Giant prehistoric kangaroos walked, not hopped
An extinct group of giant kangaroos, that died out around 30,000 years ago, got around by walking rather than hopping, says a new study.
These sthenurines, or short-faced kangaroos, included species that were more than three times the size of the largest kangaroos today. The largest, Procoptodon goliah, was 2.7m tall and weighed up to 240kg. These animals lived alongside modern species of kangaroo, but specialised on a diet of leaves from trees and shrubs.
Scientists have speculated that such large kangaroos would have had difficulty hopping, with previous studies suggesting the sthenurine anatomy was better suited to the way modern kangaroos get around slowly, using their tail as a fifth limb.
Now, detailed comparisons of limb bones from 140 species of extinct and modern kangaroos show that while sthenurines share many similarities with modern species, they also have key differences suggesting they walked rather than hopped.
Extinct giant kangaroos walked upright
Broad hips and ankle joints adapted to resist torsion or twisting, point to an upright posture where weight is supported by one leg at a time, says Dr Christine Janis from Brown University, USA, who led the study published today in the journal PLoS One.
Their broad hips also allowed for another important modification: large buttocks – a feature shared with other walking species. “These muscles are larger in humans than in [other] apes, and…prevent us from toppling over when we stand on one leg,” she says.
But, these features “don’t correlate with hopping behaviour, and are best explained by them bearing weight on one leg at a time,” Christine says.
The findings come as no surprise to some experts. “I certainly don’t see bipedal striding as somehow impossible,” says Rodger Kram, a biomechanist from the University of Colorado, Boulder, in the USA. “Modern kangaroos move their legs alternately when swimming, so the neural pathways exist even if they are seldom used.”
A new perspective on kangaroo evolution
The findings cast new light on kangaroo evolution, says Christine. “We’ve always known that [sthenurinae] skulls were different from modern kangaroos, and that they had a different diet.”
But scientists haven’t really considered a different type locomotion altogether, says Professor Mike Archer, a palaeontologist at the University of New South Wales in Sydney. “No-one had suspected that these extinct ‘ordinary’ kangaroos couldn’t hop, which makes this a very interesting study indeed.”
“One of the things that makes the question hard to answer is that there are many similarities between the skeletons of living and extinct kangaroos, and that for the most part, [Macropods] – kangaroos, wallabies and their relatives – have evolved to bipedal hopping locomotion,” says Natalie Warburton, a vertebrate anatomist at Murdoch University in Perth.
Walking may have been a key factor behind the large body size developed by some sthenurines. Small wallaby-sized sthenurines probably walked occasionally, when moving slowly, says Christine. But as they evolved toward bigger body sizes, their moving strategy evolved too.
“[Walking] allowed some species to evolve to large body sizes where hopping would have been an unlikely way of getting around,” she adds.
