Sauropod dinosaurs were large in size due to the kind of plant food they ate, a new study finds.

Sauropods are believed to be the largest land animals to have ever walked the Earth. They were herbivorous animals that belonged to a subgroup of "lizard-hipped" dinosaurs. They appeared in the Late Triassic period and began to diversify in the Middle Jurassic period, about 180 million years ago. These giant dinosaurs had a long neck and tail, but possessed a relatively small skull and brain, according to University of California Museum of Paleontology.  

Earlier studies by South African researchers have already suggested a theory that sauropods' large size was due to the nature of food they ate, but several experts did not accept the idea. Now, a team of researchers from the U.K. have argued that the South African idea still holds true.

The researchers point out that some scientists have been confused between two different issues - how much nitrogen is in the plant and the total energy in the plant food. The idea, proposed by the South African researchers, was based on nitrogen content and not the total energy of the plant food.  

They suggest that the South African idea could still be considered as an explanation for the sauropod's huge size.

According to study author David M Wilkinson, an ecologist at Liverpool John Moores University, U.K., young sauropods might have used the nitrogen content in the plants to generate metabolism, but it would have been impossible for the adult sauropods because of a risk of getting overheated, as a result of heat produced by metabolism.

"Alternatively - or in addition - it would also have been potentially beneficial for the young to be carnivorous, as this would also have helped them access more nitrogen. The large adults plausibly used their size to help process large amounts of plant food to access enough scarce nitrogen, as suggested in the original 2002 study," Wilkinson said in a statement.

"However this would potentially have caused them to have to take in more energy than they needed. A mammal (and possibly also small sauropods) would get rid of this surplus as heat, but this would not be possible for a really large dinosaur. Potentially they may have laid down fat reserves instead. So one can even speculate that they may have had humps of fat rather like modern-day camels," he added.

The findings of the study are published in the journal Functional Ecology.