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A Wilder View: Why aren’t there three-legged animals?

Triped Dog
Posted at 5:32 PM, Oct 05, 2022

MISSOULA - In science, the first question you always ask is “why?” A close second is likely ‘why not?' For example, why aren’t there three-legged animals?

You may have seen a three-legged dog or deer due to an injury, but a three-legged species does not exist today. And an animal with three legs may never have existed as it is not found in the fossil record. There has been nothing about three-legged animals all the way back.

“Why aren't there certain phenotypes? Why don’t certain adaptations seem to appear throughout the fossil record? Then we ask the question...why not?” said UC-David lecturer Dr. Tracy Thomson.

Thomson started asking this question as he was getting his Ph.D. and a professor asked for an assignment to make up a forbidden phenotype. And then he had an idea

“As I was biking home one day, I remembered reading H.G Well’s War of the Worlds in which the Martians that invade Earth are three-legged creatures. And they build their machines to have three legs. They call them tripods. And I started thinking about it and realized, there’s not three-legged animals anywhere on Earth.”

While human imagination is clearly able to come up with the idea there is a lack of any such animal in nature. To dive into this Thompson wanted to focus on the terminology behind all this.

"Humans employ bipedalism. Which is the name of the locomotion," Thompson said. "So, anything that walks on two legs is using bipedalism."

So, we can call an animal with two legs a biped, an animal with three legs a triped, and one with four legs a quadruped. From this we can break this down into multiple categories; how it walks, the number of legs used to move and the number of actual legs.

“However, we are not bipeds. Humans have four legs and therefore are quadrupeds, we have four limbs, but only use two of our limbs for locomotion,” Thompsom explained.

One of the biggest reasons we don’t see tripeds is simple — it’s hard to walk.

“There are a lot of physical problems…with gravity,” Thompson said. “How do you take your first step? Balance and motion...is problematic with three-legged locomotion that you don’t get with other forms.”

Overall, the deep issue for not seeing any tripeds walking around is genetics.

“Once a feature or a body part becomes codified in that animal's genetics it’s very hard to remove that constraint,” Thompson said.

You can do a lot of things with these genetics like adapt its arms or legs or feet. But the fact that it is a limb that is constant across animals because it is genetically ingrained in their DNA.

When we look at animals, we see symmetry. We can differentiate between a left and a right side. This symmetry is part of an animal’s genetics and is established before the genetics that makes up limbs.

“By the time we’re involving limbs we have a bilateral symmetry already implanted. And so, it’s always going to be paired. There’s always going to be two of everything,” Thomson noted.

There’s nothing that has three limbs; it’s always even numbers.

“No matter how many you have; you can have a millipede that has hundreds of limbs, but they’re always paired,” Thompson said.

But tripedalism — using three limbs to move around — is actually quite common.

“One is the parrot which has the wings that are folded up. It doesn't use the wings for walking around or climbing,” Thompson explained. “But they use their feet and their beak as kind of a third appendage. So the beak will grab onto things they’ll reposition themselves and move along.”

A recent paper published in the Royal Society cites this specific movement in parrots as tripedalism and found they will only use their beak on surfaces that are steep enough that they need to use their beak as a third limb to move around.

“You may have five legs or eight legs or four legs but you may only use two or three. And if you’re using three that’s tripdealism, and that seems to be pretty common,” Thompson said.

Thomson added that everything has a history, whether it's politics, religion, philosophy, or evolution that in some way affects what you currently see.

"If you can understand the history, it doesn’t excuse or anything, but it explains. The explanatory power of understanding history for our current understanding is needed and without that, we’re going to miss all sorts of things. And frankly just not understanding things correctly. If you want to solve a problem you have to understand how that problem got there."