h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I remember somebody arguing that HG Wells three legged creatures in The War Of The Worlds, were not possible as a life form. I said what about people who moved about on crutches, swinging their legs between them as one unit? Then there are Mudskippers, who use two fins to get about and their tails as a third 'leg.' So what do other people think about the possibility that we may find other planets that have three legged life forms rather than two or four? How viable do you think such creatures would be?

There was an episode of 'QI' in which it was said that a kangaroo uses its tail effectively as a third leg, with muscles used to help propel it forward.
See: http://news.nationalgeographic.com/news/2014/07/140701-kangaroo-animals-hopping-mammals-australia/ (although this defines it as a 'fifth leg', so it depends on whether you define a kangaroo as having 2 arms or if they're legs.)

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Getting around on three legs would be awkward. If a dog loses a leg, he/she can still get around, but it's difficult.

On the other hand, if a lifeform had wheels, then it could roll forward like a tricycle, which would be wuite reasonable.

Body parts probably tend to fall in groups of binary coefficients, 1, 2, 4, 8, 16, &c.

Cellular division follows that progression so it would seem logical.

But then again, insects have 6 appendages and squids 10, so what do I know?

Insects have 12 appendages if you count their wings and antenna.

Crustaceans often follow a similar rule.

occasional species of bottom dwelling fish use an extension of their tale along with their pectoral fins to get around on the bottom of their habitat.

Air planes often use tripodal landing gear.

Robert A. Heinlein often used tripeds for his 'Martians'.

(Amusing to visualize them running.)

Larry Niven and Jerry Pournele's 'Grayson's Puppeteers' had three legs and two heads, the latteralso functioning as hands.

Paulh, I have seen a video of a dog that lost both its back legs but could still get around very well. It looked bizarre but the dog was very happy. I think nature gives us two of everything so that we have got a spare to fall back on, if we lose one (Two legs better and Animal Farm springs to mind). As for tail use - you could say it is a pair of legs fused together and yes, I had forgotten about Kangeroos.

It has occurred to me that a monkey has five appendages if you count the tail. We lack the tail. Do we get around more or less easily than monkeys?

A spider monkey gets tremendous use of its tail for swinging through the trees.

Myself I use a rope, like Tarzan.

If the grapevines in my yard get any longer, I will *feel* like Tarzan.

The late great Ken Dodd had a three-legged chicken joke

http://www.youtube.com/watch?v=EHihHjJ4X80

When I buy chicken legs, they come in packages of 5. I sometimes wonder what a 5-legged chicken would look like.

....maybe something like a star fish, with eyes, coxcomb and beak in the middle...

Someone once suggested a transgenic creature based on a sea cucumber, but with chicken breast inside it.

I remember Rolf Harris used to sing a song called Jake the Peg, with an Extra Leg and had a third limb when he performed it, moving about the stage on it. Of course that mysterious third limb got him into trouble and jailed but it wasn't his leg

It's part of the ineffable cosmic balance. Adam lost a rib because it was made into Eve. But he had an appendage that Eve didn't have.

Okay, I can see that I'm getting further and further out on a limb, so I think I'll jump onto that nice trampoline that's right below me

Watch out for low hanging branches.

I like those, because they have the easiest fruit to pick.

"On the other hand, if a lifeform had wheels, then it could roll forward like a tricycle, which would be wuite reasonable."

Yes, getting around on wheels is quite reasonable indeed

One of my favourite book series is The Tripods by John Christopher - viable or no, they do make for effectively scary aliens

Centipedes and Millipedes seem to have overdone it in the leg department, though.

Centipede and millipede legs are organized neurologically in sets of three.

Every time the third leg in a group moves, the second preceding moves again.

Its an evolutionary point that groups them with insects rather than arachnids or crustaceans.