One might assume that all terrestrial animals have four limbs, from the bovines grazing in pastures to the African lions. However, there’s an exception to this rule. So, which creature stands out as the only five-limbed animal on Earth?
The answer lies in Australia, home to some of the planet’s most peculiar species, from the platypus with its duck-like bill to the disease-ridden koala and the monotremes such as the echidna. Despite their strangeness, all these animals have four limbs. So, which creature breaks the norm?
The answer is the red kangaroo, previously known as Macropus rufus and later reclassified as Osphranter rufus. While these animals appear to have the standard number of limbs, a 2014 study published in Biology Letters revealed that a kangaroo uses its tail for propulsion when walking.
Contrary to their reputation for hopping, kangaroos actually spend more time walking. The study found that a kangaroo’s tail exerts as much effort as a human leg at the same walking speed. This unique movement, termed “pentapedal” gait by the researchers, differs from the bipedal walk of humans. The kangaroo’s tail, composed of 20 highly flexible vertebrae and a robust muscle, is packed with mitochondria that convert nutrients into energy to power this additional limb.
To further understand this, researchers conducted an experiment with captive kangaroos. They encouraged the kangaroos to walk on a long platform equipped with special floor plates to measure the force exerted by their legs and tail. The platform was designed with a ceiling to prevent the kangaroos from hopping.
Max Donelan, a biomedical engineer at Simon Fraser University in Burnaby, Canada, and one of the study’s authors, stated, “When pentapedaling, more so than hopping, kangaroos use their tail exactly like a leg.”
How do kangaroos utilize their fifth limb? The study showed that the tail is not just for balance but plays a vital role in kangaroo locomotion. The team discovered that the tail alone generates as much propulsion force as the other four limbs combined, as reported by IFLScience.
Shawn O’Connor, another study co-author from the same university, told National Geographic in 2014, “A motor that lifts and helps accelerate the kangaroo’s body.”
Andrew Biewener from Harvard University (USA) told National Geographic that the study “confirms things I would have expected.” He added, “They’re five-legged animals when they use their tail.”
The researchers speculate that other kangaroo species, such as the giant kangaroo and the dwarf kangaroo, do not use their tails in the same manner. This led them to conclude that the red kangaroo is the only terrestrial vertebrate with five limbs on the planet.
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