An inability to chew may be one reason why giant long-necked dinosaurs such as Brachiosaurus and Diplodocus grew to be so large, scientists believe.

Because they swallowed plant material whole, they needed massive guts to allow enough time for it to be digested - hence an enormous body.

Having no chewing jaws also meant the creatures could have small heads supported by long necks.

Although this gave them tiny brains in relation to their bodies, it offered the major advantage of gaining access to vegetation other animals could not reach.

Other factors such as high growth rate, flexible metabolisms and egg laying may also have helped to keep the creatures large, according to two experts writing in the journal Science.

The four-footed sauropod dinosaurs, characterised by their long necks and tails and huge bodies, were the largest animals ever to walk the earth. They weighed up to 80 metric tons - 10 times more than an African elephant - and grew to lengths of more than 40 metres.

Environmental factors during the dinosaurs' reign cannot explain why the animals grew so vast, said Dr Martin Sander from the University of Bonn in Germany and Dr Marcus Clauss from the University of Zurich in Switzerland.

The animals had a wide range of mouth, teeth and neck designs indicating that reliance on one particular food source was not the likely answer. But one shared primitive characteristic truly set them apart: they did not chew their food, or grind it in a "gastric mill" like other plant-eating dinosaurs.

"Because gut capacity increases with body mass, the enormous gut capacity of sauropods would have guaranteed the long digestion times necessary for degrading unchewed plant parts, even at relatively high food intake," the scientists wrote.