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None of the countless species of animal in existence has three legs. Creatures such as the kangaroo and the meerkat use their tails for balance, but a tail is plainly not the same as a leg. This pattern does not apply only to mammals 鈥� other kinds of animal have an even number of legs, too. Why wouldn鈥檛 having three legs work?
鈥� A tripod is wonderfully stable, so there could be something to be said for having three legs. When insects walk, they use their legs as two sets of three. At any instant their weight is supported by three legs 鈥� two on on one side of the body and one on the other. Meanwhile, the other three legs can be moved forward to form the next 鈥渢ripod鈥�.
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鈥淎 walking insect鈥檚 weight is borne by three legs, with the other three moving to form the next 鈥榯ripod鈥欌��
All the animals mentioned in the question are bilaterally symmetrical, so it is not surprising that their limbs come in pairs 鈥� two in the case of land-dwelling mammals, three in insects, four in spiders, and various larger numbers in crustaceans, centipedes and millipedes.
In contrast, starfish are built on a radially symmetrical plan (also seen in and ), so they often have five arms. However, these are not like legs, in that they are not manipulated for locomotion. Starfish move using thousands of hydraulically operated tube feet, arranged along the undersides of their arms.
If you had to walk on exactly three legs 鈥� as opposed to the insect鈥檚 two sets of three 鈥� you would not want an asymmetrical gait with two legs on the left and one on the right, or vice versa. But an arrangement with one leg on the midline and one on each side is certainly feasible. Having recently been getting about on one leg and a pair of crutches, I can confirm that you can move quite quickly this way, though it is tiring and more difficult on slopes and steps than using two legs.
I think we have to conclude that three legs is an unlikely arrangement in a bilaterally symmetrical animal, and seems to confer no advantage in movement over two or four.
John Gee Aberystwyth, Ceredigion, UK
鈥� As a long-term user of crutches I walk with three 鈥渓egs鈥� as often as not. Quite a few gaits are possible while your weight is borne by two legs and crutches, but if you have just one weight-bearing leg you are forced to move the paired outer 鈥渓egs鈥� (the crutches) first, followed by the one in the centre (your real leg). The only latitude is in whether you move your leg just as far forward as the crutches, or past them.
Walking with crutches uses up energy at a rate that is typically closer to that of running than walking, indicating that the use of crutches is not an especially energy-efficient way of getting about. Of course, unlike real legs, crutches do not have joints and elastic tissues that can store and release energy to optimise their efficiency, so the possibility to evolve an efficient gait using three legs may well exist.
鈥淲alking with crutches uses up energy at a rate closer to that of running than walking鈥�
David Gillo Chatham, Kent, UK
鈥� Kangaroos have strong tails capable of bearing weight, and though they do not have any 鈥渢hree-legged鈥� gaits, they can move slowly with a 鈥渇ive-legged鈥� gait. First the tail and forelegs are used to support the animal while the hind legs are brought forward in unison, then the hind legs take the weight while the kangaroo shifts forward before putting its forelegs and tail back onto the ground. Because the forelegs are short, the head stays close to the ground throughout, making this gait good for grazing.
The first vertebrates to walk evolved from fish, which swim with a lateral motion, so the gait they evolved probably also involved side-to-side movement. If fish had evolved differently, swimming with a vertical tail motion like a dolphin, then the first vertebrates would have had a gait with some up-and-down motion, possibly using the tail as a 鈥渓eg鈥�. In this alternate reality, a five-legged gait similar to a grazing kangaroo could have been common, and tripedal creatures could conceivably have evolved.
Stuart Henderso Farrer, ACT, Australia