Page 478 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of Male Reproduction / 463

to suppress LH release directly. When they regress and become nonfunctional if
castration occurs after sexual maturity.
stimulated by FSH, sustentacular cells
VetBooks.ir produce a protein hormone, inhibin. Even though testosterone production by
the testis is necessary for a normal libido, it
Inhibin has a negative feedback effect on
the adenohypophysis to suppress further is not testosterone that directly affects
releases of FSH. neurons within the brain to produce a nor-
In farm animals, the feedback regula- mal libido. Within neurons, testosterone is
tion of GnRH, FSH, and LH secretion is converted to estradiol, an estrogen, and it
such that spermatogenesis is maintained at is this estradiol that actually stimulates
rates adequate for breeding throughout the libido in the male. Anabolic steroids
year. However, plasma levels of FSH, LH, (discussed next) used to promote growth
and testosterone do vary with season in cannot be converted to estradiol and thus
some species, and these have been associ- these do not increase libido.
ated with differences in sexual activity. For Anabolic steroids are synthetic com-
example, FSH, LH, and testosterone levels pounds used to increase net protein
are highest in rams while the days are synthesis and skeletal muscle mass. In
getting shorter, and this is associated with this manner, anabolic steroids are simi-
increased sexual activity and increased lar to endogenous androgens, such as
behaviors associated with male domi- testosterone, in that they can promote
nance, particularly if there are multiple the development of secondary sexual
males in the group. characteristics and exert a negative
feedback effect on the hypothalamic–
adenohypophyseal axis. As a result of
Testosterone and Its Effects this negative feedback, endogenous
testosterone production and spermato-
Testosterone is a steroid hormone that genesis are suppressed. It is not clear
enters its target cells to exert its effects. whether these can return to normal
Within target cells, testosterone is con- levels if animals receive anabolic steroids
verted to dihydrotesterone, which binds for prolonged periods.
to intracellular receptors. In addition to
supporting the maturation of spermatozoa
within the testis, testosterone promotes Erection and Ejaculation
the development and function of male
accessory sex organs, causes development Penile erection is a neural reflex initiated
of secondary sex characteristics, and by appropriate tactile stimulation of the
promotes male sexual behavior. penis, visual, or environmental stimuli
Lack of libido (sex drive) and inability (such as a female in estrus), or as a result of
to produce offspring are two of the most learned behavior. For example, an erection
obvious effects of castration and the result- may begin when a breeding stallion is being
ant lack of testosterone. However, animals led to a breeding area, but before a mare is
castrated after attaining sexual maturity present; this is believed to be a learned
may continue to display male sexual behav- response. Similarly, it is believe that learned
iors and attempt to mate for some time if responses may be responsible for lack of
they had sexual experience before castra- libido in certain males that have undergone
tion. If an animal is castrated before trauma (e.g., a stallion kicked by a mare) or
puberty, many of the masculine secondary that associate mating with some other
sex characteristics fail to develop, and the painful or unpleasant event.
castrated animal tends to resemble Regardless of penis type (musculocav-
the female of the species. In addition, the ernous or fibroelastic), penile erection
accessory sex glands fail to develop nor- requires parasympathetic stimulation
mally if castration occurs early in life and and vasodilation within the penis. Sexual