Procoptodon and its relatives couldn't do either. With increasing size, Janis and coauthors point out, there's an ever greater risk of tendon rupture while hopping. Not to mention that the extinct kangaroos, driven to extinction by climate change or hungry humans, had rigid backs that would have hindered their ability to bounce. Nor could the giants do the five-point walk. Their palms couldn't properly rotate to brace the ground, and their shorter tails lacked some of the strong musculature needed to do the big push-off.
But there is another option. Living tree kangaroos, the researchers note, sometimes walk upright along branches, and the giant, extinct kangaroos could have moved in a similar way, waddling back and forth as they held their torsos upright and shifted between one foot and the next.
They likely inherited this way of walking from their smaller ancestors. Predecessors of Sthenurus were diminutive enough for hopping to be an option, but their forelimbs were already adapted to grasping branches and made it unlikely that they could use the pentadal walk. At slow speed, Janis and colleagues suggest, they may have moved with a tottering walk. And as their descendants became bigger and bigger, this strange strut became the main way of walking for the heftier Ice Age kangaroos.
Unfortunately, these giant kangaroos didn't leave any modern-day relatives to study. Janis and colleagues hope that fossil kangaroos tracks - imprints that trace prehistoric behavior - will help test their idea. For now, we're left with their bones and the ability of our imaginations to envision the long-lost sloth-roos.
For more information on why these giant 'roos walked one step at a time, hop over to Animalist News: