Why is your dog bum to bum when mating?

When dogs mate, the "bum to bum" position is often due to the way they align their bodies during copulation. This position, where the male mounts the female from behind, allows for proper penetration and successful mating. After mating, dogs may become "tied," where their genitals swell and lock together for a short time, which is a natural part of the mating process to ensure successful reproduction. This behavior is instinctual and part of their reproductive Biology.