Giant plant-eating dinosaurs had stiffer necks than the creatures portrayed in films and on TV, new research has suggested.

Their long necks were probably more like that of an ostrich with only a limited amount of movement, according to scientists.

In the BBC series Walking with Dinosaurs, huge sauropods such as diplodocus and apatosaurus are depicted with flexible necks that allowed them to reach for tree-tops and low-growing vegetation.

But the portrayal, based on computer simulations, is likely to be inaccurate, the study authors claim.

Previous estimates of sauropod neck flexibility have focused largely on the neck bones and failed to take account of soft tissues such as muscle and cartilage, they point out.

The new research involved studying the anatomy of a distant relative of the dinosaurs, the present-day ostrich.

It showed that increasing muscle mass reduced neck flexibility.

Variations in the distance between vertebral joints and the amount of cartilage present may also have stiffened sauropod necks.

Having less flexible necks could have restricted the range of foods accessible to the dinosaurs, the US scientists writing in the online journal Public Library of Science One said.

It meant they may have had to forage more actively to meet their average dietary needs of around 400 kilograms of vegetation a day.

Study leader Matthew Cobley, from the University of Utah, said: “I believe the most important thing to take away from this study is that computer modelling of any biological system – be it anything from an individual organ to a whole dinosaur – needs to be ‘ground-truthed’ before it is accepted by the scientific community and presented to the public.

“It’s easy to be swayed by these beautifully reconstructed models of dinosaurs, but if these models aren’t based on real, empirical data taken from living animals we can actually study, they only serve to confuse.”