THE FEMALE REPRODUCTIVE ORGANS
The female reproductive organs include paired female gonads, or ovaries, which produce both female gametes (ova) and hormones; paired uterine tubes, which capture the ova on their release from the ovaries and convey them to the uterus; the uterus, in which the fertilized ova are retained and nourished until prenatal development is complete; the vagina, which serves both as copulatory organ and as birth canal; and the vestibule, which continues the vagina to open externally at the vulva but which also doubles as a urinary passage (see Figure 5-2).
Age and functional changes are particularly obtrusive where these organs are concerned. Age changes include the rapid growth and maturation associated with puberty and also the regression that occurs as the capacity for reproduction wanes with increasing age. Functional changes include those that are relatively transient and recur with each reproductive cycle as well as other, more lasting ones that are associated with pregnancy and giving birth. Unnecessary complications will be avoided if this initial account concentrates on the description of the organs of the mature nonpregnant animal; growth and functional changes are left for later comment. Even so, a few general terms are introduced at this point to help the reader.
Female mammals generally accept the male only close to the time of ovulation, a period characterized by various structural changes and by general excitability as well as specific behavioral features; the period is known as “heat” or “season” in lay language and as “estrus” more technically. Estrus recurs with varying frequency according to a program that is characteristic for each species although subject to environmental modification. In certain wild mammals the breeding season is confined to a certain part of the year, and sexual receptivity, with the concomitant structural and behavioral changes, occurs only once (monestrous species) or perhaps several times (seasonally polyestrous species) within this period. In other (truly polyestrous) species the cycle is repeated throughout the year; the adoption of the polyestrous mode often distinguishes domestic and laboratory species from their wild progenitors.
The condition in which female receptivity is continuous and not linked to ovulation occurs only in women and some higher primate species (e.g., bonobo); in most of the latter it appears to be more common among, if not confined to, menagerie specimens.The estrous cycle is divided into several phases. Estrus, the climax, is prefaced by proestrus, a period of follicular development; it is followed by a period of luteal activity divided between metestrus and diestrus. In monestrous species a lengthy period of sexual inactivity (anestrus) occurs before the cycle is renewed with a preparatory period of proestrus. In polyestrous species, proestrus follows directly after diestrus.
Proestrus and estrus together represent the follicular phase, when the reproductive condition is predominantly determined by the rising levels of estrogen produced in the batch of ovarian follicles then rapidly developing to maturity and rupture. Metestrus and dies- trus represent the luteal phase, when the dominant hormonal influence is exerted by progesterone, the hormone produced by the corpora lutea, the transient endocrine glands that replace the ruptured follicles.
Other helpful terms tell whether a female animal has or has not borne young. Animals that have borne young are said to be parous, those that have not are nulliparous, and uniparous and multiparous extend this terminology in obvious fashion. Other terms refer to the number of young habitually carried by the gravid female. A mare with its (generally) single foal is monotocous; a sow with its litter of piglets is polytocous.