The Only Living Kangaroo That Doesn't Hop

In the remnant rainforests of coastal far-north Queensland, bushwalkers may be lucky enough to catch a glimpse of a diminutive marsupial that's the last living representative of its family.

The musky rat-kangaroo (Hypsiprymnodon moschatus) weighs only 500 grams and looks a bit like a potoroo. It's part of a lineage that extends back to before kangaroos evolved their distinctive hopping gait.

Unlike their bigger relatives, muskies can be seen out and about during the day, foraging in the forest litter for fruits, fungi and invertebrates.

As the only living macropodoid (the group that includes kangaroos, wallabies, potoroos and bettongs) that doesn't hop, they can provide a crucial insight into how and when this iconic form of locomotion evolved in Australia.

Our study, published in Australian Mammalogy today, aimed to observe muskies in their native habitat in order to better understand how they move.

Why kangaroos are special

If we look around the world, hopping animals are quite rare. Hopping evolved once in macropodoids, four times in rodents, and probably once in an extinct group of South American marsupials known as argyrolagids.

In animals heavier than five kilograms, hopping is an incredibly efficient form of locomotion, in large part thanks to energy being stored in the Achilles tendon at the back of the heel.

However, the vast majority of animals that hop are really small. The only hopping animals with body masses over 500 grams are kangaroos. And Australia used to have a lot more kangaroo species, many of them quite large.

Despite the abundance of fossil kangaroos, we still don't really know why they evolved their hopping gait, especially given it only really becomes more efficient at body masses over five kilograms. Hypotheses range from predator escape, to energy preservation, to the opening of vegetation as Australia shifted to a drier climate.

Researchers looking at limb proportions have suggested that fossil kangaroos also hopped. But it's likely the ways that extinct roos moved were much more diverse than has previously been suggested.

Why muskies are key in roo evolution

Muskies are the last living member of the Hypsiprymnodontidae, a macropodoid family that branched off early in kangaroo evolution. For this reason, it is thought muskies may move in a similar way to early kangaroo ancestors.

Studies on kangaroo evolution will often mention locomotion in muskies, but only in passing. And only a single, brief, first-hand description of locomotor behaviour in muskies has actually been published, in 1982. The authors observed that muskies moved their hindlimbs together in a bound and that all four limbs were used, even at fast speeds.

So, we set out to answer the question: can H. moschatus hop? And if not, what form of locomotion does it use?

Using high-speed video recordings, we studied the sequence in which muskies place their four feet on the ground, and the relative timing and duration of each footfall.

Through this gait analysis, we determined that muskies predominantly use what is called a "bound" or "half-bound" gait. Bounding gaits are characterised by the hindfeet moving together in synchrony - just like when bipedal kangaroos hop. In the case of muskies, the forefeet (or "hands") also generally move together in close synchrony.

No other marsupial that moves on all fours is known to use this distinctive style of movement to the same extent as muskies. Rather, other species tend to use a combination of the half-bound and some form of galloping (the gait that horses, cats and dogs use) or hopping.

From all fours to hopping

We were also able to confirm that tantalisingly brief observation from the 1980s: even when travelling at high speeds, muskies always use quadrupedal gaits, never rearing up on just their back legs.

They are, therefore, the only living kangaroo that doesn't hop.

Combined with further investigation of their anatomy, these observations help us get closer to understanding how and why kangaroos adopted their distinctive bipedal hopping behaviours.

These results also signal a potential pathway to how bipedal hopping evolved in kangaroos. Perhaps it started with an ancestor that moved about on all fours like other marsupials, such as brush-tail possums, then an animal that bounded like the muskies, and finally evolved into the iconic hopping kangaroos we see in Australia today.

However, we are no clearer on how the remarkable energy economy of kangaroo movement evolved, or why hopping kangaroos got so much bigger than hopping rodents.

The next part of the research needs to focus on that and will be informed by key fossil discoveries from early periods in kangaroo evolution.

(Author: Aaron Camens, Lecturer in Palaeontology, Flinders University, Amy Tschirn, PhD Candidate in Vertebrate Palaeontology, Flinders University and Peter Bishop, Postdoctoral research fellow, Harvard Kennedy School)

Disclosure statement: Amy Tschirn received funding from an Australian Government Research Training Program Scholarship and an Australian Research Council Discovery Grant (to G.J.P) during this project.

Aaron Camens and Peter Bishop do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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