Article: Health » Canine Reproduction
Canine Sexual Anatomy and Development
In canine reproduction, the female sex of the domestic canine is often referred to as a bitch, and the male of the same species is referred to as a stud. The dog chosen for breeding is the stud. In domestic canines, sexual maturity (puberty) occurs between the ages of 6 to 12 months for both males and females, although this can be delayed until up to two years of age for some large breeds. Pregnancy is possible as soon as the first estrus cycle, but breeding is not recommended prior to the second cycle. As with other domesticated species, domestication has selectively bred for higher libido, and earlier and more frequent breeding cycles in dogs than in their wild ancestors.
Like most males of any species, the development of secondary sexual characteristics is dependent on the production of testosterone by the testes. Secondary sex characteristics include increased muscle mass, penile enlargement, anal hypertrophy, and development of a gland at the base of the penis, called the bulbus glandis. Testosterone levels are also responsible for the sex drive, which is not cyclical. Males are receptive to mating at any time, even if the female is not in estrus.
The anatomy of female dogs is similar to that of many other mammals. They have two ovaries located caudal to the kidneys in the abdominal cavity. They have a bicornuate (or two-horned) uterus which ends as the cervix, a short canal which connects to the vagina. The cervix is muscular with fibrous tissue support, and its opening closes in a stellate pattern. The vagina opens into an enlarged space called the vestibule which also receives the opening of the urethra. The vestibule makes a steep downward turn exits within the vulva. Females have a clitoris just inside the vulva. Bitches often have a vaginal stricture, which is a remnant of where the vagina and vestibule fused together during embryonic development. This structure is often asymptomatic and is broken down during mating.
As with most domesticated species, one of the first and strongest effects seen from selective breeding is selection for cooperation with the breeding process as directed by humans. In domestic dogs, one of the behaviors that is noted is the abolition of the pair bond seen in wild canines. The ability of the female domestic dog to come into estrus at any time of the year and usually twice a year is also valued. The amount of time between cycles varies greatly among individual dogs, but a particular dog’s cycle tends to be consistent through her life. Conversely, undomesticated canine species experience estrus once a year, typically in late winter.
Most female dogs come into season for the first time between 6 and 12 months, although some larger breeds delay until as late as 2 years. Like most mammals, the age that a female first comes into season is mostly a function of her current body weight as a proportion of her body weight when fully mature. They then experience estrus about every seven months until old age. Female dogs do not experience menopause, although their cycles will become irregular and fertility becomes unpredictable as they become older. Dogs over around 7 or 8 years are not usually considered appropriate for breeding, but can still remain fertile.
The Reproductive Cycle
The average length of the reproductive cycle for females is 6 months. Females reach sexual maturity (puberty) between 8 and 18 months of age. There is a tremendous variability in the maturation age between breeds, and even within a breed of dog.
The first stage of the reproductive cycle is proestrus, in which eggs in the ovaries begin to mature and estrogen levels begin to rise. During this stage males are attracted to non-receptive females. Initially, the vulva lips will swell up and become pliable, there are small amounts of bloody vaginal discharge, and signs of frequent urination and restlessness. Proestrus generally lasts 9 days.
Estrus is the next stage, in which estrogen levels are high, mature eggs are released from ovaries, and the females mentally and physically become receptive to copulation. It is only during estrus that copulation will result in pregnancy.
During proestrus and estrus, females may have a clear to bloody discharge. This stage is also known as “heat.” The length of these cycles varies greatly between individuals. Proestrus and estrus can last anywhere from 5 days to 21 days.
Diestrus is the period following mating. Diestrus lasts approximately 56 to 58 days in a pregnant female, and 60 to 100 days in a non-pregnant female. During both of these periods, progesterone levels are high. Because the hormonal profile of a pregnant female and a female in diestrus are the same, sometimes a non-pregnant female will go through a period of pseudo-pregnancy. At that time she may gain weight, have mammary gland development, produce milk, and exhibit nesting behaviors.
Anestrus is the period of reproductive quiescence. The female has no attraction to or from the male. Anestrus generally lasts four to five months.
Copulation between canines involves a male canine mounting the female from behind, as with most tetrapods, a position known informally as doggy style. When a male canine is interested in mounting a female, he will sniff the female’s vulva. If the female is unreceptive, she may sit or lie down, snap, move away, or otherwise be uncooperative or not allow the mating. If the female is receptive to mating, she will stand still and hold her tail to the side (called “flagging”). The male will often continue examining the female’s rear, before mounting her from behind while attempting penetration with his penis.
At the time of penetration, the penis is not erect; it is slender and is only able to penetrate the female because of the presence of the small bone inside it, known as the baculum. When the male achieves penetration, he will often hold the female tighter and thrust faster, and it is at this point that the male’s penis expands. Canine copulation involves the canine male first entering the female, and then swelling and becoming erect.
Male canids are the only animals that have a bulb or locking bulbus glandis, a spherical area of erectile tissue at the base of the penis. During copulation, after the male’s penis is fully inside the female’s vagina, the bulbus glandis becomes engorged with blood. Then, when the female’s vagina contracts, the penis is locked inside the female. This is known as tying or alternatively knotting. While characteristic of mating in most canids, the copulatory tie has been reported to be absent or very brief (less than one minute) in the African Wild Dog, possibly due to the abundance of large predators in its environment.
When the penis is locked into the vagina by the bulbus glandis (when the stud is “tied”), the male will usually lift a leg and swing it over the female’s back while turning around. The two stand with their hind ends touching and the penis locked inside the vagina while ejaculation occurs, decreasing leakage of semen from the vagina. After some time, typically between 5 and 20 minutes (but sometimes longer), the bulbus glandis disengorges, allowing the mates to separate. Virgin dogs can become quite distressed at finding themselves unable to separate during their first copulation, and may try to pull away or run. Dog breeders often suggest it is appropriate for those involved to help calm and settle the mating dogs once this stage is reached, if they show anxiety, through until eventual separation.
Similar canine mounting behavior (sometimes with pelvic thrusting) is also used by dominant canines of both sexes. Dominance mounting, with or without thrusting, should not be confused with copulatory mounting, in which the thrusting is short term until a “tie” is achieved. In dominance mounting, the dominant dog is merely trying to get its head above the subordinate dog. If a male dog does this, one may notice that the genitals are aimed over the subordinates tail rather than under it. Unsheathing of the male genitals is a sign that the dominant dog has “won the fight” of dominance.
Gestation and litters
Dogs bear their litters roughly 9 weeks after fertilization, although the length of gestation can vary from 59 to 65 days, with 63 days being the average.
A general rule of thumb is that a mammal will produce half as many offspring as the number of teats on the mother. This rule is altered in domesticated animals since larger litters are often favored for economic reasons and in dogs, particularly, the great range of sizes and shapes plays a role in how many healthy puppies a female can carry. An average litter consists of about six puppies, though this number may vary widely based on the breed of dog. Toy dogs generally produce from one to four puppies in each litter, while much larger breeds may average as many as 14 pups in each litter. The number of puppies also varies with the mother’s age and health, the father’s sperm count, the timing of the breeding, and many other factors.
Some veterinarians say that mating a stud and bitch every other day is the way to a full litter. Others will say that mating every day is much better. This reduces the chance of having one puppy being born prematurely or much later than the other pups of the litter.
Some breeds have been developed to emphasize certain physical traits beyond the point at which they can safely bear litters on their own. For example, an English Bulldog has wide shoulders and narrow hips, thus the breed requires artificial insemination and, later, a cesarean section for giving birth.
Since a mother can provide nutrients and care to only a limited number of offspring, humans must assist in the care and feeding when the litter exceeds approximately eight puppies.
Owners of intact bitches should be aware of pyometra, an acute infection similar in effect to appendicitis which can affect any intact bitch, and which is always very serious and often fatal. The period after heat, when the cervix is closing, is a high risk period for this disease and close attention should be paid for any signs of fever, lethargy, loss of appetite, excessive thirst, or any other indicators of sickness. Pyometra should be treated as a medical emergency if suspected. Contrary to common belief, pyometra can strike any intact bitch, whether she has been bred or not, and whether it is her first or tenth season, although it is more common as dogs become older.
Breeders and veterinarians have various methods for determining the best time to breed a dog. Breeders rely on both the visible signs of estrus and modern testing methods when they are trying to decide the best time to breed. Visible signs of estrus (or heat) include vaginal bleeding, vulvar swelling and the female dog responding to a male dog by standing and “freezing” to allow the male dog to mount her. The three most commonly used medical methods for determining estrus are: blood progesterone, blood luetinizing hormone(LH), and a vaginal smear. The first two test for the level of those hormones in the blood, and a “smear” checks the amount of cornified epithelial cells present in the vaginal secretions.
Many breeders prefer the LH and progesterone tests because they are highly accurate while the smear method can be unreliable (especially when done by a vet who has limited experience performing this test). However the smear method is still widely used because it is much cheaper, and it’s less stressful for the dog because no blood has to be drawn.
Before the advent of modern testing a breeder might mate the two dogs every two to three days during the female’s estrus to ensure that fertilization had occurred. Today, most breeders will track a female’s cycle (usually by progesterone blood tests) and breed her only during a 3-day window when she is most fertile and most receptive to the male.
These litter-mates were born to an Australian Shepherd mother who had roamed free, suggesting different fathers.
A female dog allowed to roam freely during estrus can end up producing puppies from multiple fathers. Breeders might occasionally breed a female to more than one desirable male for various reasons, in which case the only reliable way to determine parentage of the puppies is through DNA testing of the pups and mother.
Natural breeding can be easily accomplished between most dogs as long as the bitch is in estrus during the attempt. Simply placing both dogs in the same environment for a few days will usually result in a pregnancy. If, due to breed characteristics or the stud only being available for a short time, natural breeding is not desirable, artificial insemination can be used. This is often done at a veterinarian’s office. An artificial vagina is prepared, which is a conical thin latex sleeve ending in a sterile collection tube. The inside of the latex sleeve is lightly lubricated. The male is allowed to sniff a female in estrus. Experienced studs cooperate readily in the process. New studs often require encouragement in the form of manual stimulation. Generally the male will mount the female, and the collector quickly directs his penis into the latex sleeve. The male ejaculates and the semen is collected in the tube. The semen is then drawn up into a long thin pipette. The pipette is threaded through the bitch’s cervix and the semen deposited in her uterus. There is ongoing research into techniques for chilling or freezing canine semen. Currently, for high quality breeding’s, one of the mates is often flown to the location of the other for this procedure.
Amateur breeding is also referred to by some as “backyard breeding” and refer to those specifically who plan on selling the offspring on a regular basis for income.
Natural breeding can also take place in private homes.
Purebred dog breeding arose in response to jobs for which a specific type of dog was needed, whether hunting, guarding, herding, or other work and were also bred simply for their physical attractiveness and appearance as well. Most of the work qualities no longer serve any useful purpose in current American society. Different qualities were needed for these various jobs, such as a soft mouth for retrievers or a herding instinct for sheep dogs. In order to fix the genetic breed features so that two dogs of the same breed could reproduce themselves, all breeds have historically started with inbreeding. Those fixed qualities are today referred to as the breed standard. Standards are set and approved by each national breed club. Many standards are quite specific and not open to interpretation, such as height, ear type (prick versus drop), tail, coat, and other conformation features. Other aspects of each breed may be open to interpretation. Breeders may focus on conformation, temperament, or working qualities, such as hunting, pointing, herding, or rescue. Currently, the main goals for most breeders are to improve their chosen breed for show as well as producing puppies for sale as pets. Many commonly advertise their dogs on the internet,selling and shipping, often by plane, for pet and show purposes.
Professional breeders are not only aware of their breed standards, but some attempt to improve their own line by either line-breeding or by introducing new lines. Line-breeding or inbreeding can cause two main problems: the doubling up of disease-causing recessive alleles, causing the offspring to have health problems, and the loss of vigor. Dogs whose inbreeding coefficient is higher than 6.25% have shorter lifespans and are more prone to disease. Professional breeders may test for genetic defects before breeding for such things as hip dysplasia, various eye conditions, thyroid problems, and epilepsy.
Designer breed dogs are a Mixed-breed dog that are intentionally bred from parents of two established breeds. Studies have shown that cross-bred dogs have a number of desirable reproductive traits. Scott and Fuller found that cross-bred dogs were superior mothers compared to purebred mothers, producing more milk and giving better care. These advantages led to a decreased mortality in the offspring of cross-bred dogs.
Spaying and neutering
Spaying (females) and castrating (males) refers to the sterilization of animals—usually by removal of the male’s testicles or the female’s ovaries and uterus–in order to eliminate the ability to procreate, and reduce sex drive. Castration has also been known to reduce aggression in male dogs, but spaying has been shown to occasionally increase aggression in female dogs.
Animal control agencies in the United States and the ASPCA advise that dogs not intended for further breeding should be spayed or neutered so that they do not have undesired puppies. Spaying and castrating can decrease the risk of hormone-driven diseases such as mammary cancer, as well as undesired hormone-driven behaviors. However, certain medical problems are more likely after neutering, such as urinary incontinence in females and prostate cancer in males. The hormonal changes involved with sterilization are likely to somewhat change the animal’s personality, however, and some object to spaying and castrating as the sterilization could be carried out without the excision of organs.
Dogs shown in the conformation ring are not allowed to be either neutered or spayed. It disqualifies them from being shown as they must be intact and unaltered.
Female cats and dogs are seven times more likely to develop mammary tumors if they are not spayed before their first heat cycle. The high dietary estrogen content of the average commercial pet food as well as the estrogenic activity of topical pesticides may be contributing factors in the development of mammary cancer, especially when these exogenous sources are added to those normal estrogens produced by the body. Dog food containing soybeans or soybean fractions have been found to contain phytoestrogens in levels that could have biological effects when ingested long-term.
Gender-preservative surgeries such as vasectomy and tubal ligation are possible, but do not appear to be popular due to the continuation of gender-specific behaviors and disease risks.
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