HI, I'm Dr Steve
When a male dog enters a female, the bulb of the penis swells and locks his penis in the cervix. This helps prevent semen from leaking out and increases the liklihood that the mating is successful. There are many reproductive books that will have pictures of this process. It has ben proven so it isn't a theory, it actually happens. It is a very clever way to help insure the survuvability of the species. I hope this has helped. If it has, would you please click the accept button so I may receive credit for my answer. Thank you . If you have more questions, I will check later.
This is great information: don't know why but the 'curious-bug' 'bites' and I can't just 'work-around' it til I dig up an answer. Thank you, XXXXX XXXXX