was taken from Quora. Click here to view the original post.

Did you also think that dinosaurs were enormous in size?

#Geography #History #animals #Nature #Quora


What are your thoughts on this subject?
George Gleim
Good info! Weren't the carnivores bigger as well?
Oct 12, 2020 9:14PM
Francie Traschen
very interesting. the only thing i thought might be a bit off is the author's reasoning for why they got larger. s/he states, "dinosaurs became large, then they grew longer necks to support the growing need for food...". This isn't how natural selection works. There is no intent in natural selection. Probably what happened was the dinos with the longer necks were able to survive and so their genepool survived and the ones that had shorter necks died. the necks didn't grow longer to support the need for food. that would show intent. the necks of the longer ones survived and passed on their genes.
Mar 1, 2020 5:23PM
Cheryl McMeekin
Very interesting
Aug 19, 2019 9:52AM
Robert Cox
Very informative and interesting. Thank you.
Jul 12, 2019 6:23AM
Carol Bruyere
very interesting and sure explains an awful lot to me
Jun 3, 2019 5:07PM
Louis Michael Durocher,
Very educational
May 10, 2019 11:23PM
Kathleen Hartshorne
This makes a lot of sense.
Apr 28, 2019 5:19PM
Great info on this topic.
Apr 6, 2019 4:43PM
Barry G Hitchens
well foundd
Mar 16, 2019 9:56PM
Laura Dice
good one! thanks!
Mar 3, 2019 10:01PM
Tina Marie Haddad Rhodes
Makes sense...
Feb 28, 2019 9:47PM
Richard Levine
Lamarkian evolution...interesting.
Jan 23, 2019 4:41PM
Yvonne Whysall
We know they were big by the size of their natural history museum in London
Jan 23, 2019 6:41AM
Don Racette
wyodinosaur I'm 6'4" Am I aalso a dinosaur
Jan 21, 2019 5:47PM
The movies seem to mostly depict dinosaurs as larger than they actually were. That probably contributes to the idea that they were overly large.
Jan 21, 2019 4:25PM

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