Canine sexual anatomy and development
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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 ancestors. 
The reproductive cycle
The average length of the reproductive cycle for females is 2–4 weeks. 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 and there will be small amounts of bloody vaginal discharge along with 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 behaviours.
Anestrus is the period of reproductive quiescence. The female has no attraction to or from the male. Anestrus generally lasts four to five months.
As with most tetrapods, canine copulation involves the male mounting the female from behind, a position that is referred to as "doggy style" but does not have a specifically known origin. 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, lie down, snap, retreat, or otherwise be uncooperative. If the female is receptive, she will stand still and hold her tail to the side, a stance referred to as "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 canine penis is not erect, and only able to penetrate the female because it includes a narrow bone called the "baculum", a feature of most placental mammals. When the male achieves penetration, he will usually hold the female tighter and thrust deeply. It is during this time that the male's penis expands and it is important that the bulbus gland is sufficiently far inside for the female to be able to trap it. Unlike human sexual intercourse, where the male penis commonly becomes erect before entering the female, canine copulation involves the male first penetrating the female, after which swelling of the penis to erection occurs, which usually happens rapidly.
Male canines are the only animals that have a locking bulbus glandis or "bulb", a spherical area of erectile tissue at the base of the penis. During copulation, and only after the male's penis is fully inside the female's vagina, the bulbus glandis becomes engorged with blood. When the female's vagina subsequently contracts, the penis becomes locked inside the female. This is known as "tying" or "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"), thrusting behavior stops and 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 handlers to attempt to calm the mating dogs if they show anxiety once this stage is reached.
Gestation and litters
Gestation in a dog is 63 days in length, if measured from the day of ovulation. Since it is difficult to determine the exact date of ovulation, errors are often made in calculating gestation period. Canine sperm can live for 10 to 11 days in the uterine tubes (fallopian tubes) so if a bitch is bred 10 days before the oocytes (eggs) can be fertilized, she will appear to have a gestation length of 70 days. If she is bred on the day the oocytes can be fertilized, her gestation length will appear to be 60 days long.
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 favoured 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. A female dog usually has 10 teats, though this does not mean she can necessarily provide sufficient nutrition for 10 puppies in one litter.
An average litter consists of about five to six puppies, though this number may vary widely based on the breed of dog. Size of the breed is correlated with litter size. Miniature and small breeds average three to four puppies in each litter, with a maximum litter size of about 5-8. Large and giant breeds average 7 puppies per litter but can have a maximum litter size of about 15. In one study, the Rhodesian Ridgeback had the highest average litter size with 8.9 pups per litter while the Pomeranian and Toy Poodle had the lowest with 2.4 pups per litter.
The number of puppies also varies with the mother's age. In smaller breeds, both young and old age are associated with smaller litter size. In larger breeds, only old age is associated with smaller litter size. Use of artificial insemination is also associated with smaller litter size, with frozen semen having a stronger effect than fresh semen.
Some breeds have been developed to emphasize certain physical traits beyond the point at which they can safely bear litters on their own.
A large scale study in Norway showed that across all breeds, about 4% of pups will be stillborn and a further 4% will die within the first week (early neonatal mortality). Between 8 days and 8 weeks, 1% will die. Litter size, breed size and age of the bitch is associated with increased risk. High risk breeds for stillborn includes the Dogue de Bordeaux (14.2%), St. Bernard (12.3%), Chow Chow (12.1%), Pembroke Welsh Corgis (11.7%) and Dalmatian (10.6%). The Basenji, Italian Greyhound, Australian Terrier, Irish Soft Coated Wheaten Terrier and the Bichon Havanaise had few to no stillborns (0-0.6%). High risk breeds for early neonatal mortality includes the Rhodesian Ridgeback (11.6%), Dogue de Bordeaux (10.4% ), Dalmatians (8.8%) and Icelandic Sheepdog (8.7%) while the Basenji and Tibetan Terrier had no early neonatal mortality and the Border Terrier and Danish-Swedish Farmdog had <1% early neonatal mortality.
Common causes of early neonatal mortality are bacteria infection, fetal asphyxia and fading puppy syndrome. Other causes may include elective euthanasia because of congenital defects or failure to meet breed standards.
Other multi-breed studies have put stillborn rates at 6.5-7% and early neonatal mortality at 11.5-19.8%.
Female dogs are at risk for endometritis and pyometra in the postpartum period and after estrus or vaginitis. Signs and symptoms include fever, lethargy, loss of appetite, excessive thirst, restlessness, a foul smelling vaginal discharge which may or may not be bloody, infertility, or they may be asymptomatic.  Uterine infections should be treated expeditiously if suspected. Contrary to common belief, uterine infections can strike any intact bitch, whether she has been bred or not, and whether it is her first season or not although it is more common as dogs become older.
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 the male's 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.
Designer breed dogs are Mixed-breed dogs 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; however, the qualities of cross bred dogs are not predictable. For example, F2 Labrador x poodles ("labradoodle") can inherit the coat of either a Labrador, a poodle, or a remix.
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—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. Some[who?] object to spaying and castrating as the sterilization could be carried out without the excision of organs. Spaying may also be done for ethnic or religious reasons, as male dogs sometimes mate with each other (anal sex) if not introduced to a female for an extended period of time.
Increased aggression can be shown in altered females if they have previously displayed aggression prior to surgical alteration. In a study by O'Farrell and Peachy, female dogs less than 11 months of age that had previously shown signs of aggression are more likely to have an increase in aggression after being spayed. These increases in aggression may be due to the sudden change in hormone concentrations that are the result of alteration. While spaying female dogs does not "induce" aggression it can increase aggression (especially dominance aggression) and facilitates indiscriminate appetite in young altered females and can include them rapidly eating meals or eating food-associated items such as trash. When females display dominance aggression it is often directed towards owners. Behaviors displayed include: guarding of toys, food, or resting places, a tense/rigid posture, erection of ears and tail, baring of teeth, growling, and snapping or biting. These behaviors are usually an attempt to form some type of "dominance hierarchy" within the family. Some dogs may show selective dominance aggression towards one family member but not another. Dominance aggression can sometimes be caused when an owner attempts to flip the pet into a submissive position (like on its back). Dominance aggression is also not limited to humans, canines exhibiting dominance aggression can also show this behavior toward other animals.
In nearly 2/3 of the cases that involve interdog aggression castration can help decrease aggression. Castration also decreases other male-typical behavioral traits such as mounting, roaming, and urine marking. Specifically, male puppies that are neutered between 7 and 10 weeks are three times less likely to display behavioral problems compared to canines neutered at 6 months or older. Most dominantly aggressive dogs are male which causes many people to neuter their male canine companion. Removing testosterone can decrease the intensity of a canine's reaction to stimulus. Testosterone does not cause a behavior to occur but its absence may decrease the occurrence of a "bad" behavior.
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