Unlike mammals, but exactly like birds, large plant-eating dinosaurs tended to swallow their food whole rather than bothering to pre-chew it, according to a team of German researchers.

The new theory could explain why dinosaurs such as brontosaurus, which weighed up to 100 tons, had long necks and relatively tiny heads.

Until now, it was generally thought that birds swallow their food whole so as to reduce the time they are vulnerable to attack from predators. While that may be partly true, the new evidence indicates that birds are simply following in the footsteps of their gigantic ancestors, the dinosaurs, who had to devour vast quantities of food as fast as possible just to stay alive, the scientists said.

Elephants and other large modern animals that eat a lot and chew need big heads to accommodate jaw muscles and molars. But a large head would not be necessary for a creature that swallowed down food without chewing.

Long necks would also have helped big dinosaurs get to food without moving from a particular spot — again saving time.

While elephants need to spend 18 hours eating to satisfy their appetites, a giant dinosaur with normal eating habits would have had to be feeding an impossible 30 hours a day.

The researchers think they got around that time problem by simply gulping down their food to get it into their stomachs as quickly as possible.

“Chewing is a property which no large herbivorous terrestrial mammal has got rid of,” said Dr. Martin Sander from the University of Bonn, Germany in a report published in the journal Biological Reviews.

A staple part of the vegetarian dino-diet was probably horsetails, fern-like plants that were abundant in prehistoric swamps and highly nutritious, said the German scientists. Few animals feed on them today because they contain a lot of hard silicate which is bad for teeth.

But this would not have been a problem for dinosaurs that plucked and swallowed the plants without chewing. Sauropods are also known to have renewed their teeth frequently, sometimes as often as once a month.

The dinosaurs’ large gizzard—like stomachs and powerful metabolisms would have helped them cope with so much unchewed food, the researchers believe.

Dinosaurs possessed a highly efficient bird—like breathing system involving large numbers of air sacs permeating their body cavity and bones.

“In the history of species the lungs of today’s birds and of the giant dinosaurs have the same origin,” said Dr Sander.

“Two hundred million years ago, an unparalleled combination developed of primitive traits, which were new in the history of evolution. This combination made these fascinating giants possible,” he said.