In breeding, the female sex of the canine is often referred to as a bitch, and the male of the same species is referred to as a stud. In domestic canines, sexual maturity 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 third cycle. It is known that both wolves and domestic dogs have the same number of chromosomes; 39 pairs.
With the male dog, 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 tail. Testosterone levels are also responsible for the sex drive, which is not cyclical. Males are receptive to mating at any time, and are willing to mate even if the female dog is not in heat/fertile.
Female dogs in heat often show behavior as in "bending the tail", like seen in the picture below. Showing there genitals to the male dog.
This is why male dogs are the only animals that have a bulb or locking "knot", a spherical area of erectile tissue at the base of the penis, which traps the penis inside the female's vagina during sexual intercourse, as it becomes filled with blood. This is known as tying or alternatively knotting.
Once the penis is locked into the vagina by the "knot" (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 (Semen is brought into the vagina of the female) occurs, decreasing leakage of semen from the vagina. After some time, typically 5 – 20 minutes (but sometimes longer), the knot, allowing the mates to separate. Female dogs who have no experience in breeding 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 both of the mating dogs once this stage is reached, the female dog can be in pain and this can occur in aggressive behavior.
Note that similar canine mounting behavior (sometimes with humping) is also used by dominant dogs of both sexes. Dominance mounting, with or without humpin, 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 are a sign that the dominant dog has "won the fight" of dominance.
In some rare cases sexual activity occurs between two male dogs, this can happen if dogs are so influenced by testosterone/hormones that the sexual drift takes over, there is actually a case known of repeating mating behaviors between two male dogs.
sometimes male dogs show this drift in "humping" the leg or humping pillow's.