No. 2. Female Stickleback Laying Eggs in Nest.
As soon as the building operations are fully completed, the eager little householder sallies forth into his pond or brook in search of a mate who will come and stock his neatly-built home for him. At this stage of the proceedings, his wedding-garment becomes even more brilliant and glancing than ever; he gleams in silver and changeful gems; when he finds his lady-love, he dances round her, “mad with excitement,” as Darwin well phrased it, looking his handsomest and best with his lustrous colours glistening like an opal. If she will listen to his suit, he grows wild with delight, and coaxes her into the nest with most affectionate endearments. In [No. 2], as you perceive, the mate of his choice has been induced to enter, and is laying her eggs in the dainty home his care has provided for her. The father fish, meanwhile dances and capers around, in a pas de triomphe at the success of his endeavors.
One wife, however, does not suffice to fill the nest with eggs; and the stickleback is a firm believer in the advantages of large families. So, as soon as his first mate has laid all her spawn, he sets out once more in search of another. Thus he goes on until the home is quite full of eggs, bringing back one wife after another, in proportion to his success in wooing and fighting. For, like almost all polygamists, your stickleback is a terrible fighter. The males join wager of battle with one another for possession of their mates; in their fierce duels they make fearful use of the formidable spines on their backs, sometimes entirely ripping up and cutting to pieces their ill-fated adversary. The spines thus answer to the spurs of the gamecock or the antlers of the deer; they are masculine weapons in the struggle for mates. Indeed, you may take it for granted that brilliant colors and decorative adjuncts in animals almost invariably go with irascible tempers, pugnacious habits, and the practice of fighting for the possession of the harem. The consequence is, with the sticklebacks, that many males get killed during the struggle for supremacy, so that the survivors wed half a dozen wives each, like little Turks that they are in their watery seraglios. Only the most beautiful and courageous fish succeed in gaining a harem of their own: and thus the wager of battle tells in the end for the advantage of the race, by eliminating the maimed, the ugly, and the cowardly, and encouraging the strong, the handsome, the enterprising, and the valiant. This is nature’s way of preventing degeneracy.
In [No. 3] the nest is seen full of eggs, and the excellent father now comes out in his best light as their guardian and protector. He watches over them with ceaseless care, freeing them from parasites, and warding off the attacks of would-be enemies who desire to devour them, even though the intruder be several times his own size. The spines on his back here stand him once more in good stead: for small as he is, the stickleback is not an antagonist to be lightly despised: he can inflict a wound which a perch or a trout knows how to estimate at its full value. But that is not all the good parent’s duty. He takes the eggs out of the nest every now and then with his snout, airs them a little in the fresh water outside, and then replaces and rearranges them, so that all may get a fair share of oxygen and may hatch out about simultaneously. It is this question of oxygen, indeed, which gives the father fish all the greatest trouble. That necessary of life is dissolved in water in very small quantities; and it is absolutely needed by every egg in order to enable it to undergo those vital changes which we know as hatching. To keep up a due supply of oxygen, therefore, the father stickleback ungrudgingly devotes laborious days in poising himself delicately just above the nest, as you see in [No. 3], and fanning the eggs with his fins and tail, so as to set up a constant current of water through the centre of the barrel. He sits upon the eggs just as truly as a hen does; only, he sits upon them, not for warmth, but for aeration.
No. 3. Male Stickleback Watching Eggs in Nest.
For weeks together this exemplary parent continues his monotonous task, ventilating the spawn many times every day, till the time comes for hatching. It takes about a month for the eggs to develop: and then the proud father’s position grows more arduous than ever. He has to rock a thousand cradles at once, so to speak, and to pacify a thousand crying babies. On the one hand, enemies hover about, trying to eat the tender transparent glass-like little fry, and these he must drive off: on the other hand, the good nurse must take care that the active young fish do not stray far from the nest, and so expose themselves prematurely to the manifold dangers of the outer world. Till they are big enough to take care of themselves, he watches with incessant vigilance over their safety; as soon as they can go forth with tolerable security upon the world of their brook or pond, he takes a last well-merited holiday.
It is not surprising under these circumstances to learn that sticklebacks are successful and increasing animals. Their numbers are enormous, wherever they get a fair chance in life, because they multiply rapidly up to the extreme limit of the means of subsistence, and develop as fast as food remains for them. There the inexorable Malthusian law at last steps in: when there is not food enough for all some must starve; that is the long and the short of the great population question. But while provender is forthcoming they increase gayly. Sticklebacks live mainly on the spawn of other fish, though they are so careful of their own, and they are therefore naturally hated by trout-preservers and owners of fisheries in general. Thousands and thousands are caught each year; in some places, indeed, they are so numerous that they are used as manure. It is their numbers, of course, that make them formidable; they are the locusts of the streams, well armed and pugnacious, and provided with most remarkable parental instincts of a protective character, which enables them to fill up all vacancies in their ranks as fast as they occur with astonishing promptitude.
To those whose acquaintance with fish is mainly culinary, it may seem odd to hear that the father stickleback alone takes part in the care of the nursery. But this is the rule among the whole class of fish; wherever the young are tended, it is almost always the father, not the mother, who undertakes the duty of incubation. Only two instances occur where the female fish assumes maternal functions towards her young; about these I shall have more to say a little later on. We must remember that reptiles, birds, and mammals are in all probability descended from fish as ancestors, and it is therefore clear that the habit of handing over the care of the young to the female alone belongs to the higher grades of vertebrates—in other words, is of later origin. We need not be astonished, therefore, to find that in many cases among birds and other advanced vertebrates a partial reversion to the earlier habit not infrequently takes place. With doves, for example, the cock and hen birds sit equally on the eggs, taking turns about at the nest; and as for the ostriches, the male bird there does most of the incubation, for he accepts the whole of the night duty, and also assists at intervals during the daytime. There are numerous other cases where the father bird shares the tasks of the nursery at least equally with the mother. I will glance first, however, at one of the rare exceptions among fish where the main duty does not devolve on the devoted father.