Giraffes struggle to walk up steep hills because they are too top-heavy and may fall over, scientists believe.

The savannah giants are the tallest land animals in the world and can reach heights of more than 18ft, weighing almost two tonnes.

A study has found that a slope of more than a 20 degree incline is a common barrier for giraffes, according to GPS data which tracked 33 of the animals in South Africa.

A project run by the University of Manchester and the University of the Free State analysed where giraffes live in the wild and what incline that terrain has.

Data show that the animals can cope with a slope of 12 degrees if there is a good food source to motivate them for the climb, but tend to avoid anything more than 20 degrees.

“They will tolerate some steepness to access food, but simply cannot access areas above a 20 degree gradient. It’s quite shocking when you look at distribution maps,” said Jessica Granweiler, study author and a PhD candidate at the University of Manchester.

She added that although giraffes are adaptable animals they also suffer with “physiological limits”.

The work was presented at the British Ecological Society’s annual meeting in Liverpool and is set to be published in a journal.