ESTIVATION, OR SUMMER SLEEP.
By the Rev. J. G. WOOD, M.A.
I have already mentioned that the peculiar condition which we term hibernation is one which can be produced by heat as well as by cold, and that the bat passes into that state daily throughout summer. The name, therefore, is not sufficiently definite. The German naturalists more properly use two distinct terms, and employ the words “winterschlaf,” i. e., winter sleep, and “sommerschlaf,” or summer sleep.
In order to maintain the same construction in the terms, I will call the summer sleep by the name of Estivation. This word is scientifically more correct than summer sleep, because, as I have already mentioned, the condition in question is not real sleep, but a kind of trance.
As Estivation is produced in consequence of the withdrawal of food by heat, we must naturally look for it within the tropics. Many of the lower vertebrates are subjected to Estivation, but, as far as is known, no mammal estivates. It has been said that the Taurde, or Madagascar hedgehog, does so, but it is evidently a mistake. It is really one of the hibernators, like our own hedgehog; and though it assumes the trance condition in June, that month is the beginning of winter in Madagascar, and not in the middle of summer, as in England.
I will only take two examples of true Estivation, one from Africa and the other from America. The first is the well known Lepidosiren, or mud-fish, a creature which has long been an enigma to zoölogists, as no one could say definitely whether it were a fish or a reptile. Professor Owen, however, states that the structure of its organs of smell proves that it is a true, though rather anomalous, fish. It is found in many parts of Africa, and inhabits the banks of muddy rivers, being plentiful in the Nile.
Nowadays, the systematic naturalists have changed its name and called it Protopterus, giving the old and equally appropriate name of Lepidosiren to an allied species which is found in the Amazon river and its tributaries. I have, however, retained the original name, and see no sufficient ground for altering it.
It is brownish grey in color, and eel-like in shape, but has four curious rudimentary limbs, apparently useless for locomotion, though they are seldom without movement. They are, in fact, soft single rays of the pectoral and ventral fins, which represent the limbs of beings more highly organized. Each ray carries a narrow strip of membrane along nearly the whole of its length.
Along part of the back there is a very soft fin, extending over the tip of the tail, and returning on the under surface of the body as far as the base of the hind limbs. The body is always covered with viscous slime, insoluble in water, and the creature seems to be able to secrete it as it is wanted.
Essentially predacious, it does not possess rank after rank of teeth, such as we see in the pike, and the wolf-fish, and the like, but is endowed with a most remarkable dental apparatus.
Instead of separate teeth, there is in each jaw what may be called a tooth-ribbon. Suppose that we imagine the dental matter, instead of being made into separate teeth, to be rolled out into a continuous ribbon, then “pleated” into folds like those of a ruff, and so set in the jaws. Then let us imagine the projecting edge of each tooth-ribbon to be as sharp as that of a chisel, and we can realize the formidable apparatus with which the mouth is armed.
These details are here briefly given, because without them the history of its estivation could not be understood.
That the Lepidosiren was carnivorous had long been known, but no idea was formed of its voracity until some living specimens were successfully reared in the Crystal Palace. One of them was placed in the large water basin which then adorned the center of the tropical department at the north end of the Palace, but which may now be seen in the open air between the Palace and the water tower.
Though confined in a tank, it contrived to escape into the basin, and straightway began to make havoc among the gold-fish. It swam gently under them, rose with open jaws, caught the fish just behind the pectoral fins, bit out a piece, its ribbon-like teeth cutting through scale, bone, and flesh, as if they had been shears, and sank out of sight with its prey. It never bit the same fish twice, and as long as it could find fish, declined to eat anything else.
As this mode of feeding involved a gold-fish for each mouthful, Mr. F. W. Wilson, who was then in charge of the Natural History Department of the Crystal Palace, had the tank emptied, and fenced off a portion with wire grating, so that the Lepidosiren could not get at the fish. The creature was then fed with frogs, which I have seen it eat; and by reason of the perpetual supply of food, it grew so fast that it attained a length of thirty inches and weighed six pounds and a quarter, a very giant of Lepidosirens, which seldom exceed eighteen inches in length.
It lived for more than three years, and might have grown to a much larger size, but for the neglect of an attendant who forgot on one winter night to keep up the fire which warmed the water, and in consequence this interesting creature was found dead next morning.
Here then we have a carnivorous being of more than ordinary voracity, and requiring a constant supply of fish. But, during the rainless summer, the water is rapidly evaporated under the sun’s rays, the fish die, and the muddy bed of the river becomes as dry and nearly as hard as brick. What then is the Lepidosiren to do?
By Divine Providence, the heat which withdraws its food acts upon it as cold acts upon hibernating animals in this country. As soon as the drying-up process has begun, the Lepidosiren wriggles itself into the mud while it is still soft, and by dint of turning round and round, makes a sort of chamber, the sides of which are preserved from collapsing by the slime which it pours from its body.
It then doubles itself up sideways in a most curious fashion, wrapping the membranous tail over its head so as to cover it entirely. The body is not coiled in a circle, as might be imagined, but the two inner sides (mostly the left) are pressed closely against each other, so that the animal occupies a wonderfully small space. The dimensions of the chamber are soon contracted by the weight of the superincumbent mud, until at last there is scarcely the eighth of an inch of free space round the body.
In this curious refuge the Lepidosiren passes into a state of Estivation. The mud is gradually dried, and then baked under the fierce rays of a tropical sun. But the Lepidosiren lies motionless and unconscious until the next rainy season refills the river, dissolves the hardened mud, and sets the creature free to resume its predatory life.
Were it not for the Lepidosiren, the inhabitants of these countries would often be hardly pressed for food. But they search the dry bed of the river, dig up the buried estivators and live on them. So here we have Estivation as well as hibernation, indirectly beneficial to man. I may mention that most of the Lepidosirens which have been kept alive in this country were brought while still buried in their mud cells.
There is little difficulty in finding the hidden Lepidosirens, as the aperture through which they entered the mud seems almost invariably to remain open, its smooth and slime-polished sides leaving no doubt as to its identity.
I have possessed for more than four years a large lump of dry Nile mud, a hole in one of its sides showing that a Lepidosiren ought to be inside it. This morning I carefully cut it open, and there found the inhabitant, doubled up, with its tail over its head just as when it gave itself up to slumber more than twenty years ago. I expected to have seen a nearly spherical chamber, but found that the cell is cylindrical, and only just large enough to hold the creature.
The slime with which the cell is lined has been hardened into a papery consistence, and is, in fact, about as thick as the paper on which this account is printed. When a piece is torn off and held in the flame of a spirit lamp, it takes fire and it gives out a very nauseous odor, like that of a beetle’s wing case when similarly burned. This thick coating of slime is only to be found in the cell itself, and surrounding the body of the animal. I imagine that the Lepidosiren must deposit many successive coats of slime after it has taken up its position. These cells are technically named “cocoons.”
As some time elapses between the falling of the rain, when the creature awakes, and the dissolving of the cocoon, there must be some peculiar structure of the respiratory organs. Otherwise, the Lepidosiren, being a fish, and breathing by gills, must die before it can reënter the water.
This structure is of a most unexpected character. The creature has rows of gills on either side of its head, and with these it breathes while it is in the water. The swimming-bladder, however, is modified so as to act as a substitute for a lung. A branch of the artery which supplies the gills is diverted to the swimming-bladder, and as there is a communication between the interior of the swimming-bladder and the external air, the creature is able to aerate its blood sufficiently to sustain life until it can assume its normal fish life.
I may here mention that these African and American Lepidosirens, together with the Australian Ceratodus are especially interesting as being one only living survivor of a vast family which in bygone ages were extremely numerous.
The Ceratodus is a comparatively new discovery, and came on naturalists by surprise. Until lately the only known examples of this fish were to be found in the earlier secondary rocks, and when it was announced that living specimens had been found, the discovery could hardly be believed. However, there the Ceratodus is. It looks like a resuscitated fossil, and is to our known fishes what the tree-fern is to our present vegetation.
There is another interesting point about this object, showing how Estivation is connected with Scripture.
The mud of which the cocoon is made is the same as that which the Israelites, while in captivity, were forced to make into bricks. It is so tenacious, that although merely dried by the Egyptian sun, it is so hard that I was obliged to employ mallet, chisel, saw, and butcher’s knife, while making the necessary sections.
Occasionally the difficulty was increased by vegetable fibers which had become mixed with it, and which bound it together just as the cow-hairs bind builder’s plaster when honestly made. The Egyptians mixed straw with the clay of which their bricks were made, so as to strengthen it, and in order to secure a supply of such straw they did not reap their corn near the ground as we do, but cut off the ears close to the stem, leaving the stubble to be cut separately. The reader will remember that one of the grievances of the captives was, that instead of being supplied with straw, as formerly, they had to cut and fetch the stubble for themselves, and yet were forced to deliver the same number of bricks daily.
So here is my lump of Nile mud acting as a link representing nearly four thousand years between the Christian world of the present day, and the long-perished Egyptian dynasty of the Pharaohs.
Now we will pass to the opposite side of the world.
In tropical America, as in tropical Africa, the rivers are dried up in the summer, and the mud which forms their banks and bed is baked as hard as that of the Nile and other African rivers. Many of these rivers are inhabited by a fish (Callicthys) popularly called the Hassar, or Hardback. The latter name is given to it in consequence of two rows of hard, narrow scales on each side of the body. There are four long, flexible tentacles on the upper lip. It is not nearly so large as the Lepidosiren, seldom exceeding eight inches in length. Its color is greenish brown.
Unlike the Lepidosiren, which can not travel on dry ground, the Hassar is as good a walker as the Climbing Perch, a fish which not only leaves the water and traverses dry land, but can ascend the trunk of trees. All rivers have some portions deeper than others, “holes” as we call them in our rivers at home. So, when the process of drying up is nearly completed, the river is converted into a ravine along which “holes” or pools are seen at irregular distances.
As long as the holes are capable of containing water, the Hassar makes its way to them over the dry ground. But, in process of time, even the pools are dried up, and just before this happens, the Hassar works its way into the mud, and acts after the manner of the Lepidosiren. The analogy between the two fishes is made still more remarkable, inasmuch as they both furnish food to man during the time of Estivation.
The Hassar has a further interest in being one of the few fishes which make nests and watch over their young. Our sticklebacks do this, but whereas with the stickleback the double task of making the nest and guarding the young is relegated to the male, with the Hassar the latter duty is shared by the female. It begins the task of nest-making almost as soon as it escapes from its cocoon, so as to insure plenty of time for nest-making, egg-hatching, and rearing the young.
The American Alligator, which, like the Hassar, is deprived of food when the rivers and swamps have been dried, allows itself to be buried in the mud, and there awaits the return of rain.
A curious instance of this habit occurred some years ago. A party of travelers had halted on a piece of hard, level ground, lighted a fire and began to cook their dinner. But that dinner was spoiled, for before the cooking was completed the ground began to heave and swell, and out burst the head of an alligator. The unfortunate reptile was estivating exactly under the spot where the fire had been placed, and where it would have remained asleep until the next rainy season, had it not been disturbed.—London Sunday Magazine.
