Big Dinos

by Robert Coontz

from Muse, March 1999

What put the "size" in Seismosaurus?

If you ever want to embarrass a group of dinosaur experts, ask them this simple question: Why were dinosaurs so big? Here is what will happen. First, they'll tell you what an interesting question you've asked. Then they'll point out that not all dinosaurs were big. After all, the smallest ones we know about (Compsognathus and Sinosauropteryx, if you want to look them up) were the size of chickens, and even the biggest ones had small ancestors. The earliest known dinosaurs were about the size of a football linebacker with a fat tail.

Yes, you can say, but look what came along next: Diplodocus, 90 feet long and 20 tons; Brachiosaurus, 80 feet long and 80 tons; Seismosaurus, maybe 120 feet long and 100 tons. These animals (called sauropods) were all built much the same way, with long, hose-like necks, long whip-like tails, huge bodies, and four legs. The biggest ones were as heavy as box cars and as tall as oak trees, and in the 65 million years since they died out, no other land animal has come close to being that big. Why not?

"The short answer is nobody knows," says James Farlow, a geologist at Purdue University in Indiana who studies sauropods and other dinosaurs. The reason, of course, is that no one has ever seen a live sauropod, and some of the most interesting parts of them don't fossilize. That's why, when you ask scientists about dinosaurs, they often end up talking about things that they can see and study—things like elephants, dandelions, and water balloons. (Yes, water balloons. Read on.)

There are plenty of good reasons why an animal would want to be big. The obvious one is safety: the bigger you are, the harder it is for other animals to kill and eat you. A 400-pound lion would be crazy to attack a 12,000-pound elephant. What’s more, if you are an herbivore, or plant eater, being bigger makes it easier for you to digest the things you eat. Obviously, an elephant can hold much more food than a mouse can, and the elephant's barrel-sized stomach can hold it for longer, too. That means that an elephant's food spends more time sloshing around in digestive juices. As a result, an elephant can digest just about any leaves or twigs it cares to cram down its throat, while a mouse has to nibble on tender shoots.

Being big also makes it easier to store food as fat and muscle. That can come in handy during tough times such as droughts. "Big animals are better able to weather short-term calamities than small animals are," Farlow says. "If you're an elephant, you can fast a lot longer than a mouse can without starving to death." Advantages like those explain why the puny human-sized ancestors of sauropods grew into hulking giants. Over millions of years, slightly bigger sauropods were slightly more successful at finding food and avoiding predators. They lived longer, laid more eggs, and hatched more young, which inherited their parents' size. Gradually, the animals grew larger and larger until they became enormous and stayed that way.

Supergiant sauropods thudded around for 120 million years, longer than any other group of large herbivores that has ever lived on Earth. So why haven't mammals done the same? A few of them have—almost. Scientists think that the largest land mammal ever was either a kind of mammoth or a long-legged rhinoceros called Indricotherium, which was as tall as a giraffe and lived in Asia 35 million years ago. Whichever animal it was, it probably weighed no more than 15 tons—as much as three elephants, but only a seventh the size of Seismosaurus. If bigger is better, why were dinosaurs seven times better than mammals?

Here are a few of the ideas scientists have come up with.

• Because they abandoned their kids.

Big mammals such as human beings (or cattle or horses or, yes, elephants) give birth to relatively few offspring but lavish attention on them so most of them will make it to adulthood. That means there have to be a lot of adult animals around to take care of the young ones. Since caregivers have to eat, and food is limited, adult mammals can get only so big.

Dinosaurs, however, probably reproduced more like dandelions. When a dandelion casts its downy seeds onto the breeze, it doesn't lift a tendril to help them; it just sits back and lets them succeed through sheer force of numbers (and usually they do). That's how sauropods may have treated their eggs. Gregory Paul, a dinosaur researcher and writer in Baltimore, Maryland, thinks dinosaurs probably laid as many as a hundred eggs a year and left the hatchlings to fend for themselves. A lot of the babies died, but that didn't matter: there were a lot of them to start with and plenty more where they came from. As a result, there didn't need to be many adults around at any one time. And with fewer grownups munching away at the tree ferns, the ones who were around could find enough food to get enormous.

• Because there was something in the air.

Oxygen in the air, to be precise. Some scientists think that the air at the time of the dinosaurs contained a lot more oxygen gas than it does today. Since the oxygen animals breathe is used in chemical reactions that power their muscles, more oxygen in the air might have made it possible for dinosaurs to get more oxygen into their muscles. That could have made them much stronger than animals today—strong enough to carry around huge bodies. It might also explain how sauropods managed to breathe through those long, skinny necks of theirs. To see why that's a problem, try putting a drinking straw in your mouth and breathing through it. Quite a challenge, isn't it? But it would be easier to breathe if the air had more oxygen in it.

The evidence for more oxygen in the air at the time of the dinosaurs comes from bubbles in 100-million-year-old amber, tree sap that hardened during the time of the dinosaurs. By crushing the amber and analyzing the gases that come out, scientists at the U. S. Geological Survey think they can tell what the air was like back then. Other scientists who have done similar experiments, however, think the oxygen content probably was not higher. Still others suspect that dinosaurs were better at capturing oxygen because they had more efficient lungs than mammals do. That's certainly possible—after all, birds, the closest living relatives of ancient dinosaurs, have better lungs than mammals. Until some lucky paleontologist digs up a dinosaur lung, though, nobody will know for sure.

• Because they were stupid.

Scientists may never find a sauropod lung, but they have found plenty of sauropod skulls. When Gregory Paul looks at them, he sees two striking things: "Small heads and small brains." With less weight to support on top, he says, sauropods could afford to grow incredibly long necks. What’s more, because a small brain needs less blood than a big one, a small-brained animals heart doesn't need to work as hard to move the blood to it. Instead of pumping a lot of blood a little way up, as your heart does, a sauropod's heart could pump a little blood a long way up. Gregory Paul thinks that is why sauropods could grow more than 50 feet tall, three times taller than a giraffe. That might explain the long necks. But what about those gi-normous bodies? McNeill Alexander, a biologist at the University of Leeds in England, has another explanation for that:

• Because they were cold-blooded.

Yes, cold. As you probably know, mammals and birds have developed ways to keep their bodies at a fairly constant temperature. That is a useful trick, because the chemical reactions necessary for life and movement take place faster in a warm body. As a result, "warm-blooded" mammals and birds have an advantage over "cold-blooded" fish, reptiles, and amphibians, which get slow and sleepy whenever their bodies cool down too much.

Some scientists think that dinosaurs must have developed warm-bloodedness as well, in order to be more active. (Remember those velociraptors in Jurassic Park?) Others, though, think that the big dinosaurs, at least, must have been cold-blooded. Here's why: animals are mostly made of meat, and meat is mostly made of water. Therefore animals are basically water balloons. "We're sacks of water with our innards well stirred by blood circulation," Alexander says. Warm-blooded animals are water balloons with heating coils inside. Since water is probably the best-studied stuff in the universe, scientists can easily calculate how hot a sauropod-sized water balloon would get with its heater at various settings. When they do that, they find that warm-blooded sauropods would have had a lot of trouble getting rid of their excess heat. As Alexander puts it, "Elephants that were as big as dinosaurs would be well-cooked steaks." Cold-blooded animals, however, probably would be quite comfortable at sauropod scale. That may be the limit, though. Alexander suspects that anything much larger than the biggest known sauropods would have stewed in its own juices regardless. Overheating may explain why the giant dinosaurs didn't grow even bigger than they did, he says.

So if your little brother is still having nightmares about Godzilla, tell him to relax. Anything that big would have cooked itself into monsterburgers a long time ago. That is, assuming Alexander isn't mistaken. As he himself admits, "One of the joys of dinosaur research is that it's very hard to prove you're wrong."

Copyright ©1999 Robert J. Coontz, Jr.