By Dr. Becker
A recent study with the quirky title “The kangaroo's tail propels and powers pentapedal locomotion,” published in the journal Biology Letters,1 reveals that when kangaroos are walking and not hopping, they use their tail as a fifth leg. This makes them the world’s only “pentapeds.”
Kangaroos Spend More Time Walking Than Hopping
Kangaroos can hop at 12 miles an hour over long distances, and move more efficiently than many animals that run. As he hops along, the roo’s long, powerful tail whips up and down, helping him control his body position and direction.
Though hopping is how they get around quickly, kangaroos actually spend most of their time grazing and socializing on all fours, in a sort of hunched posture. Previous studies have noted that when they walk, roos use their tail as a crutch while their hind legs are off the ground. The tail moves with the front legs as the back legs swing through, and the back legs support the roo’s weight as he lifts his front legs and tail off the ground.
According to study author Max Donelan, a biomedical engineer at Simon Fraser University in Burnaby, Canada, “No one knew how much body weight support it provided, nor had anyone hypothesized about propulsion and power.”
Tail Acts as a Fifth Leg When Walking
For the study, Donelan and his colleagues built a walkway with Plexiglas sides and a ceiling high enough for red kangaroos, which are large animals weighing up to 200 pounds, to walk comfortably, but not hop. The floor of the walkway contained plates that measured the force exerted by each of the kangaroo’s limbs as he walked.
The researchers found that the kangaroos used their tails for much more than a crutch. The measurements in the walkway showed that the tail acts like "a motor to lift and help accelerate the kangaroo's body," according to study co-author Shawn O'Connor. The tail provides more propulsive force and power for walking than the front and back legs combined. In other words, the tail seems to be a better leg than the roo’s actual legs.
No Other Animal Uses Its Tail as a Leg
As far as the research team knows, no other animal uses its tail as a leg. They theorize that evolution of a tail that could be used for walking may have allowed the kangaroo’s front legs to shrink in size and weight, reducing his burden as he moves.
The truly remarkable thing is that a kangaroo’s tail is quite different, anatomically speaking, from a leg. The tail has over 20 highly articulated vertebrae, as compared to only a few long bones in the legs. Roo tails originally evolved to help the animal’s tree-dwelling ancestors move from branch to branch.
But with all those vertebrae, large muscles, and a greater density of mitochondria that converts nutrients to energy, it’s really not surprising the kangaroo tail seems built for movement.