The male reproductive system consists of a pair of testes that produce sperm (or spermatozoa), ducts that transport the sperm to the penis and glands that add secretions to the sperm to make semen (see diagram 13.2).
The various parts of the male reproductive system with a summary of their functions are shown in diagram 13.3.
Diagram 13.2. The reproductive organs of a male dog
File:Anatomy and physiology of animals Diagram summarizing the functions of the male reproductive organs.jpg
Diagram 13.3 - Diagram summarizing the functions of the male reproductive organs
Sperm need temperatures between 2 to 10 degrees Centigrade lower and then the body temperature to develop. This is the reason why the testes are located in a bag of skin called the scrotal sacs (or scrotum) that hangs below the body and where the evaporation of secretions from special glands can further reduce the temperature. In many animals (including humans) the testes descend into the scrotal sacs at birth but in some animals they do not descend until sexual maturity and in others they only descend temporarily during the breeding season. A mature animal in which one or both testes have not descended is called a cryptorchid and is usually infertile.
The problem of keeping sperm at a low enough temperature is even greater in birds that have a higher body temperature than mammals. For this reason bird’s sperm are usually produced at night when the body temperature is lower and the sperm themselves are more resistant to heat.
The testes consist of a mass of coiled tubes (the seminiferous or sperm producing tubules) in which the sperm are formed by meiosis (see diagram 13.4). Cells lying between the seminiferous tubules produce the male sex hormone testosterone.
When the sperm are mature they accumulate in the collecting ducts and then pass to the epididymisbefore moving to the sperm duct or vas deferens. The two sperm ducts join the urethra just below the bladder, which passes through the penis and transports both sperm and urine.
Ejaculation discharges the semen from the erect penis. It is brought about by the contraction of the epididymis, vas deferens, prostate gland and urethra.
Diagram 13.4 - The testis and a magnified seminiferous tubule
Semen consists of 10% sperm and 90% fluid and as sperm pass down the ducts from testis to penis, (accessory) glands add various secretion...
Three different glands may be involved in producing the secretions in which sperm are suspended, although the number and type of glands varies from species to species.
Seminal vesicles are important in rats, bulls, boars and stallions but are absent in cats and dogs. When present they produce secretions that make up much of the volume of the semen, and transport and provide nutrients for the sperm.
The prostate gland is important in dogs and humans. It produces an alkaline secretion that neutralizes the acidity of the male urethra and female vagina.
Cowper’s glands (bulbourethral glands) have various functions in different species. The secretions may lubricate, flush out urine or form a gelatinous plug that traps the semen in the female reproductive system after copulation and prevents other males of the same species fertilizing an already mated female. Cowper’s glands are absent in bears and aquatic mammals.
The penis consists of connective tissue with numerous small blood spaces in it. These fill with blood during sexual excitement causing erection.
Penis Form And Shape
Dogs, bears, seals, bats and rodents have a special bone in the penis which helps maintain the erection (see diagram 13.2). In some animals (e.g. the bull, ram and boar) the penis has an “S” shaped bend that allows it to fold up when not in use. In many animals the shape of the penis is adapted to match that of the vagina. For example, the boar has a corkscrew shaped penis, there is a pronounced twist in bulls’ and it is forked in marsupials like the opossum. Some have spines, warts or hooks on them to help keep them in the vagina and copulation may be extended to help retain the semen in the female system. Mating can last up to three hours in minks, and dogs may “knot” or “tie” during mating and can not separate until the erection has subsided.
Sperm are made up of three parts: a head consisting mainly of the nucleus, a midpiece containing many mitochondria to provide the energy and a tail that provides propulsion (see diagram 13.5).
Diagram 13.5 - A sperm
A single ejaculation may contain 2-3 hundred million sperm but even in normal semen as many as 10% of these sperm may be abnormal and infertile. Some may be dead while others are inactive or deformed with double, giant or small heads or tails that are coiled or absent altogether.
When there are too many abnormal sperm or when the sperm concentration is low, the semen may not be able to fertilize an egg and the animal is infertile. Make sure you don’t confuse infertility with impotence, which is the inability to copulate successfully.
Sperm do not live forever. They have a definite life span that varies from species to species. They survive for between 20 days (guinea pig) to 60 days (bull) in the epididymis but once ejaculated into the female tract they only live from 12 to 48 hours. When semen is used for artificial insemination, storage under the right conditions can extend the life span of some species.
In many species the male can be artificially stimulated to ejaculate and the semen collected. It can then be diluted, stored and used to inseminate females. For example bull semen can be diluted and stored for up to 3 weeks at room temperature. If mixed with an antifreeze solution and stored in “straws” in liquid nitrogen at minus 79oC it will keep for much longer. Unfortunately the semen of chickens, stallions and boars can only be stored for up to 2 days.
Dilution of the semen means that one male can be used to fertilise many more females than would occur under natural conditions. There are also advantages in the male and female not having to make physical contact. It means that owners of females do not have to buy expensive males and the possibility of transmitting sexually transmitted diseases is reduced. Routine examination of the semen for sperm concentration, quality and activity allows only the highest quality semen to be used so a high success rate is ensured.
Since the lifespan of sperm in the female tract is so short and ova only survive from 8 to 10 hours the timing of the artificial insemination is critical. Successful conception depends upon detecting the time that the animal is “on heat” and when ovulation occurs.