Three's a crowd
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Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format. New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address. THE LAST WORD Three’s a crowd 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 – other kinds of animal have an even number of legs, too. Why wouldn’t having three legs work? n 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 – 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 “tripod”. All the animals mentioned in the question are bilaterally symmetrical, so it is not surprising that their limbs come in pairs – 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 sea urchins and sea cucumbers), 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 – as opposed to the insect’s two sets of three – 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 n As a long-term user of crutches I walk with three “legs” 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 “legs” (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. David Gillon Chatham, Kent, UK n Kangaroos have strong tails capable of bearing weight, and though they do not have any “three-legged” gaits, they can move slowly with a “five-legged” 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 “leg”. 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 Henderson Farrer, ACT, Australia This week’s questions THE LITTLE DEATH I took part in a quiz in which the following question was asked: “Your bodily functions stop when you sneeze. True or false?” I answered “false”, but the quizmaster said it was true. Does he have a case, and if so, what happens physiologically when you sneeze? Jennifer O’Neill London, UK BLIND TO THE TRUTH? My mum says that if I read under the bedclothes using a torch (flashlight) I will damage my eyesight. I told her it’s an urban myth, but am I right? Ben Renton (aged 11) Leicester, UK SLENDER EVIDENCE I have just read about a company producing lingerie that claims to aid slimming. It is apparently made from fabrics containing “Active Bio-Crystals that assist in the reduction of cellulite”. Could this conceivably work? If so, how? Elaine Pettifer London, UK “A walking insect’s weight is borne by three legs, with the other three moving to form the next ‘tripod’ ” Last words past and present, plus questions, at www.last-word.com The new collection for 2010: witty, brilliant, intelligent and packed with insight Available from booksellers and at www.newscientist.com/elephants Why can’t elephants jump? “Walking with crutches uses up energy at a rate closer to that of running than walking”
Transcript of Three's a crowd
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers).
For a list of all unanswered questions send an SAE to LWQlist at the above address.
THE LAST WORD
Three’s a crowdNone 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 – other kinds of animal have an even number of legs, too. Why wouldn’t having three legs work?
n 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 – 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 “tripod”.
All the animals mentioned in the question are bilaterally symmetrical, so it is not surprising that their limbs
come in pairs – 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 sea urchins and sea cucumbers), 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 – as opposed to the insect’s two sets of three – 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 GeeAberystwyth, Ceredigion, UK
n As a long-term user of crutches I walk with three “legs” 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 “legs” (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.David GillonChatham, Kent, UK
n Kangaroos have strong tails capable of bearing weight, and though they do not have any “three-legged” gaits, they can move slowly with a “five-legged” 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 “leg”. 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 HendersonFarrer, ACT, Australia
This week’s questionsThe liTTle deaThI took part in a quiz in which the following question was asked: “Your bodily functions stop when you sneeze. True or false?” I answered “false”, but the quizmaster said it was true. Does he have a case, and if so, what happens physiologically when you sneeze?Jennifer O’NeillLondon, UK
Blind To The TruTh?My mum says that if I read under the bedclothes using a torch (flashlight) I will damage my eyesight. I told her it’s an urban myth, but am I right?Ben Renton (aged 11)Leicester, UK
Slender evidenceI have just read about a company producing lingerie that claims to aid slimming. It is apparently made from fabrics containing “Active Bio-Crystals that assist in the reduction of cellulite”. Could this conceivably work? If so, how?Elaine PettiferLondon, UK
“ a walking insect’s weight is borne by three legs, with the other three moving to form the next ‘tripod’ ”
last words past and present, plus questions, at www.last-word.com
The new collection for 2010:witty, brilliant, intelligent and packed with insight
Available from booksellers and at www.newscientist.com/elephants
Why can’t elephants jump?
“ Walking with crutches uses up energy at a rate closer to that of running than walking”
