Why are there no animals with three legs?
The paper suggests that although we might not have any visible three-legged animals, there are a few animals that use a tripod stance to rest.
Have you thought about why three-legged animals don’t exist? A new research paper tries to understand why. Tracy Thomson, a graduate student in the earth and planetary sciences department at the University of California has published a paper titled “Three‐Legged Locomotion and the Constraints on Limb Number: Why Tripeds Don’t Have a Leg to Stand On” in Bioessays.
The 'alternating tripod' stance:
The paper suggests that although we might not have any visible three-legged animals, there are a few animals that use a tripod stance to rest. Meerkats, small carnivoran animals that fall under the mongoose family stand in an upright stance that can be said to be a tripod stance. But, visibly they have four legs. Similarly, woodpeckers use their tail feathers to brace themselves against a tree-trunk.
According to the researcher, a tripod stance does not require any energy to be stable, whereas a standing upright position requires large feet and muscle work. Also, three-limbed movement is less common. Insects that have six legs, use a movement called “alternating tripod” gait in their movement. This movement uses a set of three legs for movement, two on one side and one on the opposite side.
Parrots too are a good example that shows tripodal movement. They use their beaks, along with their legs for additional grip to manoeuvre on the tree branches. Kangaroos' long feet make it difficult for them to walk like other animals and hence they use their tail and front limbs to move while grazing. Also, a few of the four-legged animals use their tails for additional gripping.
The way we evolved:
But this just causes more confusion. When so many animals use the tripod stance for movement or other purposes, why don't we have three-legged animals?
The answer might be hidden in our DNA and the way we evolved. Thomson says that almost all the animals are bilateral and the code for having two sides to everything seems to have got embedded in our DNA very early in the evolution of life, much before legs, fins, or flippers evolved in our bodies. “Once this bilateral symmetry was backed in, it was hard to change.”
Thomson adds, 'With our built-in bias to two-handedness, it can be hard to figure out how a truly three-legged animal would work. If we’re trying to understand evolution as a process we need to understand what it can and can’t do.”