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It is a colossal mystery, one an international team of researchers has been trying to solve for four years. Why did some dinosaurs get so big?

In today's edition of the journal Science, German paleontologist Martin Sander lays out a detailed explanation for how long-necked plant-eaters known as sauropods were able to grow as tall as a six-storey building, dwarfing meat-eaters like Tyrannosaurus rex and making modern giants, like elephants, seem puny by comparison.

One element of the new theory is unlikely to please parents trying to teach their kids table manners: Sauropods didn't chew their food.

Creatures like Argentinosaurus, the biggest of them all, gobbled leaves without masticating, and their massive guts guaranteed the long digestion times needed to process large mouthfuls of foliage and bark, Dr. Sander says.

He co-ordinates a project on sauropod gigantism at the University of Bonn, and he and his colleagues studied fossils, used computer models and even replicated dinosaur digestion in the lab to learn more about the most massive creatures to ever walk the Earth.

The fact that Brachiosaurus and other sauropods didn't chew their food meant that they didn't need big teeth and had relatively small, light skulls.

Over time, this allowed them to evolve such long necks, Dr. Sander says.

The necks gave them access to food that was out of the reach of other animals. Sauropods could also forage without moving their entire bodies.

The vegetarian dinosaurs were in a constant evolutionary size race with the meat-eaters.

Over time, the plant-eaters got bigger because larger animals were less vulnerable to predators, he says.

Carnivorous species also grew in size, but their growth was limited because they walked on two legs.

Plant-eaters could support more weight because they walked on four legs and didn't have to work as hard to get a meal.

The sauropods weighed roughly 50 to 80 tonnes, and to get that big, they must have been warm-blooded, he argues.

Ten-kilogram hatchlings became 100,000-kilogram adults by their third decade of life.

“If we look at these bone tissues they couldn't have a metabolism like a crocodile, it is very much like that of mammals,” Dr. Sander says. But their metabolism may have slowed down once they reached full size, he says.

The sauropod conundrum has intrigued Dr. Sander for years, and in 2004 he brought together experts from different fields to investigate.

“It is an age-old question kids have been asking for 100 years. Why did the dinosaurs get so big? Why do we not have 50-tonne elephants running around today. Why do mammals not get this big?”

He and his colleagues studied whether the Earth was different during the dinosaur era, but determined that conditions were probably less favourable for plant and animal life back then than they are today.

The concentration of oxygen in the atmosphere was much lower, for example.

The researchers concluded that the explanation must lie in the unusual biology of the sauropods.

The enormous creatures dominated for more than 100 million years, from the middle of the Jurassic to the end of the Cretaceous period, when dinosaurs disappeared from the fossil record.

One reason for that success is that the females laid lots of relatively small eggs. With big modern mammals such as elephants, the larger they get, the fewer offspring they have, Dr. Sander says.

This can make it difficult for a population to recover from drought or other catastrophes.

But the female sauropods probably produced hundreds of eggs a year.

“One fertilized female could re-establish a population quite nicely,” Dr. Sander says.

His co-author on the Science paper is Marcus Clauss, a veterinarian with the clinic for zoo animals, exotic pets and wildlife at the University of Zurich in Switzerland.

Dr. Clauss estimates that a 30-tonne sauropod would have had at least 3,000 kilograms of food in its gut, which would have fermented over a period of a few days. The animals likely grazed most of the time, and produced massive amounts of excrement.