When you hear the word ‘kangaroo’, you might picture that charismatic Australian marsupial hopping bipedally across the grassy plains. It may be surprising to hear that most of the time kangaroos spend moving is not time spent hopping. Instead, they move slowly while grazing and lazing about. This slow motion is not a hop, but instead, a gait called pentapedal locomotion. Unlike jumping, where the tail is held straight behind the body as a counterbalance, during pentapedal locomotion the tail is placed on the ground in sequence with the kangaroo's arms and hind legs. While researchers have known about pentapedal locomotion with the tail for decades, they did not know what the tail was doing – if it was functioning merely to support body weight, or as a propulsor like a leg.
Shawn O'Connor of Simon Fraser University in Canada and a team of international colleagues wanted to test the tail-as-leg hypothesis, to determine the extent to which the tail contributes to kangaroo pentapedal locomotion. They recorded the ground reaction forces of red kangaroo forelimbs, hindlimbs and the tail during this unique gait, and estimated the power generated and work done by each individual limb and the tail.
Surprisingly, the group found that the tail was functioning not just like a regular leg, but actually as a quite powerful one. While the forelimbs and the hindlimbs of the kangaroo generated some forward thrust, the tail generated more thrust than both pairs of limbs combined. However, the forelimbs and the hindlimbs played a much greater role than the tail in supporting body weight. In terms of mechanical power, the kangaroo's hindlimbs produced almost three-quarters of the total positive mechanical work. But even here the tail made a substantial contribution, accounting for most of the remaining work output. To put this pentapedal propulsion into perspective, the authors compared the kangaroo's tail with a human leg. The kangaroo tail performs as much mass-specific work as one human leg does while walking at the same speed.
Together, the force and power measurements lead to one conclusion – the kangaroo tail functions precisely as a biomechanical leg, to both support body weight and provide forward propulsion. But why walk pentapedally at all? The authors hypothesize that pentapedal locomotion may be partly the result of a body shape that improves bipedal hopping. While the huge hindlimbs and small forelimbs of the kangaroo are an asset for bipedal hopping, they give the kangaroo a very uneven distribution of weight. If not for the propulsive tail, this weight distribution might make the kangaroo fall backwards during slow ‘quadrupedal’ locomotion. So, the kangaroo tail turns out to be quite the multipurpose structure. Its utility as a balance while hopping and a propulsor while grazing has given kangaroos a literal leg-up over the quadrupedal competition.
