Two types of reproductive cycles are recognized, estrual and menstrual, with the term ovarian cycle representing the interval between two successive ovulations. These terminologies have developed in order to use certain external characteristics for accurately identifying a particular stage of the reproductive cycle and, most importantly, relating it to the time of ovulation.

Domestic animals have limited periods of estrus (or sexual receptivity); the term estrous cycle is used, and the onset of proestrus defines the start of the cycle (Figure 37-1). Primates are sexually receptive during most of the reproductive cycle; the term menstrual cycle is used, with the onset of menstruation (vaginal discharge of blood-tinged fluids and tissues) designated as the start of the cycle (Figure 37-2). The first day of the cycle for both estrual and menstrual cycles in many species begins shortly after the end of the luteal phase. In the dog a normal anestrous period separates diestrus and proestrus (the stages of the cycle are described later).

In domestic animals, proestrus usually begins within 48 hours after the end of the luteal phase; the dog and pig are exceptions, with proestrus in the dog delayed by the anestrus phase (2 to 3 months) and proestrus in the pig not occurring for 5 to 6 days. In primates, menstruation usually begins within 24 hours of the end of the luteal phase. Even though both cycles begin at the same time in relation to the luteal phase (shortly after), the time of ovulation differs. This is because, as previously discussed, luteal and follicular phases are separated in primates, with ovulation occurring at a minimum of 12 to 13 days after the onset of menses. In most domestic animals the follicular phase overlaps the luteal phase, and therefore ovulation occurs relatively earlier in the estrous cycle. Ovulation is easier to predict in domestic animals (versus primates) because estrus is usually tightly coupled to the preovulatory release of gonadotropins and ovulation. The onset of follicular development in primates can be delayed for a variety of reasons (e.g., stress), making the time of ovulation less predictable for primates than for domestic animals.

The estrous cycle has been classically divided into stages that represent either behavioral or gonadal events (see Figure 37-1). The terminology, originally developed for the guinea pig, rat, and mouse, is as follows:

The classic terminology is not particularly useful for domestic animals. The common terminologies used for domestic animals involve either behavioral or gonadal activity. The cycle can be described in a behavioral manner by indicating whether animals are in estrus (sexually receptive) or not, including the stages of proestrus, metestrus, and diestrus. The cycle can also be described with reference to the activity of the gonads if differentiation of follicles and the CL is possible. Animals can be in the follicular phase (proestrus and estrus) or the luteal phase (metestrus and diestrus).

Because the equine CL is relatively difficult to identify by palpation per rectum, horses are usually classified by sexual behavior: estrus or nonestrus. The behavioral classification is also used in other domestic species, including the goat, pig, and sheep, because of the difficulty of determining their ovarian status. The ovarian status of cattle can be determined accurately by palpation per rectum, and cows are usually classified by ovarian status: follicular or luteal. The ovarian status of the dog and cat can be determined by performing vaginal cytology (estrogen effect) and measuring serum progesterone levels. If a CL can be identified, the judgment can be made that ovarian activity is normal in the particular animal, because the CL represents the culmination of follicle growth and ovulation.

For females to begin reproductive cycles, they must undergo a process called puberty. The term puberty is used to define the onset of reproductive life. For the female, although the onset of sexual activity (in domestic animals) or first menstrual bleeding (in primates) is often used as the onset of puberty, the most precise definition is the time of first ovulation. For all species, there is a critical requirement for the attainment of a certain size in order for puberty to be initiated, in cattle about 275 kg, for example, and in sheep about 40 kg (Figure 37-3). If this critical requirement is not met because of inadequate nutrition, puberty is delayed. The age at puberty for domestic animals is as follows: cats, 6 to 12 months; cows, 8 to 12 months; dogs, 6 to 12 months; goats, 7 to 8 months; horses, 12 to 18 months; and sheep, 7 to 8 months. Classically, bitches have attained 75% of their adult size before puberty occurs.

The physiological mechanisms involving control of puberty in domestic animals are best known in sheep. One of the fundamental concepts of the onset of puberty involves an increase in the synthesis and release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which drives gonadotropin secretion (in pulsatile form) and follicle growth. Before puberty, GnRH and gonadotropin secretion are kept in check because the hypothalamus is highly sensitive to negative-feedback inhibition by estrogens. One of the keys to puberty in lambs is a maturation of the hypothalamus, which results in reduced sensitivity to negative feedback by estrogen. Puberty onset is not held back because of lack of responsiveness of the prepubertal gonads, because ovarian follicle development can be elicited by gonadotropin administration.

Changes in photoperiod are important for allowing lambs to enter puberty. It has been shown that lambs must have some exposure to a long photoperiod during their prepubertal development; the period can be as short as 1 to 2 weeks (under experimental conditions). Termination of the long photoperiod, which occurs with the summer solstice, allows the sensitivity of the hypothalamus to decrease in response to negative estrogen feedback. The minimal interval from the end of the long photoperiod exposure to the onset of puberty is 10 weeks under experimental conditions. This aspect agrees well with the timing of spontaneous puberty, in which the first ovulation often occurs in the latter part of September (in the Northern hemisphere), or about 13 weeks from the occurrence of the summer solstice. Note that this concept of the initiation of puberty does not involve decreasing photoperiod; the emphasis is on a turning point that involves the cessation of exposure to a long photoperiod.

With appropriate growth and photoperiod exposure, the secretion of gonadotropins in lambs causes significant follicle growth. This growth is maintained because of decreased sensitivity of the hypothalamus to estrogens produced by growing follicles. The first endocrine event of puberty in the ewe lamb is the appearance of a preovulatory-type surge of gonadotropins, presumably induced by estrogens produced by developing follicles (Figure 37-4). The gonadotropin surge results in the production of a luteal structure, through luteinization of a follicle(s), which has a short life span, 3 to 4 days. After the demise of the initial luteal structure, another gonadotropin surge occurs, leading to ovulation and the formation of a CL, usually of a normal life span. At this time, cyclical ovarian activity is finally initiated in the ewe lamb.

Photoperiod can have a suppressive effect on the timing of puberty in animals whose ovarian cyclicity is controlled by light. Kittens born in the spring may be large enough to enter puberty by late autumn, but puberty could be delayed a few months if the kittens are under the natural photoperiod.

Photoperiod influences the timing of puberty onset in macaque monkeys, depending on the physiological maturity of the individual. The first ovulation, or puberty onset, can occur during the late autumn or early winter, at about 30 months of age (20% of animals) or 12 months later at about 42 months of age (80% of animals). The animals undergoing puberty at about 30 months of age have an earlier maturation of the neuroendocrine system, in which significant gonadotropin secretion begins during the previous spring. Thus, there is a window of opportunity for the onset of puberty in macaques that must be entered within the favorable photoperiod of decreasing light if puberty is to occur at an earlier time; nutrition and growth are likely determinants of the earlier time for onset of puberty.

The onset of puberty usually results in the establishment of cyclical ovarian activity within a relatively short period (i.e., within a few weeks to a month in lambs). Ewe lambs can initiate normal ovarian activity at the onset of puberty, which can lead to pregnancy (if mated) at the first estrus, or they can have false starts with the establishment of limited luteal phases and cessation of ovarian activity for several weeks to a month before they resume ovarian activity. In general, the onset of ovarian cyclicity starts later and ends earlier for ewe lambs compared with adults of the same breed. The earlier cessation of ovarian activity results from an earlier response to negative estrogen feedback.

The initiation of cyclical ovarian activity in pubertal primates takes longer; the first significant follicle growth usually ends in ovulatory failure. In macaque monkeys, 3 to 6 months is usually required after the onset of menarche, or first vaginal bleeding, before the occurrence of the first ovulation of puberty. In humans, follicle growth without ovulation can occur for up to a year before the establishment of normal ovarian cyclicity, including ovulation and CL formation.

For male lambs, the onset of puberty is first keyed when lambs begin to lose their sensitivity to estrogen feedback inhibition, usually by about 15 weeks of age. For many males, this occurs during the period of increasing, or long, photoperiod, which is in contrast to the ewe lamb. Spermatogenesis (process of sperm production resulting in the presence of mature sperm) usually begins at this time, but because of the length of the process, lambs are usually not capable of successful breeding until about 30 weeks of age or more, or in concert with the onset of puberty in ewe lambs. Thus, puberty is a relatively gradual phenomenon in male sheep compared with the abrupt process in females.

Because adult ewes experience the same double gonadotropin surge at the onset of the breeding season, it has been suggested that adult animals recapitulate puberty each year as they enter the breeding season. Recent studies in adult ewes, however, indicate that refractoriness to the long photoperiod experienced by animals during the spring and summer is the most critical aspect for the establishment of ovarian activity. Thus the concept that the renewal of ovarian activity in sheep recapitulates puberty appears not to be accurate, at least in some aspects.