The Encyclopedia Americana (1920)/Cave Animals
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|Edition of 1920. See also Troglofauna and Trogloxene on Wikipedia, and the disclaimer.|
CAVE ANIMALS. The animal life of caverns falls into three categories: 1. Animals, mostly extinct, that made their dens or left their bones in caves, and in life were members of the next group. 2. Animals that temporarily, but habitually, resort to caves for refuge, or sleep, or as breeding-places. 3. Animals, degenerate, confined to an underground life throughout their whole existence. None of these classes include those making small caves for themselves, the burrowers; or which, like the mole and many insects, dwell in the soil; or, like the conies (Hyrax), the pikas and several sea-birds, spiders, etc., seek safe homes among the interstices of loose rocks.
Prehistoric Cave Beasts.— The first group will require little space, as it consists of such extinct animals as the cave bear, cave lions, cave leopard, cave hyæna, cave wolf and some smaller ones that have been given these names because their bones and portraits have been found abundantly in the floors or on the walls of caverns in Europe and Africa. Indications trusted by geologists and archæologists combine to show that these animals lived there in the latter part, at least, of the third Inter-glacial Epoch, and on through the fourth and last glacial advance, when, although central Europe was free from an ice-cap, an almost Arctic climate prevailed, with much rain. This is what is known as the Reindeer Period, when humanity was represented by the Neanderthal race (see Stone Age). The weaker part of the fauna disappeared, but those hardy carnivores, finding food still plentiful, gradually adapted themselves by increased hairiness to the cold climate; but apparently they resorted far more than previously to the shelter of caves. None of those mentioned above is regarded as anything but a larger, more vigorous variety (spelæa) of the lion, leopard, wolf, spotted hyæna, etc., except the cave bear (Ursus spelæus). This beast was the most thoroughly spelæan of all in its habits, and occupied caves before men began to do so. It was not much if any larger than the ordinary brown bear of to-day, and its claws were shorter and feebler. “Hence it would appear,” says Osborn, “that the Neanderthals had driven out from the caves a type of bear less formidable than the existing species, but nevertheless a serious opponent to men armed with the small weapons of the Mousterian period.” Probably fire and smoke were the most effective means. These bears were numerous, for game was abundant A single cavern in western France has yielded remains of more than 800 skeletons; and from these bones and from prehistoric drawings it is possible to know this animal perfectly. With the close of the last period of partial glaciation, and the return in the early Pleistocene of the moderate climate that still continues, these and the other cave-haunting beasts disappeared, largely, no doubt, killed off by the better-armed Neolithic hunters. The great bear left no descendants, for the modern European brown bear traces its lineage to an older and smaller species, the Etruscan bear, whose bones also are occasionally exhumed from cave-floors.
Caves in North America present different conditions from those of Europe. Those that have yielded animal remains, such as the Port Kennedy and Frankstown “caves” in Pennsylvania, and the Concord Fissure in Arkansas, “are hardly caverns,” says Scott, “in the ordinary sense of the word, but rather narrow fissures, into which bones and carcasses were washed by floods.” They contain a great variety of mid-Pleistocene species, at least half of which are extinct. Big Bone Lick Cave, in Kentucky, is more truly a cave, and has furnished palæontology with an immense supply of bones of recent time, including several ancient species, such as mastodons, mammoths and the ground-sloth, Megalonyx, and with certain traces of human presence. Caves in northern California are also rich in animal remains, illustrating the transition from glacial to modern faunas. Brazilian caverns hare yielded much also; and a cavern near Last Hope Inlet, Patagonia, is noteworthy for the finding in it of the bones, and large pieces of the skin, of a great extinct sloth (Megatherium) with the hair still on.
Temporary Tenants of Caves.— In modern times, as anciently, bears and other carnivores use caves as sleeping-places when it is convenient but they are exclusively resorted to by a few kinds of creatures that may properly be called cave-tenants. The most characteristic of these probably are certain bats, especially such insect-eating kinds as the leaf-nosed, the horseshoe and the true bats of the family Vespertilionidæ. Caves frequented by bats usually harbor enormous colonies, and one who enters and disturbs them will find himself in the midst of a whirring multitude that it taxes the powers of description to portray. Some caves long occupied contain vast deposits of the richest possible guano, and this has been extensively utilized in some places as a fertilizer. Such artificial caverns as the deserted tombs of Egypt are filled with bats, one species of which is popularly called tomb-bat, and abounds in the interior of the Great Pyramid.
Birds of two sorts are cave-tenants. The most singular, probably, is the large guacharo, or oil-bird, of the family Steatornithidæ, classified between the nightjars and the owls, and inhabiting northern South America and the island of Trinidad. It inhabits both sea-side and mountain caverns, going forth only at dusk to get its food, which is mainly fruit. “Visitors to the breeding-caves,” says Evans (‘Birds,’ 1900) “are suddenly surrounded by a circling crowd of oil-birds uttering loud croaking or rasping cries. . . . The numerous nests . . . are flat, circular masses of a clay-like substance placed on ledges or in holes.” Great numbers of these birds are killed by torch-light for the sake of the oil obtained from them, which is excellent for illumination or for cooking purposes. The other birds choosing sea-caves as a breeding-place are swifts of the genus Collocalia, whose nests are placed in the depths of caves on the coasts of Ceylon, and eastward and southward to northern Australia. The best known of the many species is that which produces the edible nest of which the Chinese are so fond. Huge numbers of these swifts breed in company in dark caves, where they are associated with bats; but the bats go outside at night and the swifts by day. Such caves contain, exceedingly rich deposits of guano, and return to their owners a large rental for the right to gather the swifts' nests glued to their walls and roofs. See Swifts.
Blind Inhabitants. — A special cave-fauna exists in all parts of the world, consisting of an assemblage of animals of different classes which are blind, and in most cases eyeless. This fauna is evidently of great antiquity, since it exists most plentifully in caves, such as those of the limestone district of the South-Alleghanian region, and in southern Europe, which regions are south of the region of the Pleistocene glaciation. Within such caves, formed by the action of water (see Cave), are rivers, sink-holes and deep wells, all perfectly dark, inhabited by blind amphibians, fish, crayfish and other crustaceans, and by many kinds of insects and spiders. No vegetation exists save a few scattered molds and fungi, and all the animals are carnivorous, preying on each other. Mammoth Cave in Kentucky contains about 75 species of these blind creatures, to which 40 or 50 more may be added from other southern caves; while several hundred kinds have been described from European caverns.
The most striking and interesting form in Mammoth Cave is the blind-fish (Amblyopsis spelæus). It is about four inches long, colorless and blind, the eyes being vestigial. This fish seeks the dark and shuns the light, being much disturbed by a lighted match or bright sunlight, or even by a ray of light. In well-fed adult specimens there is no external indication of an eye; but in young ones, before reaching a length of two inches, the eyes can be distinctly seen, owing to their pigment, which is lost in the adult. The optic nerve can be traced in examples under an inch in length. This will apply to the eyes of other blind fishes and blind insects, crustacea, etc. While the sense of sight is lost, that of touch in the blind-fish, as in most other cave animals, is exalted. Amblyopsis is provided with tactile papillæ, arranged in ridges on the front and sides of the head. They are said to show extreme timidity and caution in their movements.
A still higher type of vertebrate, two species of salamanders, have become adapted to cave life, losing their eyesight by disuse. The species of the genus Spelerpes frequent damp, dark situations and the entrances to caves. An allied form (Typhlotriton spelæus) is distinctly a cavernicolous as distinguished from a twilight species, and has never been found outside of caves. Its eyes show early stages of degeneration. It inhabits caves in southwestern Missouri, and occurs under rocks in and out of water. Still another salamander, whose eyes are the most degenerate known among amphibians, is the Typhlomolge rathbuni. It lives in subterranean streams, tapped by an artesian and also a surface well, near San Marcos, Tex., and in one of the caves near that town. Its remarkably long and slender legs are too weak to support its body when out of water.
The lower animals tell the same story of degeneration, total or partial atrophy of the eye, together with loss of color, and, in a more striking way, the compensation for the loss of vision by a great increase in length of the antennæ and other appendages, or by the growth of long, slender tactile bristles.
The blind crayfish (Oreonectes pellucidus) is a common cave form. It differs from its out-of-door allies in being blind deaf, slender-bodied and colorless. Other blind or eyeless crustaceans are various kinds of amphipods and isopods, both aquatic and terrestrial, of which species of Cæcidotea are the most abundant, and form the food-supply of the blind crayfish.
The eyeless beetles of caves (Anophthalmi) have no vestige of eye or of optic nerves, while their bodies and appendages are slender. They grope their way about by means of very long tactile bristles. Other beetles, such as Adelops, which have retained vestiges of the outer eye; some spiders comprising an eyeless species, and others with eyes varying in size, some much reduced, spin little webs on the walls of the chambers. Among the harvestmen some (Phalangides) have extraordinarily long legs; while the Campodea (q.v.), a wingless insect of the Mammoth and other caves of the United States and Europe, differs from the outdoor form in its antennæ and abdominal appendages, being greatly exaggerated in length. There are also mites, myriapods, primitive wingless insects (Podurans), a few flies, worms and infusorians.
Origin and History.— The blind fauna of caves, according to Packard, is composed of the descendants of individuals which have been carried by various means into the subterranean passages, have become adapted to life in perpetual darkness, becoming isolated, and thus, as long as they are subjected to their peculiar environment, breed true to their species, and show no tendency to relapse to their originally eyed condition. The absence of the stimulus of light causes the eyes, through disuse, to undergo reduction and atrophy. With this goes, in certain forms, the loss of the optic ganglia and optic nerves.
Bibliography.— Packard, A. S., ‘Cave Fauna of North America’ (Washington 1888); Eigeman, C. H., ‘Cave Vertebrates of America’ (Washington 1909); Scott, ‘Land Mammals of Western Hemisphere’ (New York 1913); Morgan, T. H., ‘Evolution and Adaptation’ (New York 1903).