Researchers Unlock Secret Behind Kangaroo Energy Efficiency While Hopping

Kangaroos may have cracked the code on how to move faster without expending additional energy. A collaborative study involving researchers from Australia, the United Kingdom, and the United States, published in the journal eLife, suggests that the unique posture of these animals plays a pivotal role in their remarkable energy efficiency while hopping.

Most animals exhibit an increase in energy expenditure as they accelerate. This phenomenon is attributed to the need for muscles to generate force more rapidly, leading to heightened metabolic costs. However, kangaroos and their relatives, known as macropods, defy this general trend. Classic treadmill experiments have demonstrated that both red kangaroos and tammar wallabies can hop at increased speeds with only a minimal rise in oxygen consumption, presenting a puzzling challenge for biomechanics researchers.

Previous investigations identified their ankle extensor muscle-tendon units as possible contributors to this efficiency. These structures can store and release elastic energy akin to springs. Yet, the existing explanations fell short of fully elucidating why larger macropods do not suffer the same energy penalties that other quadrupeds do when increasing speed. Attempts to analyze stride timing or breathing coordination did not yield conclusive insights, particularly regarding the differences between smaller and larger macropods.

The recent study focused on the kangaroo’s posture—the specific angles of their joints during ground contact. Researchers discovered that this posture actively influences the leverage at the ankle, enhancing the amount of elastic energy returned as the animal increases its speed. Should this model receive further validation, it would suggest that kangaroos can meet greater mechanical demands without necessitating additional muscular effort, effectively separating speed from metabolic costs.

To conduct their research, the team recorded three-dimensional motion and ground forces from 16 red and eastern grey kangaroos hopping at speeds ranging from 2 to 4.5 m/s on specialized force plates. They developed a scaled musculoskeletal model to analyze joint kinematics, rotations, mechanical advantages, and stress on the Achilles tendon.

As the kangaroos increased their speed, the researchers noted that they bent their legs more when decelerating. The upward bending of the ankle and the downward pressure from the toes intensified, leading to greater tension in the Achilles tendon, similar to stretching a robust rubber band. The study revealed that ground forces and twisting forces at the ankle rose accordingly. This biomechanical geometry allowed for greater energy storage upon foot landing, which was then efficiently released during takeoff.

Crucially, even though both the landing and propulsion phases increased with speed, the overall ankle work per hop remained relatively constant. The tendon assumed more of the workload, meaning the muscles did not need to expend significantly more energy. However, the researchers cautioned that because kangaroos rely heavily on their tendons, there is limited safety margin before potential failure occurs. This reliance on tendon mechanics may also impose constraints on the maximum size and agility of kangaroos.

The researchers emphasized the need for future studies to explore a broader range of body sizes among kangaroos. They suggested further assessments of tendon stress at high speeds and the need for alternative experimental or modeling approaches, especially considering the challenges of getting kangaroos in enclosures to hop faster over force plates. Understanding how posture and muscle dynamics throughout the entire body contribute to kangaroo energetics remains a key area for future research.