Reproductive system of male mammals

The reproductive systems of all male mammals have the same basic design. The reproductive organs produce sperm and deliver it to the outside of the body. The sperm can be regarded as packages of chromosomes that the animal passes on to his offspring. Hormones and nerves control and coordinate the functions of the reproductive organs.

The Brain and Reproduction

Although the brain is not usually considered to be a component of the reproductive system, part of the brain is, in fact, essential to the function of the reproductive organs because of the hormones produced there. This part of the brain is the hypothalamus, a relatively small area that acts without conscious control. The hypothalamus is located in the lower middle of the brain. It contains centers that control eating, drinking, body temperature, and other essential functions.

Hypothalamic control over reproduction in the male is primarily by way of the hormone called gonadotropin-releasing hormone (GnRH). GnRH is released from the hypothalamus to enter blood vessels that carry it to the pituitary gland, a small gland suspended just below the hypothalamus. When GnRH arrives at the pituitary, it stimulates the pituitary to produce and release two more hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH in the male are identical to hormones of the same names in the female. The names of these hormones describe their functions in the female. Like other hormones, FSH and LH are released into the blood and circulate throughout the body. FSH and LH are called gonadotropin hormones: gonadotropin means “gonad stimulating.” These are the hormones that stimulate the gonads (testes in the male, ovaries in the female) to produce sperm or eggs and to secrete gonadal hormones. In the male, the gonadal hormones are primarily testosterone and related hormones. There is a chain of hormonal commands, with GnRH from the hypothalamus at the top of the chain. GnRH stimulates the pituitary to secrete FSH and LH, which in turn stimulate the testes to produce sperm and testosterone.

In addition to the chain leading from the brain to the pituitary to the testes, information is sent back to the brain from the testes, a checks-and-balances system using principles of negative feedback to ensure that the hormones are produced in the appropriate quantities. If, for example, the hormone system gets slightly out of balance, leading to too much testosterone being produced, this excess of testosterone will be sensed by the hypothalamus. It will cause a temporary shutdown of GnRH production, leading to the system’s correcting itself, because then a little less testosterone will be produced. If testosterone levels fall too low, the opposite will happen: GnRH, and then FSH and LH, and then finally testosterone, will all increase, again resulting in a correction of the original aberration. The hormonal system is a delicately balanced network that ensures the proper functioning of the testes.

The Testes

The testes are the sites of sperm production. Within the testes are hundreds of tiny tubes, the seminiferous tubules, that are responsible for sperm production. The sperm develop gradually from round cells called spermatogonia, which are located in the walls of the seminiferous tubules. As a sperm matures, it develops a long, whiplike tail attached to an oval head. The head of the sperm contains chromosomes, the genetic information of the male that will be passed on to his offspring. The sperm of some mammals can be distinguished under the microscope by characteristic differences in their appearance.

Between the seminiferous tubules are clusters of hormone-producing cells, the interstitial or Leydig cells. The Leydig cells produce testosterone and related hormones. Testosterone is essential for proper sperm development. In addition, testosterone is responsible for the development of male body features, including, in most species, a large muscle mass, and for the growth of the reproductive organs during puberty. In some animals, testosterone is also linked to aggressive and reproductive behaviors.

The testes of most mammals are located in the scrotum, a pouch of skin and muscle that is suspended outside the abdomen. In some animals, the testes may be withdrawn into the abdomen when the animal is startled, when it is not in breeding condition, or when it is not mating season.

The function of the scrotum is to maintain the temperature of the testes at a few degrees lower than average body temperature. The capability to maintain this temperature of the scrotum is rooted in the fact that the muscles within the scrotum are responsive to temperature. Under warm conditions, the scrotum relaxes, allowing the testes to move away from the body and lose heat. In cool temperatures, the opposite occurs. The scrotum wrinkles, pulling the testes closer to the body and allowing them to stay warmer. The reduced temperature maintained by the scrotum is mandatory to produce normal, fertile sperm. Fever or other situations that raise the temperature of the scrotum can interfere with sperm production, even resulting in temporary infertility. In a few mammals (such as elephants, bats, whales, and dolphins), the testes are not located within a scrotum, but instead occupy a position in the abdomen, called intra-abdominal. It is believed that these particular animals, especially those of the Cetacea family like whales and dolphins, have an ideal internal body temperature for sperm production. In general, sperm are more cold-averse than heat-averse, so warmth is not as problematic as cold in sperm production.

The Epididymus and Vas Deferens

Sperm are removed from the testes by a system of tubes that lead out of the body. Located next to each testis within the scrotum is the epididymis, a highly coiled tube that is directly connected to the seminiferous tubules of the testes. The epididymis serves two functions: sperm maturation and sperm storage. The epididymis is drained by a long, thin tube called the vas deferens, which carries sperm out of the scrotum through the inguinal canal into the abdomen. The inner end of the vas deferens is a widened area that may serve as a site of storage for mature sperm.

The vas deferens passes in a loop next to and under the bladder, the sac that stores urine until it can be removed from the body. Immediately beneath the bladder, the vas deferens is connected by a short tube, the ejaculatory duct, to the urethra. The urethra is the long, fairly straight tube that carries either urine from the bladder or sperm from the reproductive system. A valve located in the urethra below the bladder opens and closes to prevent sperm and urine from mixing, so that only one type of fluid is in the urethra at a time.

From their site of production in the testes, sperm pass through the epididymis, the vas deferens, the ejaculatory duct, then finally, the urethra to the outside of the body. As sperm are expelled from the body along this route, they are mixed with seminal fluid to produce semen. Seminal fluid is secreted into the tubes by three sets of glands: the seminal vesicles, the prostate, and the bulbourethral (Cowper’s) glands. The sperm never enter these glands; fluid is squeezed out of them into the tubes where the sperm are located.

The Penis

The penis is designed to deliver sperm to the female system. The penis consists of a long shaft with an enlarged head, the glans. The skin of the penis, especially the glans, is extremely sensitive to touch. In some species, the penis is withdrawn into a sheath of skin except during sexual arousal.

Internally, the penis contains the outer segment of the urethra, as well as erectile tissue. This erectile tissue is designed like a sponge. The many blood vessels in the erectile tissue are capable of greatly expanding and increasing the quantity of blood that they contain. When this happens, the erectile tissues swell, and the entire penis increases in length and width and becomes stiff. This process, called erection, is an involuntary reflex: It cannot be consciously prevented or caused. Erection can result from direct stimulation of the penis, as during sexual contact, or from erotic sights or sounds. In some animals, a bone within the penis, the baculum, assists in maintenance of the erection.

Continued sexual stimulation will eventually result in ejaculation, with semen being forced out of the body by contractions of muscles in the fluid-producing glands and along the tube system. Ejaculation is coordinated by nerves that arise in the spinal cord. The normal volume of fluid ejaculated varies from species to species. In humans, it is usually two to six milliliters; it may be up to one hundred milliliters in pigs. The ejaculate of most mammals contains many millions of sperm per milliliter of fluid.

Studying Male Reproduction

The hormonal system that controls the male reproductive system is the subject of much research. The most straightforward type of hormonal research is simply descriptive: The scientist seeks to describe the levels of the reproductive hormones when the animal of interest is in different physiological and psychological states. The hormones can be measured in blood samples taken from the animals. Obtaining a blood sample from an experimental animal may pose difficulties: Some large animals may be difficult to restrain, and some small animals may not have veins large enough for an easy puncture. Another consideration is how often blood samples should be taken. Endocrinologists have become increasingly aware of the importance of the pattern of hormone release over time. In particular, it now appears that fluctuations in hormone levels within a time frame of minutes or hours may be critical in regulating the responses of hormone target sites. To obtain blood samples with such a high frequency, researchers usually implant a cannula into a vein of the animal; the cannula can be left in place for repeated blood sampling with very little stress to the animal.

Scientists interested in hormonal feedback may examine the roles of specific hormones by removing one of the endocrine glands from the system, and then examining the effects on the remaining hormones. For example, the testes (as the site of testosterone production) can be removed from an experimental animal. Blood samples after the surgery can then be assayed to determine the circulating levels of LH, FSH, and GnRH. The endocrine glands may be left in place, but the researcher may administer hormones either by injection or by implanting timed-release capsules containing the hormone under the skin. Then, blood samples taken from the animal will reveal how levels of hormones produced by the animal’s own endocrine glands have changed because of the exposure to the added hormone.

A technique that is widely used to study males of seasonally breeding species is to subject the animals to carefully controlled environmental conditions. Length of exposure to light, temperature, rainfall, nutrients in the diet, and other factors can be controlled in the laboratory to determine which acts as the cue for seasonal reproduction. The status of the reproductive system can be determined by various methods. The testes can be measured: Inactive testes are usually smaller and lighter in weight. Hormone levels in the blood can be measured: Testosterone and other hormones may decrease when the animal is reproductively quiescent, or the male can be exposed to a female to determine whether he will show mating behavior.

For some types of research, the most revealing experiments may not use the entire animal (referred to as in vivo research) but will instead focus on specific organs. Living samples of organs can be maintained in the laboratory for such in vitro experimentation. For the in vitro approach, a small piece of living tissue can be removed from an animal, and the cells suspended in a liquid that contains the nutrients necessary for their life. Under these isolated conditions, scientists can investigate several areas, such as which hormones tissues produce and the hormones that make the tissue itself respond. Organs respond optimally to a particular pattern of hormonal stimulation, and this is another important area of research. By combining the results of in vivo and in vitro experiments, scientists can piece together a complete picture of how the reproductive system functions.

Controlling Reproduction

Knowledge of how the male reproductive system functions has allowed scientists to develop technologies for controlling reproduction to enhance or curtail fertility in domestic animals. Knowledge of male reproductive physiology has been applied to the management of domestic breeding populations. Hormone measurements and sperm counts can be used to determine the optimum age at which to begin breeding young stock. Techniques for collecting and storing semen can be combined with artificial insemination of females to increase the number of offspring produced by valuable males, thus resulting in improvement of the population. These methods are particularly valuable to breeders of large animals because maintaining large numbers of males of these species (such as stallions and bulls) can be costly and difficult because of the aggressive behaviors that these males may exhibit.

The study of seasonal breeding has also been of value in agriculture. Scientists now know much about the environmental conditions that are responsible for promoting reproductive activity in many domestic species. Farmers can apply this knowledge to their breeding stock to increase production throughout the year. Another area in which reproductive studies are of vital importance is the enhancement of the breeding of captive animals that are endangered in the wild. Zoos, once considered merely spectacles for entertainment, are now seen by many as the last hope of saving many species on the verge of extinction. Zoos across multiple institutions collaborate in Species Survival Plans (SSPs), coordinating breeding efforts to ensure genetic diversity within captive populations of endangered species. They also utilize artificial insemination to avoid inbreeding and to weed out generic abnormalities. Knowledge of the conditions necessary for successful breeding of exotic animals will help to increase their numbers and, perhaps, to reintegrate them into the wild.

Certain animal species that can overpopulate, especially in residential areas, are often subject to neutering, or castration. Domesticated male dogs and cats are typically neutered in order to avoid unwanted or an abudance of offspring. This process can also change the temperament of the pet, making them less aggressive. Additionally, neutering pets can help prevent certain diseases and cancers. A potential negative side effect of neutering is weight gain due to increased metabolism and maturation. However, such changes can be monitored and food intake can be adjusted to keep the pet a healthy weight.

Principal Terms

Chromosome: A molecule of deoxyribonucleic acid (DNA) that contains a string of genes, which consist of coded information essential for all cell functions, including the creation of new life

Ejaculation: The process of expelling semen from the male body

Endocrine Glands: Glands that produce hormones and secrete them into the blood

Erection: The process of enlargement and stiffening of the penis because of increased blood volume within it

Fertilization: The union of a sperm with an ovum; fertilization is the first step in the creation of a new individual

Gamete: A reproductive cell—sperm in the male, ovum in the female; produced in the gonads, gametes contain a set of chromosomes from the adult male or female

Gonad: The organ responsible for production of gametes—the testis in the male, the ovary in the female

Gonadotropin: A hormone that stimulates the gonads to produce gametes and to secrete other hormones

Semen: The sperm-containing liquid that is expelled from the male body

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