How Kangaroo Limb Lengths Help Us Understand How Extinct Species Moved

Jenn Hoskins
26th June, 2024

How Kangaroo Limb Lengths Help Us Understand How Extinct Species Moved

Compared to the sacra of a hopping Macropus (b) and striding Simosthenurus (c), the unusually broad and massive sacrum of Protemnodon (a) suggests an adaptation for a quadrupedal gait that braced against the rotational forces of independent hindlimb movement.

Image adapted from: Jones et al. (Source), illustration by Science Graphic Design / CC BY

Key Findings

  • The study from the University of Bristol focused on the extinct giant kangaroo genus Protemnodon, which weighed over 100 kg
  • Researchers found that large Protemnodon species had limb proportions different from modern kangaroos, suggesting they did not primarily hop
  • The study suggests these giant kangaroos likely used a quadrupedal bounding gait, which was more feasible for their large body sizes
Kangaroos, members of the Macropodoidea superfamily, exhibit a range of locomotor behaviors, from quadrupedal bounding to bipedal hopping. However, the feasibility of hopping as a primary mode of locomotion in giant extinct kangaroos, particularly those exceeding 100 kg, has been a subject of debate. Recent research from the University of Bristol[1] investigates the locomotor modes of these large kangaroos, focusing on the extinct genus Protemnodon, which has been assumed to hop like its extant relatives. Previous studies have shown that large kangaroos, such as the extant species, utilize elastic strain energy stored in their hind limbs during hopping, making it an efficient gait at higher speeds[2]. However, this energy-efficient hopping has its limitations, particularly for larger body masses. The extinct sthenurine kangaroos, for example, were proposed to use a bipedal striding gait rather than hopping, due to their anatomical adaptations[3]. These adaptations included larger hips and knees, and a stabilized ankle joint, which supported their body weight differently compared to modern kangaroos. In the current study, researchers examined the limb proportions and functional indices of Protemnodon species to determine their primary locomotor mode. The analysis revealed that large Protemnodon species, which exceeded 100 kg, exhibited limb proportions distinct from any known macropodoids. This finding challenges the assumption that these giant kangaroos primarily used hopping for locomotion. The study incorporated previously published evidence on humeral morphology, which supported the hypothesis of a primarily quadrupedal mode of locomotion for large Protemnodon species. The researchers suggest that these kangaroos likely employed some form of bounding, a gait that involves a sequence of jumps with all four limbs leaving the ground simultaneously. This mode of locomotion would have been more feasible for their large body sizes compared to hopping. The findings of this study are significant as they expand our understanding of the diversity of locomotor behaviors in kangaroos, particularly in extinct species. By examining the functional limb indices, the researchers provided evidence that large Protemnodon species were adapted for a different type of locomotion, challenging previous assumptions based on their phylogenetic relationship with extant kangaroos. This study also ties together previous research on kangaroo locomotion. For instance, the positive allometry of structures in the legs and feet of Macropodoidea, which allows for efficient energy storage during hopping, was shown to be limited by body size[2]. The adaptations observed in sthenurine kangaroos for bipedal striding further illustrate how different locomotor strategies evolved in response to increasing body size[3]. Additionally, the study aligns with the findings of phylogenetic analyses that have revealed distinct evolutionary lineages within macropodids, including the close relationship between Protemnodon and extant large kangaroos[4]. In conclusion, the research from the University of Bristol provides compelling evidence that large Protemnodon species were not primarily hoppers, but rather employed a quadrupedal bounding mode of locomotion. This study enhances our understanding of the evolutionary adaptations in kangaroo locomotion and highlights the diversity of strategies that evolved in response to different ecological and physiological constraints.

GeneticsAnimal ScienceEvolution

References

Main Study

1) Hop, walk or bound? Limb proportions in kangaroos and the probable locomotion of the extinct genus Protemnodon

Published 25th June, 2024

https://doi.org/10.1007/s10914-024-09725-4

Related Studies

2) Scaling of elastic strain energy in kangaroos and the benefits of being big.

Journal: Nature, Issue: Vol 378, Issue 6552, Nov 1995

3) Locomotion in extinct giant kangaroos: were sthenurines hop-less monsters?

https://doi.org/10.1371/journal.pone.0109888

4) Late pleistocene Australian marsupial DNA clarifies the affinities of extinct megafaunal kangaroos and wallabies.

https://doi.org/10.1093/molbev/msu338


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