The long necked sauropod dinosaurs were the largest land animals ever to walk the Earth - but why were they so large? A decade ago a team of plant ecologists from South Africa suggested that this was due to the nature of the plant food they ate, however these ideas have fallen out of favour with many dinosaur researchers.

Now Liverpool John Moores University's (LJMU's) Dr David Wilkinson and Professor Graeme Ruxton of University of St Andrews, Scotland, argue that this idea still has legs.

The results have been published in the journal Functional Ecology published by the British Ecological Society this month. They suggest that this mistake happened because some scientists confused two different issues in thinking about this problem; namely how much energy is in the plant with how much nitrogen is in the plant - the South African ideas were based on nitrogen content not the total energy in the plant food.

Dr Wilkinson and Professor Ruxton now argue that this South African idea about long necked sauropod dinosaurs being large based on the nature of the plant food they ate, is still a contender for explaining their size. As well as arguing that these ideas have been prematurely discarded the new work goes on the further develop this theory.

Dr David M Wilkinson who is an ecologist from the LJMU School of Natural Sciences and Psychology explains: "This new study makes a first attempt to calculate in more detail the implications of this idea. It suggests that it may have been to the advantage of young sauropods trying to get enough nitrogen to have a metabolism rather like modern mammals, but that this would have been impossible for the adults because of the danger of such large animals overheating from all the heat that such a metabolism would have produced."

"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.

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."

Listen to Robbie Wilson's podcast interview David Wilkinson on why sauropod dinosaurs were so large.

David M. Wilkinson and Graeme D. Ruxton (2012). 'High C/N ratio (not low-energy content) of vegetation may have driven gigantism in sauropod dinosaurs and perhaps omnivory and/or endothermy in their juveniles', doi: 10.1111/1365-2435.12033, is published in Functional Ecology on Wednesday 12 December 2012.