The Giant Kangaroos That Didn't Hop

Ask anyone to describe a kangaroo and the word 'hopping' is likely to feature in their reply. It's a quintessential part of the popular image of kangaroos, and with good reason: every known species uses it to get around (with the possible exception of the musky rat-kangaroo, which may have lost the ability). That hasn't always been the case, though. Kangaroos were more diverse until the end of the Pleistocene around 30,000 years ago, and new research suggests that many of the extinct species may have given up walking for a different way of getting around. Have a look at this shambling monster:



This reconstruction is based on an analysis of the anatomy of several extinct species of 'giant' kangaroos (some species weighed as much as 240kg!). Differences in their limb bones and other aspects of their morphology has led researchers from Brown University and the University of Malaga to suggest that these kangaroos were bipedal and had lost the ability to hop.

Contrary to popular perception, modern kangaroos don't spend most of their time hopping around. Hopping is energetically efficient at high speeds, but it's not ideal for the slower speeds needed during foraging. Kangaroos often move using a 'pentapedal walk' in which they place their forelimbs on the ground and then use the tail as a 'fifth leg' for balance while they push their hindlimbs forward. The researchers think that this pentapedal walk was central in the evolution of bipedal giant kangaroos.

The key to this change is that some kangaroos started using their forelimbs for browsing. This led to anatomical changes which made it impossible for them to put their hands on the ground palm-down or to support their weight on their forelimbs, ruling out the pentapedal walk as a way of getting around. These animals probably still hopped when they had to move quickly, but shuffled about on two feet when moving slowly. This led to changes in their spinal anatomy to make them better adapted to upright walking, and these morphological changes would have made it possible for them to grow larger. Eventually, some species may have evolved to be so large that hopping was no longer a feasible way of moving around.

I originally decided to write about this research because I really wanted to share the lovely kangaroo drawing, but there's more to the story than just a cute picture. I assumed the paper would be about kangaroo ancestors from before they evolved to hop, but it turned out to be about extinct kangaroos which (probably) lost the ability to hop.
It's always interesting to see the winds and wends that evolution traces. This story is also an excellent reminder of how evolutionary history acts to constrain the options available as a species evolves. In the greatest story ever told, as in life, the past is always with us.

Ref
Janis CM, Buttrill K, Figueirido B. Locomotion in Extinct Giant Kangaroos: Were Sthenurines Hop-Less Monsters? PLoS ONE 9(10): e109888. (2014) doi:10.1371/journal.pone.0109888

The image is Figure 1 from the paper and is distributed under a CC-BY license.