It's certainly Australia's iconic animal, yet boffins have claimed the kangaroo hasn't always performed its most famous trick: hopping.

Once upon a time, the Skippy the Bush Kangaroo's ancestors may not have actually bounded along, instead favouring a gait akin to a drunken human.

"At best, they'd have been really clumsy hoppers," said study author Professor Christine Janis, a professor of ecology and evolutionary biology at Brown University.

She found that an extinct species called the sthenurine could not have possibly hopped along because its spine and tail were too rigid. These massive beasts were also fatter than modern day beasties, weighing some 220kg.

This weight meant they were just too chubby to hop.

"Hopping is a tricky gait, and modern kangaroos are near the limit, in terms of size," Janis continued.

The modern day 'roo uses its flexible, springy tail as a kind of third leg. Previous research found that Skippy and his bush-based buddies' tails are so strong that they provide a stronger force than the front and hind legs combined.

However, Prof Janis focused on the slower amble kangaroos use when walking slowly, rather than hopping at speed. Modern day 'roos use their tail to help them move slowly, whereas the ancient beast was too heavy to rely on this method.

"They would have had to do something else to get around slowly," Janis added.

It is likely that the Sthenurine evolved from a hopping beast, but just got too fat and had to waddle instead.

A study on the research has been published in PLOS ONE under the name Locomotion in Extinct Giant Kangaroos: Were Sthenurines Hop-Less Monsters?. ®