In Search Of The Best Tail For Balance

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. Later in the hour, an update on the flu and why it’s been such a brutal sickness season. But first, it has been a busy news week, and we’re going to start out with some counterprogramming. We are talking tails.

If you have ever met a cat, you’ve probably, at some point, been amazed by how acrobatic they are. They can reorient themselves effortlessly, even in midair. It turns out that a lot of that twistiness comes down to having a top-tier tail. Writing this week in the Journal of the Royal Society Interface, researchers explore the mechanics of different tails, from reptiles to mammals, and consider how a better tail could help build a better robot. Joining me now to untwist all of this science are Dr. Talia Moore, Assistant Professor of Robotics and Mechanical Engineering at the University of Michigan in Ann Arbor, and Dr. Ceri Weber, a postdoc in Kim Cooper’s lab in the Department of Cellular and Developmental biology. At UC San Diego. Welcome to you both to Science Friday.

TALIA MOORE: Thank you. I’m so thrilled to be here.

CERI WEBER: Hi. I’m happy to be here.

FLORA LICHTMAN: Talia, you’re a roboticist. Why do you care about tails?

TALIA MOORE: Yeah. Tails are just a multifaceted appendage. You can do a lot of different things with your tail than you can with your legs. And so it just kind of gives you this extra oomph, a little pizzazz to the things that a robot can do.

FLORA LICHTMAN: Ceri, you’re a developmental biologist. What do tails mean to you?

CERI WEBER: I love that tails can be looking so different and do so many different things, but they have a really shared similar embryonic origin. So they’re kind of made the same way. They happen at the same point in development, whether it’s a human or a cat or a mouse or a giraffe. But they all look really different. They’re different sizes, they’re vertebrae are different shapes. And I just am mystified by how evolution and ecology and genetics can make this organ look so different.

FLORA LICHTMAN: You’re mystified by the mystery of the tail.

CERI WEBER: I am. It’s not a mystery I had ever considered, I have to say, until this paper. So thank you for giving me something new to think about.

FLORA LICHTMAN: Talia, what did you find with this new study?

TALIA MOORE: In our paper, we have a simulation environment to try and understand how tails can be used as inertial appendages. And the way I like to explain that is, if you’re a tightrope walker and you’re walking on your tightrope, you have this really long stick. And if you start to lose your balance, you rotate that stick, and it kind of rotates your body so you can get back onto that tightrope. And that stick is your inertial appendage.

And a paper that I wrote a while back showed that lizards can do this with their tail to rotate their body when they’re jumping in mid-air. And that tail– we modeled it as a single, rigid rod, and that inspired a lot of robots. But they’ve all been kind of single, rigid rods, moving in one plane.

FLORA LICHTMAN: Which is not actually true for many tails. Right?

TALIA MOORE: Right. And so we started looking at mammals and seeing, wow, their tails– they’re making all these complex curves in 3D space. And we started to investigate, what if the joints in the tails and the curve of the tail is actually what’s helping these tails act as better inertial appendages, even though they’re skinny and don’t have the moment of inertia themselves?

So we did this simulation to try and compare the effectiveness of different types of tails, which has never been done before. Is this shape of your tail better than this shape of your tail, and going to be able to move your body more effectively? So we came up with this optimization program. And then, once we figured out that joints were really helpful, then we said, OK, what if we allow the bones to change size, but the total tail length to be the same?

And so when we did that optimization, we actually found that there’s this really sharp crescendo and then a sharp decrescendo. So the bones get a little bit longer and then they get shorter. And when we look at the real tails that Ceri shared with us, we find that animals that use their tails as inertial appendages have that exaggerated crescendo-decrescendo more than the tails of animals who don’t use their tails as inertial appendages.

FLORA LICHTMAN: OK. So where the tail attaches to the body, the bones are short, and then they get longer, and then they get shorter again.

TALIA MOORE: Exactly.

FLORA LICHTMAN: Ceri, after all these simulations, I mean, you were in the museum archives, tail-spotting.

CERI WEBER: Yes.

FLORA LICHTMAN: What were you looking for?

CERI WEBER: So I had also noticed this crescendo pattern in mice, in the rodents that we study in the laboratory. And I wanted to know if this pattern was in all mammals. And so I was just a menace, measuring as many tails as I could get my hands on in the museum archives.

And then we were talking with Talia about this paper, because we’d already been collaborating and talking about tails, and it was really, really exciting to find that they had found this pattern in the simulations that I was finding these patterns in the museum archives. But it turns out there’s lots of different patterns, and we don’t know what those patterns do yet. And we’re really excited to figure it out.

FLORA LICHTMAN: Ceri, in your opinion, what is the top tail in the animal kingdom? The tail of all tails?

CERI WEBER: I can tell you my personal favorite, which is the silky anteater. And that one stands out because most of the tails that we’ve measured, the complete ones, average out around 30 vertebrae– that tends to be what most of them are making, whether they’re prehensile or they’re using them for inertial maneuvering. But the silky anteater tail that we measured had 41 vertebrae in it, which– it’s the longest one in our data set. I remember when I pulled the skeleton out of the box, I was in awe of how many vertebrae–

FLORA LICHTMAN: Did you gasp?

CERI WEBER: –were in the tail. I did. I did gasp. But it’s a very cool animal. It’s an anteater, but it’s mostly living in trees and it has a prehensile tail, so in pictures of it, you can see it grasping and holding onto branches. It’s very cool.

FLORA LICHTMAN: So, Talia, should we expect robots with anteater tails? Like, is that what you’re thinking?

TALIA MOORE: Yeah, I think there are some folks who are working on prehensile tails right now. I saw one at a conference last year, and so I think they’re closer than you think.

FLORA LICHTMAN: Really?

TALIA MOORE: Mm-hmm.

FLORA LICHTMAN: I mean, a robot with a tail sounds creepy to me.

TALIA MOORE: There are actually a lot of robots with tails that are out in the world right now, and I think they’re pretty cute. The benefit of these tails is they can make tighter turns, they can recover from external perturbations. So if something bumps them as they’re walking, the tail helps them recover. And there’s even a little RC car with a tail on it that can drive up off of a ramp and do a barrel roll with its tail.

FLORA LICHTMAN: Well, this is the thing I’ve been wondering since reading your paper– should we be attaching tails to more things?

TALIA MOORE: Yes. Why not?

[LAUGHTER]

FLORA LICHTMAN: Would that be helpful? If I had a tail, what would I be able to do that I can’t do now?

TALIA MOORE: Yeah. So as I was walking the dog this morning, I slipped on the ice. And if you had a tail, you’d probably be able to prevent yourself from falling in a really damaging way. You’d probably be able to regain your balance pretty quickly.

FLORA LICHTMAN: Would I be an Olympic gymnast if I had a tail?

TALIA MOORE: Yes. I was watching the Olympics, and those ladies on the balance beams– I watched them using their arms and legs as inertial appendages, and it was amazing to see.

FLORA LICHTMAN: Ceri, if tails are so great, why do I not have one?

[LAUGHTER]

CERI WEBER: It probably has to do with how us humans stand and how we walk and move. This is a huge area of study in bioanthropology and primatologists, and understanding why great apes and humans don’t have a tail anymore. And I would argue that we do.

We have a tiny little tail bone. We have a few tail vertebrae that fuse into our pelvis. And actually, as embryos, we make a tail, and it just goes away, probably through apoptosis, or cell death, and it retracts a little bit into the body. So making a tail is a part of all of our embryonic programs. We just don’t make one outside of our bodies.

TALIA MOORE: And even though we don’t have an external tail, we still have a lot of those muscles that control the tail. And so I’m learning a lot about those when I go to physical therapy.

CERI WEBER: And the pelvic floor– that’s where a lot of them have ended up.

FLORA LICHTMAN: Wait, really?

CERI WEBER: Yeah.

TALIA MOORE: Absolutely.

FLORA LICHTMAN: So if I’m doing my pelvic floor exercises, I’m actually activating my tail muscles?

CERI WEBER: Yeah, you are.

TALIA MOORE: We’re doing dissections, and we’re looking at all of the muscles that control mammal tails, and I’m working those same muscles in physical therapy.

FLORA LICHTMAN: A lot of women are going to be interested in this.

CERI WEBER: Yes, definitely.

FLORA LICHTMAN: Talia, can knowing about different tails help you make a super tail for your robot? Like, something even better than what you see in nature?

TALIA MOORE: Yes. So this is exactly what I teach in my bio-inspired robotics course. So if we understand the principles– like, the underlying physics, principles, and the mechanics of how the animals are doing something, and we understand their evolution, we can identify that maybe this animal tail is constrained, because maybe of its lineage, it has this certain constraint.

Maybe it’s used for 25 different things, and so it’s not going to be the best at anything. But if we understand that underlying principle, we can abstract that and transfer it to an engineering system and make something that’s even better than the biological system. So in the first lizard paper that we had, we were able to make a tail that was more effective than both a lizard that we were studying in the lab and a simulation of a velociraptor.

FLORA LICHTMAN: Really?

TALIA MOORE: Yeah.

FLORA LICHTMAN: Why do you all care about this?

TALIA MOORE: Yeah. My background is biomechanics. I want to know how biology– like, biological things– move in the world, and what are the underlying principles of movement that are fundamental and can be applied to all sorts of different species. And so it’s really cool to me that many different animals have evolved a tail that is capable of really good inertial maneuvering.

And it’s not just tails. Like, there are other animals. There’s a study showing that bats use their wings as inertial appendages and praying mantises use their arms and legs as inertial appendages. So I’m really excited to learn about the diversity of the morphology of these inertial appendages and how the morphology affects their effectiveness as that inertial appendage.

FLORA LICHTMAN: What about for you, Ceri?

CERI WEBER: I think tails are such a cool, diverse appendage that have not been given a lot of attention in biology and how they develop and they take shape, especially because they are so diverse. And our namesake– we are vertebrates. We are defined by our axial skeleton and our spine. And it is really diverse.

Like, you think of giraffes and humans have the same number of vertebrae in our necks, but giraffe necks are really, really long. And that’s just because the vertebrae become much longer. So there’s all of this incredible diversity in our vertebral skeleton, and I think the tail is an amazing model to study what sort of ecologies and movements and genetics drive those differences. And I just think there’s more to the tail than we’ve been giving it credit.

FLORA LICHTMAN: Well, thank you. I could nerd out for tails forever, so I appreciate it. Thanks for coming on the show.

CERI WEBER: Thanks for having us.

TALIA MOORE: Thank you so much.

FLORA LICHTMAN: Dr. Talia Moore, Assistant Professor of Robotics and Mechanical Engineering at the University of Michigan in Ann Arbor, and Dr. Ceri Weber, a postdoc in Kim Cooper’s Lab in the Department of Cellular and Developmental Biology at UC San Diego.

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