Stallion Anatomy and Physiology

Breeding horses is a precarious business at best. Despite great strides in research, conception rates overall are not high. Some researchers place them as low as 55-60% of all mares bred. In many cases, blame is placed on the mare. She is too old, she has uterine cysts, scar tissue, abnormal perineal conformation, a malfunctioning endocrine system--and the list goes on. However, it takes two to reproduce, and there are times when the problem can lie with the stallion. Before we can discuss what those problems might be, or what is involved in getting maximum reproductive efficiency from a stallion, we must understand the unique anatomy and physiology of the unaltered equine male. Only then can we understand what difference size of the testes might make, why stallion libido and sperm production are seasonally oriented, why anabolic steroids have a deleterious effect, why the older stallion can produce and store more sperm than a young stallion, what effect fever or injury has on sperm production, why the testes of some young colts don't descend from the abdominal cavity to the scrotum, what effect frequent breeding has on sperm quantity and production, and the question of whether placing a stallion under artificial lights will stimulate hormonal production.

The information that follows comes from published information emanating from such institutions as Colorado State University ("Management of the Stallion for Maximum Reproductive Efficiency"), Kansas State University (M.J. Arns, PhD), and Texas A&M University (J. W. Evans, PhD).

Central to the stallion's reproductive system are the testes. It is within the confines of these two organs that spermatozoa are produced (the process is called spermatogenesis), along with the key male sex hormone, testosterone.

Each testis is suspended by the spermatic cord that extends from the abdomen to its attachment on the testis.

The stallion's testes are normally ovoid (like a walnut) in shape and will measure 80-140 millimeters in length and 50-80 millimeters in width and will weigh about 225 grams. Size of the testes is an important consideration in evaluating a breeding stallion. Generally speaking, the larger the testes, the greater the potential for heavy production and storage of spermatozoa.

The testes are housed in the scrotum, which is an outpouching of the skin. It is comprised of two scrotal sacs--one for each testis--and is separated by a septum. The scrotal sacs lie on either side of the penis.

The scrotum itself consists of four layers. The external layer is the skin, which contains an unusually large number of sweat glands that are involved in a cooling process during hot weather. Just under the skin is the tunica dartos muscle, which is a layer of smooth muscle that forms the outermost component of each scrotal sac and also aids in controlling testicular temperature. The third layer is loose connective tissue that allows the testis great mobility--up, down, and sideways--within the scrotal sac.

The innermost layer of the scrotum is called the parietal vaginal tunic. It is a membrous sac that extends from the abdominal cavity through the opening in the abdominal wall--the inguinal canal--through which the spermatic cord passes. This innermost layer extends all the way from the abdominal cavity to the bottom of the scrotum and forms a covering for the spermatic cord, testis, and epididymis. The space within the vaginal cavity, between the testis and the parietal vaginal tunic, contains a watery serous fluid that serves as a lubricant and facilitates movement of the testis within the sac.

The testes are actually formed within the abdominal cavity. In the normal colt, both testes should descend into the scrotum between 30 days before birth and 10 days after birth. In some colts, this does not happen and one or both of the testes might remain within the abdominal cavity. This condition carries the term cryptorchidism.

The reasons for failure of one or both of the testes to descend are several and varied. They include: insufficient abdominal pressure to properly expand the vaginal process; stretching of the gubernacular cord (the fetal ligament attaching the epididymis to the scrotum, present during the descent of the testis), insufficient growth of the gubernaculum and tail of the epididymis so that they are unable to expand the inguinal ring sufficiently to allow entrance of the testis, and displacement of the testis to a position where the pressure of the intestines prevents gubernacular tension from pulling the testis into the vaginal process.

If the testis does not descend during the first two weeks after birth, it is doomed to remain in the abdominal cavity unless removed surgically. The reason is that the internal inguinal ring closes during the first two weeks following birth and the canal through which the testis must pass is blocked.

(Generally, if there is a problem, it will more often be with the left testis rather than the right. For some reason, there is a relatively slow rate of descent for the left epididymis and left testis.)

If only one testis is retained, the stallion usually will still be fertile, although sperm production is generally lower than that for stallions with both testes descended. If both of the testes are retained within the abdominal cavity, the stallion will be infertile because spermatozoa can not be produced under those conditions. However, the stallion with two retained testes will normally exhibit "stallion-like behavior."

Unfortunately, cryptorchidism is believed to be an inheritable trait. Some breed associations prohibit the showing and/or breeding of cryptorchid stallions.

At birth, each testis will normally weigh between 5 and 10 grams. They will remain at that weight and size during the first 10 months of life. A slight growth occurs between 11 and 16 months of age, and rapid development of both testes starts at around 18 months of age. The testes, in some cases, don't achieve full maturation until the stallion is 12 to 13 years of age.

Sperm Production

The testis is the male gonad and, as mentioned earlier, is the site of production for both spermatozoa and the predominate male sex hormone, testosterone. It is covered by a thick lining of connective tissue called the tunica albuginea. Fused to the outer surface of this capsule is the thin visceral vaginal tunic. Supporting strands of connective tissue extend from the tunica albuginea to divide the testis into lobules.

The non-capsular part of the testis is called the parenchyma. It consists of seminiferous tubules (capable of producing and conveying semen) and interstitial tissue situated between the seminiferous tubules.

Located within the seminiferous tubules are Sertoli cells. They are involved with the production of spermatozoa. Located in the interstitial tissue between the seminiferous tubules are Leydig cells. Their prime function involves the production of testosterone. They also produce estrogen, but just what function this hormone has in the stallion's reproductive system is still something of a mystery. The Sertoli cells are contained within a lining of the seminiferous tubules, which is known as the germinal epithelium.

During the first 18 to 24 months after birth, the testes do not produce sperm and the colt is infertile. At about 18 months of age, the testes grow and develop rapidly and several months later they gradually begin to produce sperm. In the adult stallion, billions of spermatozoa are produced daily in the convoluted seminiferous tubules. This figures out to be at a rate of about 70,000 each second.

Although there is this prolific production of sperm on a second by second basis, the development of each individual sperm requires between 55 and 57 days. The process is called spermatogenesis. This process can be divided into three phases. First is spermatocytogenesis. This is the initial differentiation and subsequent division of the germ cells know as spermatogonia to increase their number. The second step is known as meiosis--the process where genetic rearrangement occurs between homologous chromosomes and where spermatogonia are reduced in chromosome number of division to form spermatids. The final step in the process is referred to as spermiogenesis, where spermatids differentiate into mature spermatids.

Once spermatids are released from the seminiferous epithelium into the lumen of the seminiferous tubule, they are referred to as spermatozoa. Each of the above three phases requires between 18 and 19 days.

When spermatozoa are released within the seminiferous tubule, they travel through a series of ducts to the epididymis, which is lightly attached to the upper surface of each testis. It is here that the spermatozoa complete the maturation process and are stored.

The epididymis is divided into three sections: the head (caput), body (corpus), and tail (cauda). It is while traveling through the epididymis--from head to tail--that the spermatozoa reach maturity and acquire the ability to fertilize an egg. The migration of the spermatozoa through the epididymis requires approximately eight days. Once the migration and maturation process is complete, most of the spermatozoa are stored in the tail of the epididymis until ejaculation.

However, if the stallion is not being used for breeding, another process is involved. Even though there are not frequent ejaculations, the stallion will continue to produce sperm. When the epididymis is filled to overflowing, the excess is voided in the urine.

(The fact that approximately 65 days are involved in the development and maturation of spermatozoa is highly significant for stallion managers. For example, if a stallion suffers an injury to the testes or has a high fever that can result in the death of stored sperm, the full implications might not be manifested until two months later.)

The minimum number of spermatozoa produced within a 24-hour time period is known as daily sperm production (DSP). This will vary among stallions and is strongly influenced by testicular size. Daily sperm output (DSO) refers to the number of spermatozoa that can be collected per 24-hour period from a stallion and is determined by collecting the stallion daily for seven days.

The number of spermatozoa produced varies from stallion to stallion, but the important thing to remember is that there are only so many spermatozoa available for ejaculation. Some horsemen have been under the opinion that frequent ejaculations by a stallion will stimulate a more rapid production of spermatozoa. That is a misconception, because the time required for spermatogenesis and maturation in the epididymis is independent of ejaculation frequency.

In other words, whether a stallion is collected daily or every other day for a week will not have an effect on the total number of spermatozoa produced during that week. The stallion, quite simply, has an individual productive capability and the number of times he is collected or used in live cover has no bearing on sperm production.

It thus becomes obvious that a good management tool is a monitoring of the stallion's daily sperm output. Having this knowledge at hand enables the stallion manager to know how many mares can reasonably be booked to an individual stallion.

Hormonal Function

At this point, it might be a good idea to back up and take a look at the role played within the testes by the second group of key cells found there--the Leydig cells, which produce both testosterone and estrogen. To reiterate, testosterone is the key male sex hormone. It is necessary for normal sexual behavior and proper testicular function.

Hormonal production begins with action of the hypothalamus at the base of the brain. The hypothalamus releases the gonadotropin releasing hormone (GnRH). This hormone travels through portal blood vessels to the anterior pituitary gland. At the anterior pituitary gland, GnRH simulates the secretion of two gonadotropins--follicle stimulating hormone (FSH) and luteinizing hormone (LH). These two gonadotropins are carried by the bloodstream to the testes, where FSH exerts its effect on the Sertoli cells and LH exerts its effect on the Leydig cells.

The highest levels of testosterone found in the stallion's body are in the testes. These concentrations are necessary for the testes to carry out their function of producing sperm.

The Sertoli cells, in addition to their other functions, produce proteins that bind testosterone and are responsible for maintaining proper testicular concentration. Testosterone concentrations control the release of GnRH and the gonadotropins FSH and LH through a negative feedback system. Simply put, when testosterone levels are high, the system cuts back on testosterone production due to inhibition of the hypothalamus and anterior pituitary gland. When testosterone concentrations are low, there is no such inhibition and production is at a much higher level.

It is for this reason that administration of anabolic steroids to a stallion has such negative effects on the reproduction process. The stallion's system recognizes the steroids as an over abundance of testosterone and the signal goes out to cut back on natural testosterone production.

The stallion on anabolic steroids will exhibit normal "stallion-like" behavior, but testicular size will be reduced and sperm production will be severely impaired.

Most experts agree that sperm production will continue to be impaired as long as the anabolic steroids are being administered, and that the stallion will not return to normal sperm production until at least two months after treatments cease.

Light Effects

As is the case with mares, hormonal happenings within the stallion's system are season-related. When daylight periods are short, the pineal gland, located within the brain, releases melatonin. Melatonin, researchers believe, inhibits the hypothalamus from releasing GnRH, which subsequently decreases LH and testosterone production.

This means that during the winter months, the stallion's reproductive function is suppressed due to low testosterone concentrations. During this period, both testicular size and sperm production are reduced. Although it varies from stallion to stallion, it is not uncommon--because of low levels of testosterone--for the stallion to have a much lower sex drive in the winter than during the spring and summer months.

Although there is debate on the question, it is believed by some researchers that utilization of lights (photostimulation) during the short days of winter can have much the same positive effect on stallions as it does on mares which are brought into estrus during a period when their reproductive system normally shuts down.

However, it is believed, a key to the positive use of lights involves allowing the stallion to be exposed to short daylight hours before administering photostimulation. Unless the stallion's reproductive system is allowed to shut down for at least a short period, it will be impossible to "restart" it with lights.

Stallions exposed to 16 hours of light beginning in December might have testicular size and sperm production at near normal within 60 days. However, there has been some indication that stallions will begin to "shut down" earlier in the year if they are started earlier in the season under lights.


The production of sperm might also be affected by temperature, so nature has provided the stallion with thermal regulatory features designed to maintain a relatively constant temperature within the testes.

The testes are suspended within the scrotum by the spermatic cord and associated cremaster muscle. During cold weather, the muscles involved with both the scrotum and the spermatic cord contract, drawing the testes closer to the body for additional warmth. During extremely hot weather or when the stallion might have a fever, those same muscles relax, allowing the testes to drop away from the horse's body and thus lower the temperature.

At the same time, during episodes of hot weather, the scrotum stretches and the numerous sweat glands on the skin help regulate the temperature through the evaporation of perspiration.

Also involved in the thermal regulation process is heat transfer between blood from the arteries entering the testes and venous blood leaving them.

Proper regulation of heat within the testes has a profound effect on sperm production. If the stallion's temperature should reach 105° Fahrenheit for even a short period of time, it could result in the death of millions of spermatozoa. This becomes significant when one realizes that the total length of time for the production and maturation of spermatozoa totals approximately 65 days.

Anatomy of Travel

Now that we've gone into some detail about the testes and epididymides and the key role they play in sperm and testosterone production, as well as sperm storage, it is time to take a look at some other organs and ducts that are involved in delivering sperm so that it can fertilize the egg.

First, a look at the route the sperm will take. From the tail of the epididymis, where the majority of sperm is stored, it will be delivered to the deferent duct, which is a continuation of the epididymal duct and runs from the tail of the epididymis through the spermatic cord to the pelvic urethra.

As the deferent duct approaches the pelvic urethra, it widens into a structure termed the ampulla of the deferent duct. The ampulla is about 18 millimeters in diameter, compared to 4 to 5 millimeters for the deferent duct. During ejaculation, sperm is moved along through the ducts and into the urethra by muscular contractions.

Along the route through the ducts that the sperm will travel are the two vesicular glands, which are elongated, hollow pouches about 15 to 20 centimeters long and 5 centimeters in diameter. Fluid secreted by the vesicular glands contributes a major portion of the seminal plasma in an ejaculate.

Next along the route is the prostrate gland, which is a single, firm, nodular gland with two narrow lobes connected by a thin, transverse isthmus. The secretion of the prostate gland is thin and watery and likely helps to cleanse the urethra during ejaculation. It also contributes to the seminal plasma.

Next we come to the two bulbourethral glands that lie on either side of the pelvic urethra near the ischial arch. Their secretion also contributes to the seminal plasma. (The role of seminal plasma is not fully understood. It is believed that, in addition to serving as a means to transport sperm through the male tract, it also contains buffers, nutrients, and stabilizing factors for spermatozoa.)

The vesicular glands, prostate gland, and bulbourethral glands are termed the accessory sex glands. They contribute most of the fluid to the ejaculate, but are not essential for normal fertility of the sperm. Their ability to function normally is dependent on the presence of testosterone in the peripheral blood.

As the sperm completes its journey through the deferent ducts, fortified along the way with seminal plasma, it ultimately arrives at the urethra, a long mucous tube that extends from the bladder to the free end of the penis. The urethra serves as a joint excretory canal for both urine and semen. (Semen is the term used to describe the whitish fluid of the male reproductive tract consisting of spermatozoa suspended in secretions of the accessory glands.)

The pelvic portion of the urethra is overlaid by a thick striated muscle that contracts vigorously during ejaculation. The penile urethra is surrounded by the corpus spongiosum, which is an area of cavernous, erectile tissue.

The penis is the male organ of copulation and consists of three parts. They are the root, which attaches the penis to the pelvis via two strong ligaments and a pair of muscles; the body or shaft, which is the main portion of the penis; and the glans penis, which is the enlarged free end of the penis.

The major portion of the penis is formed by the corpus cavernosum penis. It is spongy, erectile tissue that becomes engorged with blood during erection. The corpus spongiosum is a small area of spongy erectile tissue that immediately surrounds the urethra. It also becomes engorged with blood during erection. The glans penis is filled with a multitude of nerve endings and becomes engorged and erect during sexual excitement.

When not erect, the stallion's penis is approximately 50 centimeters in length and 2.5 to 5 centimeters in diameter. The penis is contained within the sheath or prepuce, which forms two folds around the free end of the penis. Because of the presence of a number of sebaceous glands within these folds, it is not uncommon for there to be a buildup of smegma--the oily product of the glands.

An undue buildup of smegma within the folds can result in soreness, which can lead to ejaculatory dysfunction. A simple washing with warm water can quickly solve the problem. (Detergents and disinfectants are not advised as they can remove natural microflora present on a stallion's penis, which, in turn, might allow potentially pathogenic organisms to take up residence.)

During erection, the erectile tissues of the penis become engorged with blood and its size will increase about 50%. However, the glans penis "flowers" or "bells" and increases in size between 300-400% during erection.

Release of Semen

There are three basic processes involved in release of semen--erection, emission, and ejaculation. Erection is the lengthening and stiffening of the penis. Emission is the movement and deposition of sperm and fluid from the deferent duct and tail of the epididymis as well as fluids from the accessory sex glands, into the pelvic urethra. Ejaculation is the actual expulsion of the semen out through the urethra. In the stallion, ejaculation occurs as a series of strong pulsatile contractions so that several successive "jets" of semen are spurted forth.

Semen emitted during these spurts is described, as to content, as occurring in fractions. The first fraction emitted is the presperm fraction and is considered to be of bulbourethral origin. Its function is to cleanse the urethra. The second fraction is the sperm-rich fraction, containing approximately 75% of the spermatozoa. The third fraction is the sperm-poor fraction and is primarily of vesicular origin and contains the gel portion.

As mentioned earlier, the size of the stallion's testes and the horse's age are significant in sperm production and storage respectively. The number of spermatozoa available for ejaculation depends upon the reserves in the tails of the epididymides, deferent ducts, and ampullae. Stallions which have large sperm reserves in the tails of the epididymides successfully can impregnate more mares in a short period of time.

A study conducted at Colorado State University indicated that age of the stallion has a strong influence on sperm storage. The study involved 54 stallions of light horse breeds. It was found that the sperm reserves in stallions 2 to 4 years of age were significantly lower than were the reserves for stallions 10 to 16 years of age.

Stallions in the 2 to 4 years of age category had in storage an average of 29 billion spermatozoa each. Stallions in the 5- to 9-year-old category had an average of 41 billion spermatozoa. Stallions that were 10 to 16 years of age had an average of 45 billion spermatozoa.

The normal stallion is not sexually mature until at least 6 years of age or perhaps older as reflected by testicular size and, consequently, sperm production.

Size of a stallion's testes has a profound influence on sperm production--the larger the testes, the greater the capacity for production--and age of the stallion has a significant bearing on testes size. Another study was conducted at Colorado State to determine testes size in relation to age.

Average testicular size for stallions 2 to 3 years of age was 96 millimeters in width. For stallions 4 to 6 years of age, the average width was 100 millimeters, and for stallions 7 years old or older, the average width was 109 millimeters.

The conclusion of the Colorado State researchers was that the average 3-year-old stallion could be used in breeding once daily during the breeding season, providing testicular size is normal and sex drive is adequate. The reproductive capacity of older stallions, it was concluded, should be adequate to permit use two or three times daily during the spring and summer.

As can be seen from the foregoing, the reproductive system of a stallion is both complicated and beautifully synchronized by nature. Having a basic understanding of the anatomy and physiology, plus being aware of each individual stallion's sperm production capability, can help the stallion manager to make better decisions concerning proper care of the horse and the number of mares to be booked each breeding season.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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