More than 30,000 years ago, giant kangaroos walked the Earth. Three times the size of today's largest species, these colossal beasts were over 10 feet tall and weighed 240 kg — or 529 lbs. Because of their size, it was hard to tell if they would have moved around like kangaroos do today: by hopping at high speeds and walking on five limbs (arms and tail included) when they slowed down. Now, a study published in PLOS One suggests that these giants may have developed an upright posture — a posture that would have allowed them to put one powerful, gigantic foot in front of the other. In short, they probably didn't hop at all.
A rigid back limited their ability to hop
In the study, researchers examined the bones of over 45 different species of kangaroos, including bones from the extinct giant kangaroo, Sthenurus stirlingi. This helped them identify some notable differences between current and past species. For instance, the study shows that giant kangaroos had larger knees and hips than the species we see today, as well as stabilized ankle joints. Moreover, they found that the backbone of the giant kangaroo was a lot more rigid than those of other species, a characteristic that indicates that the muscles that flex the spine were probably a lot smaller, proportionally speaking. "This would limit the ability to hop," the researchers write, but would have braced the backbone against the rotational forces generated by putting one foot in front of the other.
Because of these differences, the researchers think that as kangaroos became larger, some species started walking on two limbs at slower speeds, instead of bending down to walk on their arms, legs, and tail. And as their backs became more rigid, their tails shortened, because they didn't need them for support, and a long tail would have gotten in the way. Eventually, the largest species stopped hopping altogether.
Imagine a 500-lbs kangaroo walking around on two legs
What's notable about this finding — besides the visual of a 500-lbs kangaroo walking around on two legs — is that, with the exception of the musky rat-kangaroo, every single species of kangaroo alive today hops. This indicates that hopping is probably an efficient way of getting around, even for larger species. It's also a good explanation for why scientists took so long to suggest that giant kangaroos weren't capable of moving around that way. When researchers look at the bones of extinct specimens, they often use information from living relatives to interpret what they see. Generally, this works pretty well. But every once in a while, you encounter a giant kangaroo. And that kangaroo turns out to be hop-less.