How did the kangaroo learn to hop? Insights from its smallest relative

Australia’s landscapes are home to one of nature’s most fascinating creatures – the kangaroo. Unique among large animals, kangaroos depend on hopping as their main form of locomotion.

This method of locomotion, known as saltation, allows them to cover vast distances with efficiency and speed. But how did kangaroos develop this distinctive way of moving?

Understanding their evolutionary history requires looking at their ancestors, particularly the musky rat-kangaroo, the only macropodoid that does not hop.

Kangaroo relatives that don’t hop

Amy Tschirn is a PhD student and researcher in the College of Science and Engineering at Flinders University, and co-lead author of the study.

“The musky rat-kangaroo, as the most primitive living macropodoid (a group that includes kangaroos, wallabies, potoroos and bettongs), can offer insight into evolution within the group, including the origin of bipedal hopping locomotion,” said Tschim.

“As the only living macropodoid that doesn’t hop, the musky rat-kangaroo provides a crucial insight into how and when the iconic hopping form of locomotion evolved in Australia.”

Studying the musky rat-kangaroo helps scientists understand how early macropodoids moved before developing bipedal hopping.

This tiny marsupial, found in the dense rainforests of Queensland, may hold the key to unlocking the evolutionary steps that led to modern kangaroos.

Musky rat-kangaroos in the wild

To study the movement of musky rat-kangaroos, researchers observed them in their natural habitat in the Atherton Tableland, a region in far-northern Queensland.

By filming adult muskies in the wild, they documented how these animals move and whether any form of hopping appeared in their locomotion. The findings revealed something remarkable.

Unlike any other marsupial that moves on all fours, musky rat-kangaroos use a distinct style of movement called a “bound” or “half-bound” gait. This means their hindfeet move together in synchrony rather than alternating as in most quadrupedal animals.

This movement pattern is unusual among marsupials and could provide crucial insight into how macropodoids transitioned toward hopping.

Role of quadrupedal movement

Observations also confirmed that musky rat-kangaroos never hop, even when moving at their fastest speeds. This means that hopping likely developed later in macropodoid evolution.

“There remains no evidence of hopping in this species. Even when travelling at high speeds, muskies always use quadrupedal gaits, never rearing up on just their back legs,” said Harvard’s Dr. Peter Bishop, co-lead author of the research paper.

“These results support the hypothesis that a shift to an asymmetric-gait-dominant locomotor repertoire was a functional prerequisite in the evolution of bipedal hopping in macropodoids.”

Since musky rat-kangaroos lack the ability to hop, scientists believe that the transition toward bipedal movement must have started with an intermediary stage. The bounding gait of these small marsupials may represent a stepping stone in this evolutionary process.

Bounding led to hopping in kangaroos

The musky rat-kangaroo’s movement patterns align with previous studies on quadrupedal Australian marsupials.

Many use symmetric gaits at lower speeds and shift to asymmetric gaits at higher speeds. Unlike most, however, the musky rat-kangaroo exclusively uses asymmetric gaits, even at slow speeds.

Research suggests that early macropodoids likely adopted a bounding gait before transitioning to bipedal hopping. Small potoroids, close relatives of kangaroos, demonstrate this transition by using both quadrupedal bounding and bipedal hopping at different speeds.

This pattern hints that bounding could have been the crucial precursor to the hopping behavior seen in modern kangaroos.

Body structure and the kangaroo hop

The musky rat-kangaroo also retains several physical traits that distinguish it from true hopping macropodoids. Its forelimbs and hindlimbs are proportionally closer in size compared to kangaroos, which have much longer hindlimbs adapted for hopping.

The musky rat-kangaroo also has a mobile first toe, something absent in larger macropodoids. These features suggest that its anatomy is better suited for quadrupedal movement, making it unlikely to develop hopping.

Studies on limb proportions across marsupials and rodents indicate that animals capable of bipedal locomotion generally have shorter forelimbs relative to their hindlimbs.

The musky rat-kangaroo falls somewhere in between, suggesting it represents an early stage in macropodoid evolution before the full transition to bipedal hopping.

From bounding to hopping

The transition from quadrupedal movement to bipedal hopping remains one of the most intriguing mysteries of kangaroo evolution. By studying the musky rat-kangaroo’s anatomy and locomotion, researchers can piece together the story of how hopping emerged.

“These results 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, then an animal that bounded like the muskies, and finally evolved into the iconic hopping kangaroos we see in Australia today.”

This suggests that early macropodoids may have first adopted bounding gaits, allowing them to generate forward momentum using their powerful hind legs.

Over time, as these animals became more specialized, they may have started relying solely on their back legs, leading to the development of true hopping.

Future of kangaroo evolution studies

The research, published in Australian Mammalogy, provides an important foundation for future studies on kangaroo evolution.

While the mystery of hopping is still being unraveled, fossil discoveries from early macropodoids could provide more clues about how and when kangaroos made the final leap into bipedal hopping.

Scientists also hope to answer another key question: why did hopping kangaroos grow so much larger than rodents, which also use saltation? Understanding this could reveal more about the evolutionary pressures that shaped Australia’s unique wildlife.

By continuing to study living species like the musky rat-kangaroo and examining fossils from early kangaroo ancestors, researchers move closer to understanding one of the most distinctive adaptations in the animal kingdom.

The study is published in the journal Australian Mammalogy.

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