
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 whilst 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. Unlike human sexual intercourse , where human males become erect first and then enter the female, 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.