If you’ve seen a squirrel while out for a walk lately, or even nipped over to your friend's house to feed the cat while they’ve been out of town, it might not even have crossed your mind that all terrestrial vertebrates have four legs. From the cows in the field to the lions on the African continent, all modern-day land-dwelling vertebrates have four limbs. All that is except one.
Of course it would be an Australian animal. Australia is arguably home to among the weirdest and most funky creatures on Earth, from the downright confusing duck-billed platypus to koalas with STIs and monotremes like echidnas left, right, and center. Despite their weirdness, these critters all still have four limbs – so what is this mystery five-limb animal, I hear you cry?
Drumroll please… the red kangaroo (previously known as Macropus rufus when the study was published, later reclassified to Osphranter rufus ). Before you get all high and mighty about kangaroos having the traditional number of arms and legs, hear this out. A study published in the journal Biology Letters in 2014 found that a kangaroo walking around uses its tail to propel itself forward - like a limb.
While kangaroos might be famous for hopping around Australia, the team actually found that they spend more time walking about. The authors discovered that the tail of a kangaroo works as hard as the leg of a human walking at the same speed. They called this a “pentapedal” gait as opposed to a bipedal human one. The tail is full of 20 highly articulated vertebrae and thick muscle, and the tissues are packed full of mitochondria helping to convert nutrients into energy to power the use of this extra limb.
To find out more, the researchers created an experiment using kangaroos that were bred in captivity. They encouraged them to walk along a long platform with specialized plates in the floor that could measure the force exerted by their legs and tail; the platform had a ceiling to prevent the kangaroos from simply hopping along it.
"We found that when kangaroos are walking pentapedally—which they spend more time doing than hopping—they use their tail just like a leg," said paper author Max Donelan, a biomedical engineer at Simon Fraser University in Burnaby, Canada to Science.
The results showed that, far from being a simple balance aid, the tail was a crucial part of how the kangaroos were walking. The team found that the tail was responsible for as much propulsive force as all four other limbs combined. "A motor to lift and help accelerate the kangaroo's body," study co-author Shawn O'Connor of Simon Fraser University in British Columbia, Canada told National Geographic in 2014.
The study "confirms what I would've expected," Harvard University's Andrew Biewener told National Geographic. "They are five-legged animals when they're using their tail."
The team thinks that other kangaroo species, like the greys and smaller wallaby species, aren’t using their tails in the same way, leading the authors to conclude that the red kangaroo is the only five-legged vertebrate on the planet.