Fertility and Fecundity

FERTILITY AND FECUNDITY. Fertility is the power to beget living offspring; fecundity that of the individual to pro duce functional gametes (marrying cells, sex-cells, in the higher animals' ova and sperms). The reproductive rate of a mating or of a generation is determined both by the fecundity of the indi viduals composing it and by the fertility of their matings.

Fecundity

is affected by factors which determine or influence : (I) The number of gametes available at the time of fertiliza tion. In certain forms the female is monotocous (producing one young at a birth), only one ovum being extruded from the ovary and becoming available for fertilization at each period of "heat." In others she is polytocous, several or many extruded ova being available for synchronous fertilization by the abundant sperm. In both forms the ripening of the ova and the number extruded are conditioned by environmental factors. As a general rule, the aver age number of offspring in a litter in any species of mammals is inversely proportional to the average size of the animals of that species. In ungulates, twins are exceptional. The sow, however, is remarkable in having very large litters. In small mammals and rodents large litters are the rule, but the bats are exceptional, only one young ordinarily being produced at a time, doubtless to avoid extra weight when flying. Broadly speaking those species in which the gestation period exceeds six months produce but one young at a time and the number of teats is an approximate indi cation of the average size of the litter. In a general way, fecundity is in direct relation to the chances of death; those species which have many enemies produce many eggs but there are numerous exceptions to this rule. The initial fecundity gradually waxes to a climax and thereafter gradually wanes. In polytocous ani mals the first litter as a rule is relatively smaller as are those which are born towards the end of the reproductive life of the indi vidual. In the fowl the greatest fecundity is exhibited during the first year and diminishes rapidly and progressively thereafter. It is probable that artificial selection is the cause of this antedating of the peak of fecundity in the fowl. Increased nourishment is followed by an increase in the number of ova shed in the poly tocous forms, e.g., the effects of "flushing" (special feeding) in sheep. Insufficient food and especially a deficiency of the acces sory food substances, vitamins B and E (see VITAMIN) lead to an imperfect production or to the non-production of gametes by both male and female. The number of sperms is an important factor in efficient fertilization, for a certain concentration is re quired in the rabbit. Ordinarily, spermatozoa are present in amply sufficient numbers in the male ejaculate. Hammond (1925) found that in the case of the male rabbit repeated intercourse was followed by no reduction in fertility, though the periods be tween copulations became increased. Lloyd Jones and Hays (1918), however, found that excessive intercourse led to a reduc tion both in pregnancies and in litter size.

(2) The frequency of ovulation. There are monoestrous forms in which the female has but a single oestrous cycle within one sexual season, and polyoestrous in which she has two or more. Since fertilization can only occur during the oestrous cycle the reproductive rate is limited by the frequency of these cycles. In most wild mammals the male experiences a sexual season as well as the female and generative activity is entirely restricted to such times. In man and most domesticated animals, however, the male is capable of sexual intercourse at all seasons, though an in creased sexual activity may be evidenced at certain times of the year. Since in the human female there is no restricted sexual season fertilization can occur at all times throughout the year. Long continued lactation would seem commonly to exert an in hibitory influence on the oestrous cycle. In the pig, early weaning leads to a more frequent recurrence of oestrous cycles and an increased number of litters.

(3) The length of life of the individual and the length of its reproductive phase. Longevity is an hereditary character. Fecund ity is exhibited during the period of life between puberty and senescence. The total number of opportunities for fertilization is influenced by hereditary and environmental factors which affect the time of attainment of puberty, the time of the onset of the climacteric, and the length of life. The protection and sanitation of domestication and civilization can prolong life and delay the decline of reproductive vigour.

(4) The functional ability of the gametes. The gametes of the mule and of other interspecific hybrids are in the great major ity of cases inherently imperfect and incapable of fusing in proper fertilization. Commonly the sterility in these cases is due to irregularities in the mechanism of division of the germ cells and in some to the fact that the chromosome number and form of the two parental forms are widely different (see CYTOL oGv). Poisons, toxins and X-rays can render the gametes imper fect. The intra-abdominal undescended testis (e.g., in so-called "rigs" in horses) cannot proceed to the elaboration of functional sperm for this cannot occur at a temperature as high as that within the body. Certain wild animals when removed from their natural habitat and kept in captivity become partially or completely sterile even though kept in a healthy condition and in their native countries. For reasons as yet unknown the generative system fails to discharge its functions : there is either an absence of sexual desire or an inability to elaborate functional gametes. Some normal stimulus to sexual activity, possibly a psychological one, is lacking.

Fertility.

The union of the gametes and the proper develop ment of the resulting individual is embarrassed or prevented by factors which affect : (I) The act of sexual congress, e.g., homosexuality, inter sexuality and other structural, physiological and psychological anomalies.

(2) The actual union of the gametes after efficient congress. For reasons as yet unknown, certain individuals are infertile inter se but when mated with other individuals beget offspring. In human societies a differential fertility-rate distinguishes the social grades. The average size of the family among the rela tively socially unsuccessful is on the whole greater than that of the successful. Much of this difference is due to differences in the exercise of control, abstinence, and the use of contraceptives. It has not yet been demonstrated that any of it is due to a differ ential fecundity distinguishing the social grades.

(3) The ante-natal nurture of the offspring. Hereditary and environmental factors can bring about the death of the embryo and foetus in utero. In polytocous forms the number of fertilized ova is frequently in excess of the nutrition available for them with the result that atrophy and absorption of the surplus embryos and foetuses occur. In other cases the death of the offspring is due to the action of hereditary lethal factors (see HEREDITY). An instance of such lethal factors is that which eliminates the homozygous yellow mouse. The mating yellowXyellow invariably yields yellows and greys in the ratio 2:I. Such yellows are con stitutionally heterozygous : the homozygous yellow dies in utero as the result of the action of the hereditary factor for yellow present in duplicate. Haemophilic (bleeder) female children are comparatively rare : it is thought that the homozygous haemo philic usually dies early in pregnancy; only the female can carry the factor for this sex-linked character in duplicate (see HERED ITY) . An examination of the sex-ratio among new-born children, still-births and abortions, reveals the fact that during intra uterine life and parturition more males than females succumb. The male is constitutionally less able to withstand unfavourable conditions.

(4) The post-natal viability of the individual. The same heredi tary and environmental factors as those which operate ante natally are responsible for the fact that more males than females succumb during the ante-pubertal period. During the age group 10-15 years more females than males die, a reflection of the dangers that beset the onset of puberty in the female. (See REPRO