RECORD: Darwin, C. R. 1875. [Letter on animal tails.] in R. L. Tait, The uses of tails in animals. Hardwicke's Science Gossip 11, no. 126 (1 June): 126-127, p. 127.
REVISION HISTORY: Scanned by Angus Carroll, transcribed and edited by John van Wyhe 6.2011. RN1
NOTE: Darwin's letter is here rendered in bold to make it more distinct from the surrounding article. The copy scanned is from the collection of Angus Carroll.
THE USES OF TAILS IN ANIMALS.
MR. LAWSON TAIT recently delivered a lecture on this important subject, before the Birmingham Natural History Society. We give a summary of the Lecturer's remarks: — It is not difficult to imagine how the prehensile tails of monkeys, opossums, and other animals, or the fly-switches of the horse or cow, have been useful in the struggles of these animals to master their surroundings; but there are some forms of the appendage which puzzle us to see how they can ever have been available as assistants in survival, and still more how they are still perpetuated in their apparently purposeless forms. Amongst them the bushy tail seen in the fox, dog, cat, &c, has long attracted my attention, and no intelligible meaning of it suggested itself to me till I came into possession of a cat which is perfectly deaf, and on whom I can, therefore, perform many experiments which would be impossible in an animal possessed of hearing. Like all cats, he is very fond of a warm place, and when he is asleep nothing but a touch or a very strong vibration communicated through what he is lying upon will wake him. If he goes to sleep before a big fire, he sleeps lying on his side, at full length, with head, tail, and limbs all stretched out. But if I place screens between him and the heat, he gradually coils himself up, apparently without waking, covering his limbs with his tail and head, so that as little surface is exposed as possible for loss of heat. If, in addition to screening off the heat, I directed a gentle current of cold air on him by means of a bellows, he soon buries his nose in the fur of his tail, or between his tail and thigh, so that almost his whole face is protected. On re-admitting the heat, the whole movements are reversed, and he resumes his extended position. The use of the tail is clearly, therefore, completely analogous to that of the respirator worn by people with delicate chests, the object being to abstract from the expired air, by means of fur in the one case and wire-gauze in the other, the heat which is being taken out with it; so that the cold inspired air shall be raised in temperature before it reaches the lungs, and thereby conduce to a conservation of the bodily heat. Some interesting considerations bear on this. Animals provided with bushy tails seem to be so as a matter of correlation of growth, their bodies being always provided with thickly-set and more or less soft fur. I cannot find an animal with a bushy tail which cannot, and does not, lie curled up when asleep. I went round the Zoological Gardens at Dublin on a very cold morning in February, and found the civet cat, and some other bushy-tailed animals, coiled up with their noses buried in the fur of their tails. In the squirrel this use of the tail is very marked and in birds the same object is accomplished by their burying their heads in the down, of the
shoulders. Animals provided with bushy tails are all solitary in their method of living, so far as I can find; and, therefore, an essential for their survival is some method by which variations of temperature shall be resisted. The use of the tail for this purpose is, I think, best of all illustrated in the great ant-eater (Myrmecophaga jubata), in which the hairs of the tail reach a very great size, and cover up the animal when reposing, so that he looks like a bundle of dried grass. It may also serve as a protection by mimicry in this case. Mr. Wallace states,1 also, that he uses his tail as an umbrella in a shower, and that the Indians divert his attention from themselves by rustling the leaves in imitation of a falling shower, and whilst he is putting up his umbrella they kill him. Some of the Myrmecophaga have the lower end of the tail naked, and use it as a prehensile organ, whilst the upper part remains covered with long hair, and is used as a respirator. In other edentulous animals, living in tropical countries, where they are not subjected to extremes of temperature, the long hair is replaced by scales, as in the pangolins, or the tail is absent, as in the sloths. Amongst the rodents two very curious contrasts in the matter of tail are presented by the guinea-pig and the squirrel. The former is gregarious, and any one who has kept a hutch of guinea-pigs must have seen how they protect themselves from loss of heat by packing themselves in rows arranged heads and tails; whilst the squirrel is solitary, and in his nest, during his winter sleep, coils himself up and covers his face with his tail. The same is seen in the jerboa, and in the dormouse during hibernation. Of the Carnivora, those which have bushy tails are all solitary in their method of living, though the wolf and jackal hunt in packs, and those with the bushiest tails are most exposed to low temperatures, as the Arctic fox and sable. Of the Quadrumana the marmosets afford a striking instance of a bushy tail as a probable provision for protecting these delicate creatures from depressions of their temperature. I have received an interesting letter2 from Mr. Darwin on this point, in which he says:— "Your view is new to me, and has only to be suggested for its probability to be recognized. I presume that of course you would thus account only in part for the retention of a tail, and for its modification. Your view does not preclude the conjoint use of the tail for other service, as for gliding through the air when flattened, as in the squirrel, or as a signal to beasts of prey, in accordance with Mr. Belt's ingenious suggestion in his 'Nicaraguan Travels,'3 with respect to the great bushy and conspicuously-coloured tail of the skunks. I wish we knew the use of the extraordinary hairy tail of the yak, which inhabits such cold regions, whether it serves solely as a fly-flapper. If poor Dr. Falconer4 had been alive, he could have told us." In reply I said that I had missed the yak in my search for animals with bushy tails. But I find that he also has a long additional fringe of hair nearly touching the ground. When he lies down with his limbs drawn up to, or under him, as all ruminants do, his tail and fringe would act as a rug, preventing loss of heat from the limbs and damage to them from frost-bite — as the tissues outside the bone are thin, and there is nothing but a rather weak circulation to resist loss of heat. The yak lives close to the line of perpetual snow, which is the condition in which such epithelial appendages as he has would most conduce to survival, and, therefore, that in which they would be most easily evolved by natural selection. This seems to open out a curious study of the mechanical arrangements which exist in animals for the conservation of heat — a very important feature in the struggle of life. Some forms of tails are yet a puzzle, notably the tails of rats and mice, for which I have as yet found no reasonable explanation.
1 Alfred Russel Wallace. Wallace 1853, p. 452: "During rain it turns its long bushy tail up over its back and stands still; the Indians, when they meet with one, rustle the leaves, and it thinks rain is falling, and turning up its tail, they take the opportunity of killing it by a blow on the head with a stick."
2 Darwin wrote this letter c. 14-15 March 1875 in response to a letter of 12 March  (Calendar 9885) by Lawson Tait [born Robert Lawson Tait] (1845-1899) Birmingham surgeon, gynaecologist and anti-vivisectionist. On 16 March  (Calendar 9889) Tait wrote to thank Darwin for his reply. This printing is the only known fragment of Darwin’s reply to survive. Much further information on the connections between Lawson Tait and Darwin can be found by searching the texts on Darwin Online, here. See also the printed items in Darwin's papers by Tait here.
Colour seems to be advantageous to another animal, the skunk, in a manner of which we have had many instances in other classes. No animal will voluntarily attack one of these creatures on account of the dreadful odour which it emits when irritated; but during the dusk it would not easily be recognised and might be attacked by a beast of prey. Hence it is, as Mr. Belt believes, that the skunk is provided with a great white bushy tail, which serves as a conspicuous warning.
Darwin seems never to have written on the evolutionary origins of yak tails, though he did remark, in Variation (1868) vol. 2, p. 206 "According to Pallas the Mongolians endeavour to breed the Yaks or horse-tailed buffaloes with white tails, for these are sold to the Chinese mandarins as fly-flappers".
4 Hugh Falconer (1808-1865), physician and palaeontologist.
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