The female dog's (bitch) vagina muscles tighten up to ensure that all semen is put into her. When a male dog ejaculates, the semen drips out little by little. Some say that the "tie" is to ensure that another male is not able to get to the bitch.
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The dog's penis, when fully erect and engaged in the mating, swells at the base, which is referred to as the 'knot'. When this round portion is inserted into the bitch's vagina, the bitch has a restrictive muscle near the opening which holds the dog's penis in place while ejaculation occurs. This is to insure that not only do no other dogs get at the female until the odds of pregnancy have decreased, and also to keep the dog's semen inside the female as it has a rather low viscosity (meaning it's more 'watery') and therefore increase the odds that it will impregnate the bitch.
The penis of the dog contains a bone that allows it to penetrate the bitch's vagina before the penis becomes erect.
Once it has penetrated, the penis then becomes engorged, including swelling at the base, which is known as the bulbus glandis, that then locks the two animals together for the duration of the mating in what is known as a dog breeding tie.
The tie or lock can last up to an hour but is usually 10 to 20 minutes long.
However, it is always advisable to supervise the mating so that the male dog's reproductive parts aren't inadvertently damaged by the two coupled dogs twisting or dragging each other about. Some bitches will struggle and need gentle but firm restraint and reassurance to keep them still and calm.
Don't try to separate the dogs during the tie! It can injure either or both animals. After some time, they will part naturally.
The next thing that we are going to do now begin to get him aroused. I find that the best way is to first gently stroke his cock through his sheath until it begins to enlarge and slip out.
You should continue to gently stoke him with your hand while you begin to slowly move you mouth back and forth over his cock. As you do this, his cock will continue to enlarge until he reaches full erection.
As he gets closer to his climax you will notice that at the base of his cock there is a very large bulge known as his "knot". This knot is used to hold his cock inside a female (dog or human) until he has finished ejaculating. If you are considering going further then you should make a mental note of the size of his cock and knot. The average large dog has a cock, when aroused, that is 5 to 7 inches long and 1 and half to 2 inches wide. The knot for a dog whose cock is 6 inches long and 1 and a half wide can be two inches long and 4 inches wide. A dog is different than most mammals as from the time they begin to become aroused until they begin to get soft they will have some form of ejaculate coming from their cocks. At first arousal there is a clear thin fluid that tastes like iron and has the consistency of water. This is his precum that is for lubrication so his knot will slip into the female's vagina. At full arousal is when he actually will produce his sperm and you can tell when this happens as his cum will begin to have a slightly salty taste to it. You should be aware that his cum will never be as thick as a man's but he will produce about twice as much as a man will.