Biomechanical limits of hopping in the hindlimbs of giant extinct kangaroos

The locomotor abilities of animals depend upon their body size. Today, kangaroos are the largest hopping mammals, but some of their Pleistocene relatives were larger still—more than twice as heavy as the largest extant species. So, is there an upper size limit of bipedal hopping? Here, we integrate scaling data from extant species with direct observation of the hindlimb bones of giant fossil species to improve our understanding of the mechanical limitations faced by kangaroos during hopping. We test two potential limiting factors on hopping —bone strength, and tendon size. We find that (a) the metatarsals of giant kangaroos would be capable of resisting the bending moments involved in hopping, and (b), the calcanea (ankle bones) of giant kangaroos could accommodate tendons large enough to resist the loads generated during hopping. Thus, contrary to previous analyses, we do not find strict physical limitations on hopping in giant kangaroos. While hopping may not have been their primary mode of locomotion, our findings suggest that it may have formed part of a broader locomotor repertoire, for example for short bursts of speed.