Ancient giant kangaroos could hop despite their enormous size

Even the giant kangaroos that roamed Australia thousands of years ago could jump, according to a new bone analysis.

Some of the kangaroos living during the Pleistocene were more than twice as heavy as those living today. One group, the sthenurini, were so bulky that it was thought they couldn’t hop – they only had to walk on their hind legs.

“Sthenurines are what most people talk about when they talk about giant kangaroos. They’re the really weird ones,” he says Megan Jones at the University of Manchester in the UK. “They have these really short, boxy skulls and one toe on each foot. A big male red kangaroo is the biggest you’ll get today, about 90 kilograms, but the biggest sthenurine was about 250 kilograms.”

The giant was Procoptodon goliaththe largest species of kangaroo known to have existed, reaching a height of about 2 meters. It died out about 40,000 years ago.

However, it was always debated how much stress his legs could take. To try to get a better handle on this, Jones and his colleagues collected bone measurements of 67 species of macropods, a group that includes extant kangaroos, kangaroos, potoroos, bettongs and rat kangaroos, as well as extinct lines of giant kangaroos.

Based on measurements of the leg bones, including the femur, tibia and calcaneus — the bone where the Achilles tendon inserts — and body weight data, the researchers estimated how big the attached tendons would be and how much force they could handle.

“The Achilles tendon in today’s kangaroos is pretty dangerously close to breaking, but that serves a purpose,” says Jones. “It allows them to store a lot of elastic energy to push into the next jump. If you took today’s kangaroo and made it bigger, you’d be in trouble.”

But he says ancient kangaroos aren’t just scaled up. They have shorter feet and, for example, a wider heel bone. The researchers’ calculations show that this would help the bones of the giant kangaroos to withstand the bending moments associated with jumping and accommodate tendons large enough to withstand the loads generated during the activity.

“It’s proof that they weren’t mechanically prevented from jumping,” says Jones. “Whether they jumped is another question.”

Hopping almost certainly wasn’t their primary mode of locomotion, but they could have used it for short bursts of speed, he says.

“The study supports what is now a solidifying picture of the iconic kangaroo hop as a functionally adaptive component of a surprisingly variable gait repertoire,” he says Benjamin Kear at Uppsala University in Sweden. This repertoire has been key to the ecological success of macropods for many millions of years, he says.

This flexibility is still evident today. Although we might think of red kangaroos as always hopping, they can also walk and use their tail as a fifth limb, Jones says. “And tree kangaroos basically do everything under the sun: walk, hop, bound, quadrupedal and even bipedal.”

topics:

• development/
• animal behavior