Giant ancestors of modern-day kangaroos — which previous research has estimated could weigh up to 250 kilograms — may have been able to hop in short bursts, according to research published in Scientific Reports. These findings challenge those of previous studies suggesting that giant kangaroos weighing more than 160 kilograms were too heavy for their ankles to withstand hopping.
Megan Jones and colleagues studied the hindlimbs of 94 modern and 40 fossil specimens from 63 kangaroo and wallaby species — including members of the extinct giant kangaroo group Protemnodon, which lived during the Pleistocene (between 2.6 million and 11,700 years ago). For each species they used published estimates of their weight and the length and diameter of their fourth metatarsals — an elongated foot bone key for hopping in modern kangaroos — to calculate whether they could have withstood the strain of hopping. The authors then compared the heel bone structures of giant kangaroos with those of modern kangaroo species. They estimated the size of the tendon required to withstand the forces required to facilitate hopping in giant kangaroos and calculated whether their heel bones would have been large enough to accommodate tendons of this size. The authors predict that the metatarsals of all giant kangaroo species would have been robust enough to withstand the physical stress caused by hopping, and that their heel bones would have been big enough to allow for the tendon width required for hopping.
Together, the findings suggest that the hindlimbs of all giant kangaroos were robust enough for them to be able to hop. However, the authors propose that it is unlikely giant kangaroos would have relied on hopping for all locomotion owing to their large body sizes making this inefficient over longer distances. They note that sporadic hopping is already seen in many of today's smaller species, such as hopping rodents and smaller marsupials. They speculate that short, quick bursts of hopping may have helped some giant kangaroo species evade predators, such as those belonging to a group of extinct marsupial lions known as Thylacoleo.
