Photoperiod, Lactation, Nutrition, and Animal Interaction Are Important Factors That Affect Reproduction
Photoperiod
Photoperiod controls the occurrence of reproductive cycles in a number of domestic species, including cats, goats, horses, and sheep. The result is that these animals have an annual period in which they have continuous (cyclical) ovarian activity, as well as another period of no ovarian activity, termed anestrus.
The response to photoperiod is different among these species; cats and horses are positively affected by increasing light, and goats and sheep are positively affected by decreasing photoperiod (Figure 37-7).A positive response to a change in the photoperiod usually occurs relatively soon after the occurrence of the summer or winter solstice (i.e., within 1-2 months). A negative response to a change in photoperiod usually requires a longer duration for an effect (i.e., 2-4 months to suppress ovarian activity after the
occurrence of the particular solstice). The net result is that in the absence of pregnancy» cyclical ovarian activity usually occupies more than half the year for these four seasonally breeding species.
In cats» cyclical ovarian activity can range from Iate January through October (Northern Hemisphere). In horses, the usual range of ovarian activity is from March through October. Conversely, sheep and goats have ovarian activity from late July through February or March (depending on the breed). As indicated previously» progesterone priming immediately before follicle development is required for sexual receptivity in sheep. The full length of the reproductive season of sheep is not manifested externally because (1) the first ovulation is not preceded by the presence of a CL, and (2) the last follicle phase may be delayed because of a negative photoperiod, with the priming effects of progesterone lost before follicle growth.
The main translator of photoperiod is the pineal gland, which produces melatonin in response to darkness.
The central nervous system pathway involved with the translation of light includes the retina, Suprachiasmatic nucleus, superior cervical ganglion, and pineal gland. Whereas melatonin has been previously described as “antigonadal,” this is obviously not true, because both short and long phases of darkness, with resultant short and long duration of melatonin secretion, can have a positive effect on reproductive cycles. In sheep, however, exposure to increasing darkness may be important only for maintaining ovarian activity. The onset of ovarian activity is thought to occur in response to the development of refractoriness to the long photoperiod. The development of photorefractoriness to a long photoperiod as a requisite to ovarian cyclicity is consonant with the fact that sheep can begin cyclical ovarian activity even before the onset of the summer solstice.Of the seasonal breeders, the cat is the most sensitive to photoperiod change; estrus, in conjunction with the presence of mature antral follicles, can occur as early as January 15. Initial follicle activity likely begins at least IO days before the first expression of estrus, or 15 days after the winter solstice. Thus a total photoperiod change as brief as 15 minutes can be perceived and translated by the cat into ovarian activity.
The suppressive effects of photoperiod can be overcome by exposure to artificial lighting regimens. This is relatively easy in the case of cats and horses, in which environments with photoperiods are compatible with ovarian activity (i.e., at least 12 hours of light per day). If the photoperiod is established before the end of ovarian activity in the autumn, cyclical ovarian activity continues through the time associated with anestrus. If mares are allowed to become anestrous in the autumn, it can take a minimum of 2 months of exposure of mares to increased light to reestablish ovarian activity. The usual time for placing mares under lights is December 1 (Northern Hemisphere), with cyclical ovarian activity expected by early February.
It is usually not possible to place sheep and goats in lighttight barns to increase their exposure to dark in order to overcome the suppressive effects of increasing light. One recent development in this regard has been the oral or systemic (implant) administration of melatonin to sheep during the spring. This exposure to melatonin has resulted in an early onset of ovarian activity and increased the number of multiple ovulations above that normally observed at the beginning of the breeding season.
Lactation
Lactation can have suppressive effects on ovarian activity. In pigs, suppression of ovarian activity is complete; sows do not come into estrus until after piglets are weaned. Cats can have ovarian activity suppressed throughout lactation, although they occasionally come into estrus during the latter part of lactation. Ovarian activity tends to be suppressed in lactating beef cows, with the first estrus and ovulation not occurring before day 45 postpartum. The suckling process appears important to ovarian suppression; dairy cows are not suppressed by lactation unless it involves a large nutritional deficit.
Goats and sheep usually begin lactation during a photoperiod that is increasingly suppressive for ovarian activity, and therefore the reestablishment of ovarian activity in these species is confounded by the photoperiod. Ewes delivering in the autumn ovulated as early as postpartum day 12 (average, postpartum day 23), indicating that lactation has little suppressive effect on ovarian activity in sheep. Mares usually ovulate by postpartum days 10 to 15, with lactation having no suppressive effect on ovarian activity regarding this ovulatory interval.
One of the concepts of lactational suppression of ovarian activity involves the importance of suckling with its related stimulation of prolactin synthesis. Inhibiting factors for prolactin synthesis, including dopamine and the GnRH- associated peptide, need to be suppressed in order for prolactin synthesis to proceed.
The sensory input from suckling suppresses the production of these prolactin-inhibiting factors. Because both dopamine and GnRFLassociated peptide are essential links in the synthesis of gonadotropins, their reduced output results in reduced ovarian activity through decreased gonadotropin synthesis and release.Pheromones
Pheromones are chemical compounds that allow communication among animals through the olfactory system. When sexual behavior is affected, the compounds are called sex pheromones. Pheromones arise from several tissue sources; the most prominent sources for animals are sebaceous glands, the reproductive tract, and the urinary tract.
Some of the first experiments that demonstrated the potency of male odors to influence reproductive behavior were done in mice. One syndrome, called the Whitten effect, involves the synchronization of estrus in female mice through the sudden introduction of a male (or male odor through bedding), with a large number of animals cycling within 3 days of introduction of the male. The effect of the pheromones in this case is to stimulate the synthesis and release of gonadotropins. Another syndrome, called the Bruce effect, involves the blockage of pregnancy development by the introduction of a different (strange) male in proximity to a recently bred female. The effect of the odor of the strange male is to block the release of prolactin, the hormone responsible for the maintenance of the CL in association with pregnancy in rodents. Regression of the CL in this case produces fetal loss. Thus, pheromones can strongly affect reproductive cycles.
Pheromones are important for the attraction of the male to the female at the time of sexual receptivity. Sexual attractiveness of the female evolves from the pheromones that she elicits on a limited, cyclical basis in association with estrus. For example, methyl-p-hydroxybenzoate, isolated from the vaginal secretions of dogs in proestrus and estrus, has produced intense anogenital interest by males when applied to anestrous females.
Females are also influenced by male odors; sows in estrus assume a breeding (rigidity) stance when exposed to the urine of males. Androgens can serve as pheromones, or they can influence the production of substances within the kidney that influence female sexual behavior. The attractiveness of the female to the male involves a change in perception of the male by the female resulting from a changing physiological state within the female, not because of changes that are occurring in the male.The classic way for males to delineate their territory has been for them to mark the area with urine. In general, pheromones that affect sexual behavior tend to have a musk type of odor. The classic pheromone used by humans is perfume, which is derived from civetone, a cyclical 17-carbon compound obtained from the civet cat.
The Whitten effect has been used to manipulate the estrous cycles of animals. In sheep, males are introduced into a flock of ewes before the breeding season to advance, or ensure, ovarian cyclicity at the beginning of the breeding season. Whereas it was previously thought the effect of the introduction of a male was short-lived (i.e., a gonadotropin response could only be obtained within the first few days from ewes that had antral follicles), it is now clear that the interaction of rams with ewes over extended periods of the anestrus results in earlier ovarian activity.
As discussed, pheromones can account for some of the effect of the male. More recent studies, however, have shown that sight of the male by the female, as well as physical contact, are important factors that influence gonadotropin secretion and thus ovarian activity.
The WTiitten effect has also been used to influence the onset of puberty in pigs. I he introduction of males into groups of gilts beginning several weeks before the expected time of puberty (180-200 days) has been used to ensure, or advance, the onset of puberty. The dormitory effect, the well-recognized synchronization of menstrual cycles in roommate women, occurs in bitches kenneled together as well.