Question: Why are there no animals with 3 legs?
Austin Elliott answered on 20 Jun 2012:
There are a couple of answers. One would be that it is simply difficult to run fast, or manoeuvre well, on three legs, compared to how easy it is with two or four. Have a look when you next see an animal that’s lost a leg in an accident but recovered (I’ve seen cats, dogs and even a deer). They manage, but not as well as the four-legged ones. A dog with three legs is OK because its owners feed it – if it had to hunt and catch its food, it would have a much tougher time. So I reckon a three-legged animal would be at a disadvantage in evolutionary terms compared to a four-legged one.
The other reason is that nature tends to favour body plans with symmetry, so that (e.g.) for you, the right half of your body is quite similar to the left half. You could imagine an animal with three-fold-symmetry, I suppose, but again I can’t see how it would be very fast or manoeuvrable.
Nike Dattani answered on 20 Jun 2012:
Tripod fish (http://en.wikipedia.org/wiki/Tripod_fish) rest on three “rays” , but rays are not technically legs.
Kangaroos can rest their weight on either their legs, OR their tail … so their tail is like a third leg.
Animals tend to be symmetric: if something’s on the left side, it’s also found on the right side … therefore the number of legs and arms is usually an even number 😉
Kirsty Ross answered on 21 Jun 2012:
It also goes back to the ancestor of all animals (and by animals I am mainly talking about vertebrates or animals with backbones). The ancestor was probably closely related to the modern hagfish. It had bilateral symmetry and enclosed its spinal cord inside bone. It also had a cranium. This explains why you can pretty much draw a line down your body and the two sides will roughly match.
Most animals that have legs are descended from a group of fish called the lobed-fin fish. These fish had paired fins under their body, roughly where the shoulders and hips are in modern animals. There are one or two surviving species from that era, one of which is the coelacanth and a few species of lungfish. They are the rare survivors of a once flourishing group of animals and have survived pretty much unchanged since then. As the fins moved under the body, it became possible for them to support the animal’s weight and so they were able to venture out onto land. As they did so they needed to adapt to the changing environment and became amphibians and on into other types of mammals. Because the ancestor lobed-fin fish had two pairs of fins, that is why most animals also have four limbs rather than three. You can find out more about vertebrates here.