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Tripedalism seems physiologically inefficient, otherwise we would probably see at least one tripedal species on Earth. And yet the Culture universe has at least three (!) tripedal species: the Chelgrians, the Homomda, and the Idirans.

Is there a joke or reference that I am missing?

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    Actually evolution says that the species of earth are not totally free to change basic morphology. Once some worm chose bilateral symmetry, then all descendants are stuck with it. So the species of earth are not a fair selection.
    – Oldcat
    Commented May 20, 2014 at 21:25
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    I seem to recall that at least one of the tripedal species have a bilateral symmetry with fused middle limbs.
    – dmckee --- ex-moderator kitten
    Commented May 20, 2014 at 23:14
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    The Chelgrians evolved from six-legged, tiger-like animals. Their forelegs became arms and their middle two legs fused to become a front, middle leg. Seems clumsy to me. :)
    – Chris Peterson
    Commented May 21, 2014 at 3:26
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    There are plenty of "sub-optimal" life forms on Earth. For example plants would have better energy absorption with black leaves, but they happen to have a green pigment for photosynthesis, and now they're stuck with it. We humans have appalling back problems and difficulty with childbirth because we are large-brained bipeds who evolved from ancestors with four legs and smaller brains. Whales commonly have vestigial hind legs inherited from their land-dwelling antecedents. (And so on, and so forth.)
    – Royal Canadian Bandit
    Commented May 21, 2014 at 11:03

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To the best of my knowledge: No.

  • It may be as simple as wanting a clearly alien body shape. On Earth, animals invariably have two, four, six, or eight legs, so giving something three legs is an easy way to signal that it's not from our world.

  • It is suggested the Homomda were patrons/mentors to the Idirans. So there may be some connection between the species in the distant past. (The Homomda might similarly have influenced the Chelgrians, although I don't think this is made explicit.)

  • The Culture stories are vague on why there are so many humanoid species in the galaxy. An awful lot of them have the basic body plan of two arms, two legs, and one head with no explanation as to why they look like that. In the novella The State of the Art, a Culture ship visits Earth in the 1970s and most of the crew can circulate among humans with little or no alteration in their appearance.

I am fairly sure the reason for the plethora of two- and three-legged species in the Culture universe is not addressed anywhere in the stories. It may have been intentionally left as a Cosmic Mystery. Sadly, Iain M Banks is no longer around for us to ask, so it may have to remain so.

The essay A Few Brief Notes on the Culture further indicates Banks was deliberately leaving this unexplained:

...there are two untold stories implicit. One is the history of the Culture's formation, which was a lot less easy and more troubled than its later demeanour might lead one to expect, and the other is the story which answers the question; why were there all those so-similar humanoid species scattered around the galaxy in the first place?

Each story is too complicated to relate here.

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The Chelgrians were not originally tripedal (they had six limbs), so the only the Homomda and the Idirians were notable tripedal races. Since the majority of the races described in the books were humanoid/bipedal it would appear that tripedal races were actually quite rare - although arguably they were not as "alien" as some of the more exotic morphologies such as the Affront.

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