“Almost but not quite a decade ago, researchers from UC Berkeley equipped a little robotic car with an actuated metal rod with a weight on the end and used it to show how lizards use their tails to stabilize themselves while jumping through the air. That research inspired a whole bunch of other tailed mobile robots, including a couple of nifty ones from Amir Patel at the University of Cape Town,” reports IEEE Spectrum.

“The robotic tails that we’ve seen are generally actuated inertial tails: a moving mass that goes one way causes the robot that it’s attached to to go the other way. This is how lizard tails work, and it’s a totally fine way to do things. In fact, people generally figured that many if not most other animals that use their tails to improve their agility leverage this inertial principle, including (most famously) the cheetah. But at least as far as the cheetah was concerned, nobody had actually bothered to check, until Patel took the tails from a collection of ex-cheetahs and showed that in fact cheetah tails are almost entirely fluff. So if it’s not the mass of its tail that helps a cheetah chase down prey, then it must be the aerodynamics.”

“The internet is full of wisdom on cheetah tails, and most of it describes ‘heavy’ tails that ‘act as a counterbalance’ to the rest of the cheetah’s body. This makes intuitive sense, but it’s also quite wrong, as Amir Patel figured out.”

Patel describes seeing an autopsy of a cheetah. and how “…once we removed the fur and skin from the tail during the autopsy, it was surprisingly skinny! We measured it (and the tails of another 6 cheetahs) as being only about 2 percent of the body mass – much lower than my own robotic tails. But the fur made up a significant volume of the tail. So, I figured that there must be something to it: maybe the fur was making the tail appear like a larger object aerodynamically, without the weight penalty of an inertial tail.”

“A few years ago, Patel started to characterize tail aerodynamics in partnership with Aaron Johnson’s lab at CMU, and that work has lead to a recent paper published in IEEE Transactions on Robotics, exploring how aerodynamic drag on a lightweight tail can help robots perform dynamic behaviors more successfully.”

For a 200 lbs robot, tail weight of 2% of body weight only equals 4 lbs, so we don’t know yet whether our future robot overlords will be equipped with the heavier lizard like tails, or whether some ‘chaser’ style bots designed specifically for tracking human escapees from the lithium mines will instead use the lighter cheetah-style tale to optimize their aerodynamics.