LOVE-MAKING AMONG FISHES
Germinal Variations—Fishes and mate—hunting—Some remarkable Sexual
differences displayed by the Teeth of Rays—The Double-eyed Fish—The
Coloration of the Dragonet—Some curious facts about Salmon—The strange
use of the kidneys in the Stickle-back—The Stickle-back and parental
duties—Siamese Fighting-fish.
Mammals, Birds, Reptiles and Amphibia, as has already been shown,
all exhibit practically the same line of conduct in regard to their
mate-hunting instincts; all use like modes of expression. And this is a
very significant fact. It becomes more so when we turn to the fishes,
for here again we meet with the same behaviour, and here again we meet
with the same rules in “secondary sexual characters.”
An instance or two of the latter distinction between the sexes should
suffice. As a rule, among fishes, the males are smaller than the
females: commonly there is no other external distinguishing feature
between them. In many cases, however, the males are more or less
strikingly different, thereby showing a departure in the nature of a
higher degree of complexity, or “specialization,” just as obtains among
the birds. And the same sequence to this also obtains. That is to
say, as has already been remarked in the case of the Mammals and the
Birds, the new features first appear in the males, leaving the females
and young of both sexes unmodified. A singular illustration of this is
afforded by some of the Rays, or Skates, as they are often called. In
the Thorn-backed Ray (_Raia clavata_) the teeth of the adult male are
sharp-pointed and directed backwards, while those of the female are
broad and flat, forming a sort of mosaic or pavement. The young male
agrees with the female in this respect. In the Common Blue Skate (_Raia
batis_) the teeth are pointed in both sexes, though more so in the
adult male. In the Spotted Skate (_Raia maculata_) the teeth are fully
pointed in both sexes. Here, then, the normal course in the evolution
of new characters is followed, but it is remarkable that the teeth,
which are so intimately related to the capture of food, should be thus
affected. Whether the change of teeth is associated with a change of
food, or whether neither pointed nor pavement teeth affect the feeding,
is unknown.
Still more remarkable is the case of the Double-eyed Fish (_Anableps_).
In this fish there is an intromittent organ in the shape of a tube
which is formed by a continuation of the urinogenital ducts down the
front of the anal fin. In the hinder half of this organ a bend is made
either to the right or left. Out of seventeen males, this bend was to
the right in eleven, to the left in six. Further, there is a small
fleshy tubercle at the side of the anal fin-ray, at the middle of its
length. When this prominence is on the left side, the organ bends to
the right; when it is on the right, the bend is to the left. In the
females the genital opening is covered by a special scale, which is
free on one side, left or right, and not on the other. Thus copulation
is possible only from the side, and a left-sided male can only
conjugate with a right-sided female, and vice versa. Here is one of the
most extraordinary cases of specialized secondary sexual characters
known. How do the sexes distinguish their complemental mates? It is
important that they should, for unions are otherwise impossible.
In the Dragonet (_Callionymus lyra_) the male differs conspicuously
from the female in being much the larger—an exception to the rule—and
in having the fin-rays enormously elongated. Further he wears a
conspicuously resplendent livery, but this is strictly a “nuptial”
livery, the colours waning as soon as the period of sexual activity is
past. That these colours play the same part as with the birds is clear
from the observations of the late Saville Kent. “The male,” he says,
“resplendent in his bridal livery, swims leisurely round the female,
who is reclining quietly on the sand, his opercula distended, his
glittering dorsal fins erect and his every effort being concentrated
upon the endeavour to attract the attention of his mate.... The
female, at first indifferent, becomes at length evidently dazzled by
his resplendent attire and the persistency of his wooing. She rises
to meet him, the pair so—far as is practicable with fishes—rush into
each other’s arms, and with their ventral areas closely applied, ascend
perpendicularly towards the surface of the water.” In the course of
this ascent the ova and sperms are shed, and fertilization takes place.
The difficulties in the way of the study of the behaviour of fishes
during the critical period of mate-hunting are many and obvious.
Something may be inferred from the nature of the secondary sexual
characters which they exhibit, and more definite information can be
obtained from such species as can be kept in aquariums. From these
two sources enough has been gleaned to show that these cold-blooded
creatures, in many cases, exhibit the same emotions and the same means
for their fulfilment as the higher vertebrates. And it is significant
that wherever anything approaching what may be called “Courtship”
obtains, the males commonly exhibit secondary sexual characters,
whether in the form of ornament or of armature; while among species
which consort in shoals during the breeding season no such distinctions
are present. The ova and milt are shed and fertilization takes place as
they escape.
Comment is frequently made in works on Natural History on the fact that
among fishes the males are commonly smaller, often conspicuously so,
than the females. Among mammals the males are the larger; but among
birds this is by no means always the case. It is somewhat surprising to
find this discrepancy among the birds of prey, where, as in the case
of the Sparrow-hawk, the male is little more than half the size of his
mate; commonly, however, there is little or no difference. Among the
fishes the differences are often much more marked, as for example in
the Conger-eel, wherein the male never exceeds a length of two feet
six inches or a weight of one pound; females, on the other hand, may
exceed eight feet in length and attain a weight of one hundred and
twenty-eight pounds, though such giantesses are rare, but specimens
of fifty pounds and upwards are frequently met with. The explanation
of this may lie in the fact that among fishes it is no uncommon thing
to and males becoming sexually mature long before they have attained
their full stature. With the Salmon, for instance, ripe spermatozoa
have been found in individuals of not more than a few inches in length,
and in this species also the male is the smaller. Ova take longer
to attain maturity, for in addition to the germ-plasm they must be
provided with a more or less extensive amount of food-material in the
shape of yolk. The formation of this is inhibited until the demands
on the system for the building up of the body have begun at least to
lessen.
Mate-hunger among the fishes seems generally to find peaceable modes
of expression, either in “display” or in consorting in vast shoals,
though, so far, the factors which govern their conduct in this matter
are as yet unknown. But here, as with the higher vertebrates, there
are some species which adopt more violent methods. A good illustration
of such conduct is furnished by the Salmon, which, during the period
of sexual activity, develops a curious modification of the lower jaw,
which is produced forwards and upwards to form a hook-shaped projection
of fibrous tissue. When the mouth is closed this hook is received
into a cavity formed within the fore-part of the roof of the mouth.
It has been described as a weapon of offence. But this it can hardly
be. On the other hand it has been suggested that it serves to protect
the jaws when charging a rival, for the shock on such occasions is
considerable. It answers, in short, like to the fibrous mass of tissue
which protects the fore-part of the head in Whales like the Black Whale
(_Globicephalus_) and the Bottle-nose Whale (_Hyperoodon_), serving as
a battering-ram. In the Pacific Salmon (_Onchorhynchus_) both jaws are
hooked, so that when the mouth is closed the hooks cross one another
as in the beak of the Crossbill. In this Salmon, too, the front
teeth attain a considerable length, while the body becomes laterally
compressed and a hump forms at the shoulders. Little, however, seems to
be known as to the nature of their battles.
The combats of the Salmon of our own islands, however, are evidently
severe, and this has long been known, for Darwin speaks of as many
as three hundred, all with one exception males, being found dead in
the Tyne during the month of June, killed by fighting. Such battles
are fought, it is to be noticed, not so much for the possession of
females—for it is a polygamous fish—as for the privilege of fertilizing
the eggs as they are shed. The absence of a “display” here is a
noticeable feature, and it is on this account, probably, that the
reproductive period is not associated with the appearance of any form
of resplendent livery. On the contrary, the marvellous silvery sheen
which adorned both sexes on their arrival at the spawning ground from
the sea has entirely vanished by the time that the consummation of the
journey has been attained, and in its place is naught but a slimy,
dingy copper-coloured hue. But no sooner has the reproductive period
passed than the silver lustre makes its appearance once more.
These facts are the more interesting when contrasted with what obtains
among other fighting species which must woo the females. Take the
case of the common freshwater Stickle-back. In this species the body
is invested with an armature of bony plates and spines in place of
scales, while the males are arrayed in vivid hues of red and blue. Any
survey, however, of the reproductive activities of this little fish
must take into account certain quite remarkable prenuptial actions
and instincts. Briefly, before the male commences his search for a
mate he constructs a nest of fine fragments of aquatic weeds, which
are held together, not by interweaving as with birds’ nests, but by
a sticky and copious secretion from the kidneys. According to some
authorities, this secretion is to be regarded as a pathological product
caused by the undue pressure of the ripening testes. It is difficult
to accept this interpretation, for it might with as much reason be
argued that the copious secretions of the salivary glands of the edible
Swift—which builds a nest constructed entirely of hardened saliva—are
also pathological in character. But be this as it may, the nest
completed, the male seeks a mate, or mates, for polygamy is the rule of
his tribe. In his search for these he has constantly to do battle with
other males, whom he endeavours to disembowl by swift rushes contrived
to t rip open his rival as he passes, by means of one or other of
the erectile spines which project from his back and belly. With the
females whom he desires he uses the arts of peaceful persuasion,
swimming backwards and forwards before her in his endeavour to excite
her amorous instincts. At last he persuades her to enter his bower and
deposit a few eggs, fertilizing them immediately they are laid. The
first to enter leaves by forcing a passage through the opposite wall
of the nest, a happy contrivance, for thereby a current of water can
be constantly driven through, leaving fresh oxygen to the developing
eggs. One female after another is inveigled into the bower, until the
complement of eggs is complete. These, singularly enough, are now taken
charge of by the male. He it is who creates life-sustaining currents
which bathe the eggs, by the rapid vibrations of his breast-fins, and
he it is who protects them from their most persistent enemies—the
females who laid them. As soon as the fry appear the duties of the male
are still further increased. He must guard them from their mothers,
and other foes, and he must prevent their too extensive wanderings.
Such as stray too far afield are sucked into the mouth and brought
back again to the nursery, where they are set at liberty by a reversal
of the sucking action. That the male of a polygamous species, and
with all the attributes of a polygamous species—pugnacity and vivid
coloration—should take upon himself the duties which under like
circumstances among the higher vertebrates are undertaken by the female
is a very remarkable and puzzling feature. In this species, in short
the male plays successively a polygamous and a polyandrous rôle.
Strange as these facts are, they are not apparently without parallel
among fishes, for certain of the labyrinth-gilled fish present many
features in common, though as yet proof seems to be wanting. Thus
the small Siamese “Fighting Fish” (_Betta pugnax_) is endowed with
so ferocious a nature that it is kept, as the Malays keep fighting
cocks, for the amusement of native sportsmen, two fish being pitted
against one another and large bets being made on the result. In a
state of quiescence it presents no very remarkable coloration, but if
two be brought together, or if one sees its image in a looking-glass,
it becomes thrown into a paroxysm of rage, the fins are raised and
the whole body becomes irradiated with metallic colours of dazzling
beauty. There can be no doubt but that a like play of colour occurs
during moments of sexual excitement; it is highly probable that it is
polygamous. Of its breeding habits, however, little or nothing seems to
be known. Not so, however, in the case of a closely-related species,
less pugnacious in disposition, but almost as vividly coloured, in so
far as the male is concerned. Now in this species a nest of froth is
made and the eggs, after deposition therein, are jealously guarded by
the male. Hence, on these facts, we may assume with a fair amount of
certainty that the closely-related “Fighting Fish” displays like habits.
That the Reptiles, Amphibia and Fishes have much in common with one
another, and with the higher vertebrates, in the manner of their
love-making is indisputable. We find no evidence anywhere that the
first faint throbbings of the sexual pulse in the female are quickened
to fever beats after the efforts of several successive wooers, each
more demonstrative than the last, to arouse this state—the conditions
required by the Sexual Selection theory. But successful mating depends,
in each year, on the sexual fitness of the male himself, and the mate,
or mates, which for that year he has taken “for better or worse.” It
is possible, of course, that a male, ambitious but impotent, will
be forsaken by his mate; it is possible that a female of low sexual
vitality may fail to respond to the most impassioned displays; in
either case no offspring result, and thus the failures are eliminated.
It is possible that here, as with the higher vertebrates, coition
may by no means always be immediately preceded by display. But the
“display” has done its work. It has stimulated the sexual appetite, as
the sight of tempting food stimulated the bodily appetite.
But both the Amphibia and the Fishes reveal a lower plane of the sexual
instincts, when the sexes, dominated by some imperious instinct,
gather in hordes, commingling to shed their precious germs into the
surrounding water, there to effect the work of fertilization and the
achievement of new birth. The all-important union of these germs is
no mere work of chance, as it might seem, but the sperms seek the ova
with unerring surety, guided, in this case, by that very efficient
substitute for instinct, chemotaxis, or the attraction which certain
chemical substances have for lowly organized living bodies. In
this case the allurement is furnished by the ova. It is surely no
unreasonable surmise that here we have the beginnings of the complex
phenomena which the earlier chapters have revealed. On this lower plane
we are probably confronted by instinct alone, but from this level
upwards intelligence plays an increasingly important part.