CHAPTER 84 Reproductive Physiology of the Ram
An understanding of basic ram reproductive physiology forms the foundation to the designing of practical approaches to management of a breeding program and improved reproductive efficiency. In addition, this knowledge has the potential to improve our ability to advance flock genetics through indirect selection for reproductive merit.
ORGANIZATION OF THE REPRODUCTIVE SYSTEM
The cryptorchid condition can be used to illustrate some important concepts about the developmental organization of primary and secondary sex organs and reproductive development in the ram. Histologically, seminiferous tubules of cryptorchid testes are juvenile in appearance and devoid of germ cells1; however, despite the fact that these rams are sterile, few differences in other male traits occur. Growth rate, male-type body and carcass characteristics, and the accessory sex glands are little affected. A similar situation is seen in rams with short scrotums who, although usually sterile, nonetheless reach normal physical maturity.
The cryptorchid and short scrotum conditions can be used to gain information about male reproductive development. First, reproductive competence and fertility in rams depend on migration of the testes to the scrotum at birth. The scrotum is where the testes reach full maturation and where spermatozoa acquire fertilizing potential. In hot humid areas such as the southwestern desert in the United States, the scrotum affords some protection against the deleterious effects of heat, although even with this efficient cooling mechanism, temperature-induced sterility is often a problem. Although scrotal descent is essential for fertile sperm production, it is not needed for all endocrine functions of the testes. Testes formed during in utero development are fully capable of secreting androgens.2 It is this organizational effect of androgen, occurring 30 to 40 days after conception,3 that directs differentiation of the reproductive tract toward the male system.
Gonadotropin-releasing hormone plays a pivotal role in male reproduction, directing the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland, and ultimately affecting testicular function. Although marked gender differences are observed in the secretion of LH, and presumably of GnRH,4 no differences in neuroanatomy (number or appearance of GnRH neurons) have been found between male and female sheep.5 This suggests that androgens in utero alter the secretory capacity of the GnRH neuronal network and thereby establish distinct male and female patterns of release.
Fetal testicular androgen secreted during early development organizes hypothalamic and extrahypothalamic neurons involved with the expression of sexual behavior in the adult ram. Masculinization, or the ability to express male sexual behavior, is established in utero between days 20 to 80 of gestation.3 Androgens given to pregnant ewes by mid-gestation masculinize female offspring. As adults, such ewes not only fail to show typical estrous behavior and ovulate, but display courtship and mounting behavior typical of rams.
ACTIVATION OF THE REPRODUCTIVE SYSTEM
The appearance of spermatozoa and the onset of mating behavior that herald the initiation of puberty in rams usually occur at 5 to 6 months of age, depending somewhat on breed and season of birth.6 Spring-born rams from temperate climates show a gradual increase in testicular size that parallels changes in growth rate with a more rapid phase of testicular growth occurring in the fall. Changes in GnRH-induced LH secretion drive the final maturation of the testes, including stimulation of testosterone secretion. Onset of mating activity is synchronized by changes in steroids. The rise in LH secretion that occurs during the peripubertal period is likely caused by (1) a steroid-independent drive of hypothalamic GnRH secretion and (2) a change in feedback inhibition of testosterone on the GnRH/LH axis.7
The importance of GnRH and LH to drive events that lead to puberty is further shown by differences in age at which inadequately nourished rams reach sexual maturity,8 because low planes of nutrition are associated with inadequate increments in LH release.
Sexual Behavior
The activation of mating behavior in rams occurs at puberty, thereafter occurring seasonally in the mature adult. Ram lambs castrated at birth rarely show onset of courtship behaviors because of the absence of testosterone. Replacement of testosterone in a castrated male will elicit onset of sexual behavior and, at the appropriate threshold dose, copulation. Proficiency in courtship and mating has learned components that can be observed when lambs are weaned. Ram lambs frequently mount other males when they are put in all-male groups (an important consideration for disease transmission). Rams that are reared with females can achieve an earlier onset of mating activity. The frequency of various courtship behaviors varies by season of the year, occurring less frequently and less intensely in periods that are typical of anestrus in ewes. The seasonal changes that occur in mating behavior should not be confused with situations in which rams fail to perform sexually. Sexual performance is highly variable among rams, irrespective of season. Both sexual inactivity and homosexual behavior have been described, and approach an incidence of 10% to 20% in populations of rams.9 In a series of studies designed to elucidate physiologic correlates to these abnormal behaviors, differences in gonadotropin secretion and receptors in the brain for estradiol were found.10–12
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