Pick from the Past
Natural History, November-December 1930


Caudal Appendages Adapted by Nature
to the Needs of Her Creatures

ET us trace, in a general way, the development or formation of the tail in the animal kingdom. One of the simplest and most generalized animals known,

The raccoon’s tail am ringed all ’round;
the ’possum's tail am bare;
The rabbit’s got no tail at all;
but the white spot’s always there.

the minute, one-celled Amoeba proteus, is without constant body form. Therefore, there is no tail, just as there can be no head, no mouth, no eyes, no ears, and no legs—the mass of living substance or protoplasm constituting the species serving as all of these and more. In many other simple one-celled forms, however, the body is constant in outline and, in addition, in many of them one part of the body is always behind and another is always in front as the organism moves about. Here we find longitudinal orientation of the protoplasmic mass which constitutes the body and hence, as regards the tail, the foreshadowing of future developments.

The free-living, freshwater flatworm Planaria, a larger, more advanced, many-celled animal, offers a very perplexing problem when we attempt to divide it into general sections or parts according to the popular method. This curious organism is always cross-eyed, if its tiny "eye-spots" are really to be regarded as eyes, and its mouth is situated near the middle of the body. Now, if we consider the fore part of the animal as the head, including the mouth as usual, we find that most of the planarian is head,
In fish, amphibians, reptiles, birds, and mammals, there is probably no part of the physiognomy that has been subjected to a more interesting series of variations and uses than this appendage.
but if we regard the portion back from the "ears" or sensory lobes as tail, much of this head really proves to be something else. Thus, at this point we are still unable to positively separate head, body, and tail.

This same indefinite condition or status of the tail holds to a greater or lesser degree throughout the lower or invertebrate series of animal groups, although it is true that certain of these show caudal developments which are worthy of note. The crayfish has a series of five terminal plates which the animal often uses in swimming rapidly backward through the water. Numerous insects, such as the cricket, have conspicuous posterior hairs or filaments which some may regard as a tail. Bees have developed a strong sting behind. Scorpions, which also sting with an apparatus at the terminal part of the abdomen, likewise show no real tail—and so on.

In the back-boned animals or vertebrates the tail appears as a definite structure. Here, in the fish, amphibians, reptiles, birds, and mammals, there is probably no part of the physiognomy that has been subjected to a more interesting series of variations and uses than this appendage.

Take for instance, among the reptiles, a fossil form, the ancient Ichthyosaurus or "fish-lizard," was obviously specialized for pelagic life like the porpoise and the whale, but the most primitive modern species, the tuatara of New Zealand (Sphenodon punctatum) shows none of this. Its tail is merely strongly compressed along its length, as are those of many other reptiles which live in marshy or semiaquatic habitats—such as the crocodiles and their allies, or certain large South American lizards of the family Teiidae (Dracaena guianensis and Crocodilurus lacertinus). Such tails are long and strong, being obviously adapted to aid in swimming movements and in defense. A similar type of modification is found in the marine snakes which occur so commonly in the vicinity of the islands of the Pacific and Indian oceans. These creatures usually have the elongate body and other general characters of the serpents, but their tails, instead of being rounded as in the terrestrial species, are often much compressed, so as to form a helpful rudder which may be waved from side to side while the animal swims about.

A small lizard, the common spiny-swift of central Kansas, reminds one of a pleased dog as it quivers its little tail in anticipation of food.

The tails of the turtles, like those of the horned lizards (Phrynosoma and Moloch), are almost too small and insignificant, as reptile tails go, to warrant discussion here, being reduced to but a relatively small fraction of the total length—usually less than thirty per cent.

Many reptiles vibrate their tails when disturbed, but none are better equipped by nature to capitalize this tendency than the venomous rattlers, which, unlike most other poisonous snakes, usually warn their would-be victims before attempting to molest them. The snakes of the genus Coluber, known generally as the racers, are unusually nervous and will frequently vibrate the tail when disturbed. Exhibiting this same tendency, a small lizard, the common spiny-swift of central Kansas (Sceloporus undulatus thayerii), reminds one of a pleased dog as it quivers its little tail in anticipation of food.

The Old World chameleons have a prehensile or clinging type of tail somewhat similar to that of the opossum. This is a rather unique development among the reptiles, although it is said also to have taken place in a rare iguanid lizard of the West Indies which is known to scientists as Xiphocercus valencienni.

Inhabiting the southwestern United States and the Mexican mainland are the poisonous gila monsters, which have large, rounded tails. These are covered with beadlike scales and are said to act as reservoirs for food, fat being stored in them during times of prosperity and used from them during the periods of inactivity known as hibernation and aestivation.

The tails of many lizards including those of various geckos, skinks, and "glass-snakes," are very delicate and consequently fragile and easily broken. This feature often spares the life of these animals, for an enemy in pursuit, having succeeded in grasping the tail alone, usually gets a section of just that, and nothing more. If the tail of a skink is injured in any way, so that it starts to come off, the section back of the injury is very apt to be shed whether it is firmly held or not. I have personally observed this phenomenon in the field in the case of an adult Sonoran skink (Eumeces obsoletus), which severed its long tail by a series of quick muscular contractions after a slight cut had been made accidentally near the base. After the tail has been broken, the wound heals readily and new or regenerated tissue grows. The part that develops in this manner, although not as long as the section lost, often becomes fully as long or even longer than the remaining stub of the original tail. In many regions various adult skinks and, in fact, certain other lizards, too, nearly all have regenerated or incomplete tails, and middle-sized or even young examples frequently show this state as well. Regeneration often produces freaks, as many scientists have shown, and extra tails are sometimes added after an injury. Two tails are found rather frequently in lizards, but three-tailed examples, although occasionally seen, are decidedly rare.

Amphibians have been divided into three main groups, the tailed and the tailless sections, and the section comprised of the little-known, wormlike caecilians which we need not consider. The tailed amphibians do not undergo complete change in outline between the young and adult stages, but are much the same in general appearance throughout their existence, whereas the tailless amphibians, named with reference to the adult anatomy, have a primary or tadpole stage in which the tail is present as in the other group and a secondary or adult stage in which the tail is lost. All degrees of transition between these two extremes may be seen in a carefully selected developmental series of our common frogs and toads. It seems appropriate at this point to call attention to the fact that in certain salamanders, particularly in some plethodontids, the base of the tail is constricted for loss at times of danger, thus serving the same protective purpose as the tails of the lizards mentioned above.

Fishes are specialized for aquatic life and possess flattened, rudder-like tails that aid in swimming as do similar tails in other groups. The fish-tail, although essentially constant in function and compression, is subjected to numerous variations in detail and outline. Therefore, the structure and appearance of the tail has been deemed of great importance in the classification of the group.

Woodpeckers have developed the practical habit of clinging to vertical surfaces and bracing themselves by means of their tail feathers, which in turn have become stiffened and thereby definitely modified for the purpose.

In the birds, the tail is formed internally by a short bony support, the pygostyle, and by the surrounding muscles, and externally by dermal elements, particularly feathers. It is in the feathers that the most obvious and interesting avian variation exists.

Perhaps in no group of animals have the tails become more specialized for purposes of ornamentation or display than in the birds, particularly in the males. In this regard, it is only necessary to mention such striking examples as the lyre bird, the bird of paradise, and the peacock. On the other hand, woodpeckers and chimney-swifts have developed the more practical habit of clinging to vertical surfaces and bracing themselves by means of their tail feathers, which in turn have become stiffened and thereby definitely modified for the purpose.

The wide variations shown by the tails of mammals are thoroughly interesting. For instance monkeys of the New World, belonging to the family Cebidae, nearly all have prehensile or grasping tails, but monkeys of the Old World, curiously enough, have only ordinary, non-prehensile ones, which may, perhaps, be considered decorative, but which perform no useful arboreal or gymnastic duties.

Then, there is the very short tail of the "cottontail" rabbit, which is covered by thick, soft, white, downy fur, as the common flame implies, and is consequently incapable of any monkey-like uses, but for all that it is still useful enough. Since the back and sides are brownish and much darker in color, this fluffy little ball is very conspicuous when the rabbit is running. Therefore, an enemy in pursuit of "bunny," be it man or beast, usually finds his attention fixed upon the "cottontail" rather than upon the prospective victim as a whole. This frequently results in the rabbit’s escape, for when it suddenly ceases its zigzag run and as suddenly squats to hide the tail from view, the pursuer frequently finds himself completely baffled, for with the disappearance of the "white spot" the whole animal seems to disappear.

The flying squirrel has a large, flattened, hairy tail, which serves as a balancer in his soaring jumps from tree to tree, but the opossum has a long hairless, prehensile tail that could not possibly aid its owner in the same manner.

In contrast to the rabbit’s type, the tail of the jumping mouse is sparsely haired and of an inconspicuous color. Moreover, instead of being short, and rounded, it is very elongate, measuring about twice the length of the body.

The flying squirrel has a large, flattened, hairy tail, which serves as a balancer in his soaring jumps from branch to branch and tree to tree, but the opossum has a long, hairless, prehensile tail that could not possibly aid its owner in the same manner. Instead, the opossum often grasps objects by winding its tail about them. This trait appears early in life, for each baby opossum thus clings tightly to "mamma" opossum’s tail as she swings it over her back and goes for a quiet evening stroll; while later, in the adult, the entire weight of the body may be easily supported from some convenient limb by this remarkable appendage alone.

There is an old saying that "if you pick up a guinea pig by the tail, its eyes will drop out." Since the caudal appendage is so short, being a mere vestige, guinea pig eyes seem perfectly safe. How fortunate it is, however, that the guinea pig does not feel called upon to show its feelings in the same way as does the dog, for it would have to wag the whole end of its body energetically to express a happy emotional state.

The muskrat and the beaver have much in common, for both of these rodents live in the vicinity of water, both are very fond of vegetable food, and both build more or less elaborate houses or dens for themselves, but when it comes to tails-the resemblances seemingly end. The muskrat’s tail is flattened vertically along its length, and while it may serve as a rudder in swimming, it is too weak to be used as a beaver uses his. The beaver’s tail, which is flattened cross-wise into a broad, paddle-like appendage, has developed an unusually strong set of muscles, so that it may be maneuvered both easily and effectually. This tool is used particularly in swimming and in personal broadcasting when danger threatens. Loud danger signals are often sent out to fellow beavers by a vigorous slap of the alert one’s tail on the surface of the water.

In remarkable contrast to the types just discussed, the whale and the porpoise have fleshy, bilobed tails, which suggest the fins of fishes but are set cross-wise of the body rather than perpendicular to it. In addition, they lack the bony supporting rays. Like many fishes, both the whale and the porpoise are specialized for life in the open seas.

Again, Nature has been especially generous with the kangaroo, in so far as his tail is concerned, whereas it has decidedly slighted the world’s champion heavy-weights, the elephant, the rhinoceros, and the hippopotamus. So strong is the kangaroo’s large tail that in times of danger it may support the entire weight of the body while the animal kicks viciously with its hind feet. At other times it is regularly used as a prop in sitting or as a spring or propelling force in locomotion. Exactly contrary to this condition, the elephant’s most useful appendage was placed in front of the body instead of behind, while the unlucky hippopotamus was cheated at both ends.

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