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._