Periodicity in Reproduction
After reaching sexual maturity, a stage which is usually less marked in the male than in the female, most animals exhibit a periodical recurrence of reproductive activity. This, as applied to the species, may be called the breeding season. During the breeding season the individual animal may react to a single sexual crisis, or it may undergo several rhythmical waves of sexual activity, while between breeding seasons the pairing instincts and behavior are in abeyance.
The onset of the breeding season is probably due to a variety of causes, both external and internal, differing largely in various groups of animals. There is some underlying, common ground, however, for it is apparent that the reproductive cycle of most plants and animals is timed more or less to accord with the changing seasons, and to occur at a time that is favorable for the development of the forthcoming young. This is usually the spring or early summer, particularly among insects, annual plants, and cold-blooded animals.
A host of aquatic forms, for example, respond sexually to the rising temperature of the water in the spring of the year. Ocean fishes shift in schools into warmer shallow or surface waters to spawn, while amphibians and reptiles, arousing from their winter lethargy, proceed at once to increase the census returns in their cold-blooded world.
Birds stream northward in their annual “Canterbury Pilgrimage” as soon as the cold of winter in our northern latitudes has given way to the breath of spring. At this season they show considerably more purpose than in the more leisurely fall migration. The factor of changing temperature, however, does not wholly account for the remarkable nuptial flight of birds in their spring migration.
Mammals also generally exhibit enhanced vitality and courtship behavior at the spring season. According to Bissonnette it is the gradual increase in the daily light ration which brings about this increased sexual activity of birds and mammals. Even man, whose breeding season has been extended to include the entire year, feels the spell of spring, so that Tennyson stated a biological fact when he sang:
In the spring a livelier iris changes on the burnished dove;
In the spring a young man’s fancy lightly turns to thoughts of love.
The breeding season of fishes in the valley of the Nile is during the annual period of inundation. Lungfishes, which pass the dry season burrowed inactively in the mud, breed at once when the rainy season begins, being at that time restored to activity. It is rather remarkable that hibernating animals, as well as aestivating lungfishes, when they awake starved and hungry after their dormant sleep, proceed immediately to the business of propagating the species before they attend to their own individual needs. This is a good illustration of the imperative law of reproduction, which places the preservation of the species above the welfare of the individual.
In the same self-effacing way the salmon of the Pacific Coast, when they leave the ocean for the long perilous run up rapids, waterfalls, and past countless dangers for many hundred miles to their breeding grounds in the upper reaches of the Columbia and Yukon rivers, ascetically forego feeding and devote themselves entirely to the great adventure of reproduction. It is quite probable that some internal factor, perhaps a physiological urge set in motion as a consequence of rhythmical metabolic processes, must account for the astonishing behavior of the salmon. It is certainly not entirely due to the fact that their gonads have swollen to a degree demanding immediate action, because in the “Silver Horde” of the salmon run fishes of all stages of sexual development are found. Moreover, it has been pointed out by Jacobi that the gonads of the eel, Anguilla, which migrates in the opposite direction, from fresh to salt water for its one breeding season in a lifetime, do not become enlarged until it reaches salt water.
The breeding rhythm of some animals is even gauged to a certain time of day. Thus amphioxus in the Mediterranean region always spawns at sundown, and the famous “pololo worm” of the South Pacific, Eunice viridis, produces its myriads of eggs and sperm in quantities sufficient to color the water for miles about, at daybreak of a particular day falling in the last quarter of the moon in September and October.
Many invertebrates breed once for all, the act frequently marking the end of their life cycle. Animals like most marsupials and seals breed annually; the walrus once in three years; and elephants at considerably longer intervals.
There is a curious correlation between the breeding season of bats and a favorable time of year for the appearance of the future offspring. Pairing is effected in the fall, a mucous plug sealing the uterus after copulation so that the sperm can winter over in the vagina. After copulation has occurred the sexes go their separate ways to hibernation quarters, the males in one place, the females in another, where they literally “hang up” for the winter. Since the period of gestation in the bat is only two months, the young would normally be born during the somnolent hibernation period—an impossible state of affairs with no available insect food on the wing—were it not for the fact that fertilization is not consummated for several months after copulation, the sperm remaining viable in the vagina of the female throughout all that time.
Among mammals, the female during the breeding season passes through an oestrous cycle, or “heat,” in which preparation is made for the fertilization of the egg. If only one oestrus occurs during the breeding season, as in the case of the bear, the animal is said to be monoestrous. Polyoestrous animals, on the other hand, are those which, like rodents, have recurrent oestrous periods following each other throughout the breeding season.
Domestication frequently works changes in the periodicity of reproduction. Many wild animals refuse to breed at all in captivity, while domestic animals, such as cattle, have been changed from a monoestrous to a polyoestrous condition. The breeding season has been greatly extended in poultry, for example, to include practically the entire year.
In the human female the breeding season is not dependent upon external factors, but continues uninterrupted from the time of puberty until the menopause at the age of 45 or 50 years, throughout which time the oestrus, unless interrupted by pregnancy or some abnormal condition, recurs rhythmically approximately every four weeks.
Associated with the oestrus in humankind the walls of the uterus undergo marked periodic modification, throughout the sexual life, in preparation for the possible implantation of an ovum. These changes include enlargement of the uterine glands and accumulation of fluid in the mucosa which becomes distended to several times its ordinary thickness, a state which can be maintained for only a few days. If no fertilized egg is implanted, the mucosa undergoes further changes, leading to a gradual breakdown of the superficial layers which are discharged to the outside together with secretions and 40 to 60 cc of blood, as the menstrual flow. Menstruation, which lasts four or five days, is followed by repair of the mucosa and the next period of preparation. Counting the first day of menstruation as the first day of the oestrous cycle, the egg is usually discharged from the Graafian follicle on the twelfth to sixteenth day. If the descending mammalian egg meets an ascending sperm in a Fallopian tube, it may become implanted in the wall of the uterus that has been prepared for it and there undergo development. Under these conditions destructive changes in the uterine mucosa do not occur, menstruation being in abeyance during pregnancy and lactation. This monthly recurrence of the reorganization of the uterine mucosa begins and ends earlier in life in the tropics than among inhabitants of colder countries. The human menstrual cycle has been diagrammed by Corner as shown in Figure 411.
Ethnologists find indications of a former primitive breeding season in man coupled with the annual feasts and orgies of savages and in the yearly festival of the Saturnalia of classical times, when great sexual license was known to prevail.