ARTICLE REVIEW! Tail-assisted pitch control in lizards, robots, and dinosaurs.

Anyone who has interacted with me for more than two minutes can probably guess that dinos are my favorite herps. They appeal to me, in part, because they are total enigmas: we can’t observe their locomotion, their parental care, their courtship behavior—for the most part, we can’t even observe their bodies, just a footprint here and a vertebra there. They’re big, rocky mysteries, and solving those mysteries has required some really innovative techniques, which is generally how the most exciting science happens.

One particularly interesting question being investigated right now is a pretty basic one: How did dinos use their tails? We don’t always have a good idea of this—for instance, there was a long time where we thought of the sauropods like Brachiosaurus standing with their necks up really high and their tails just hanging down behind them, but a new picture has emerged in which sauropods let their neck and tail form a long, continuous line extending along the length of their bodies, and then when they ate from tall trees, they went up on their hind legs and used their tail as a stabilizing tripod. And, of course, in other species, tails were used differently because other dinos were shaped differently, ate different food, and basically had a whole different niche to occupy in which a tail would be useful, but not as a tripod. So there are a lot of questions around why different species of dinos had tails and what they used them for.

So in January, Nature published a super interesting paper* in which Libby et al. studied a jumping lizard to (a) build a robot with a tail, and (b) infer something about how some theropods used their tails. The type of motion they were investigating was using a tail as a stabilizer. The physical principle behind this is the conservation and redistribution of angular momentum. Whether we’re talking about a jumping lizard, a jumping lizard robot, or a jumping velociraptor, once it takes off, the angular momentum it had in its body at take-off is stuck in its body while it’s in the air; by rotating its tail mid-jump, it can transfer the angular momentum from its body out into its tail, stabilizing its body so it can land smoothly. It’s like when you lose your balance and you flail your arms around in circles. This kind of tail control is more likely to have been found in the lighter, more agile theropods (like raptors) than in the bigger theropods (like T. rex), so to apply their findings to dinos, they used a velociraptor as their model.

Their experiment had four parts. The authors

1. recorded red-headed agama lizards (Agama agama) jumping to observe how they used their tails to control the angular momentum in their bodies
2. developed a model to describe that kind of control
3. built a lizard robot to replicate that kind of control, and
4. tested their system on a mathematical model of Velociraptor mongoliensis biomechanics.

For the first and second parts of the experiment, they took five A. agama lizards and had them run along track and leap from some surface to a wall. The surface was either rough (sandpaper) or smooth (cardstock), so different types of leaps were required from the lizard. The lizards were recorded as they jumped, and they used these recordings, along with the lizards’ mass and length, to analyze the lizards’ kinematics and from there compute the average angular momentum over each lizard’s leap. They also determined the angle through which a lizard without its tail would move, and then compared that to the computed angular momentum per lizard. So this gave them a complete set of equations with which to tell a robot lizard how to move.

For the third part of the experiment, they built a little robot A. agama, which looked a lot more like a tiny model car with a tail attached to it than a lizard, and had it go through the same types of leaps (except it landed on the ground instead of the wall) with the tail control on and with the tail control off. When the tail control was on, the tail swung up in order to stabilize the robot as it landed; when the tail control was off, the tail remained in the same position over the whole leap. The robot’s body went through smaller angles when its tail control was on than when it was off because of the stabilizing effect of the tail (73% smaller angles with tail control than without).

This figure from the paper shows the overall experimental setup for the lizard and robot jumps.
(a) Lizard jumping off sandpaper. (b) Lizard jumping off cardstock.
(c) Robot without tail control. (d) Robot with tail control.

So these three parts of the experiment gave them an overall model for how a tailed, lizard-like organism uses its tail when it jumps which they could apply to the robot, and then they could use the results of the robot jumping without tail control to get experimental data on how a lizard would jump without its tail, which is kind of circular, but the three parts do reinforce each other, so that is good.

The fourth part of the experiment involved another model, this one of the biomechanics of V. mongoliensis, which the authors developed based on fossil evidence. Using this model and the kinematic model they developed from the data they gathered from the lizards, they found that velociraptors could use their tails really effectively as balancing tools while jumping, because their tails could bend at the base by around 90°, so they could wobble their tail all around to capture the angular momentum from their bodies and stabilize themselves for some really impressive jumps.

So that’s super cool! They could figure out how a velociraptor jumped from trees to attack unsuspecting prey from some judicious application of math and a video camera! That is super awesome. Also, the authors mentioned that one of the applications of this study could be the development of more stable and effective search-and-rescue robots, so if ever you find yourself, like, lost in the Alps on a ski trip, your salvation might look like this:

Rawwrrr, I am here to help!

*This is the link to the paper, but be careful! If you're not on a school computer, you may not have access. I don't know if you know the shortcut, but I will tell you in case: for CofC-ers, you can type .nuncio.cofc.edu after nature.com in the URL and get redirected to the library's off-campus access page, where you can sign in using your cougars log-in, and then use the library's account at Nature to read the full text. For non-CofC-ers, I have no idea what you should do.