Why do dogs get stuck together when they mate?

Dogs get stuck together when mating because the male dog's penis swells inside the female dog's vagina, causing the two dogs to lock together in a breeding tie. The dogs remain locked together for two to 30 minutes until the bulbus glandis on the male's penis returns to normal size.

During the breeding tie, the male dog ejaculates semen into the female dog and the penis swells up to block the seminal fluid from leaking from the vagina.

If both dogs are not encouraged to stay calm during mating, the forced separation can cause serious damage to the genitals and may complicate the breeding process.