My dogs start off mating like many mammals, the male mounting the female. But they then turned away from each other during the process.
Why do they do this? What purpose would it serve?
When dogs mate, the male dog gains entry to the female by mounting her from behind. The bulbis glandis, a part of the penis, swells and lodges inside the vagina of the female. The female has strong sphincter muscles at the opening to the vagina, which contract around this swelling, further securing the penis inside the vagina.
The dogs then turn, maintaining this contact and face away from each other in what is known as a tie. The dog penis is able to turn to accomodate this rotation.
As it takes 20 minutes for the entire mating process to complete, it is theorised that the evolutionary reason for this turn is so the dogs are not left defenceless in the original mount position, with the males dog's back and genitals exposed. Both dogs' faces and jaws are facing outwards, forming quite a formidable defence against would-be predators or other dogs wanting to mate with the female.
After ejaculation the swelling subsides and the vagina muscles relax and the dogs are able to release and break.