Mounting (aka: humping) is a complicated subject. One we don’t fully understand yet.
Mounting behavior occurs for many reasons. It can be triggered by high levels of excitement, especially in younger dogs and sometimes turns into an obsessive/compulsive disorder. Mounting is also tied to sexual hormone influences.
Mounting is generally a normal behavior, unless it gets so prevalent that it becomes obsessive/compulsive. In those cases, there are medications that can help such as fluoxetine (Prozac Rx) or clomipramine (Clomicalm Rx). Most of the time medications are not the answer. It is OK to discourage this behavior, (if it doesn’t slow down) using distracting commands.
Work on preventing humping by teaching a dog skills like sit or down. For example if the dog is going into a state of arousal, they can be cued to stop the behavior and calm themselves down before mounting occurs. With the skills you’ve taught your dog, you will be giving them something else to do besides mounting. In the event mounting occurs you may need to interfere, but do so very carefully so you do not further excite the dog or cause the dog to feel threatened.
When mounting is occurring, if you are concerned that one dog is getting annoyed and cannot defend himself or get away from the humper, step between the dogs and separate them with your body. This is only if the dog has not been taught skills with distractions, and is not responding to your cues. If the humping dog cannot calm down on his own and is bothering other dogs, crate him or pull him aside and wait until his arousal level lowers. If the “humper” is not particularly bothering the other dogs – let ’em go for it. Look at it as mother nature’s way of giving him an opportunity to practice for sex.
There is evidence that some female dogs exhibit “male” characteristics, such as marking and mounting, which is believed to be associated with higher levels of testosterone. Is this conclusive? Not sure. Maybe the dog learned the behavior from watching other dogs.
Researchers are tossing the whole dominance theory and rank reduction stuff to the wind. The most recent thought is that humping is simply a social behavior/motor pattern that dogs have. All it means is that the dog is excited or in an aroused state. Females do it too. Dogs do not understand rank because they do not relate to the experiences of others. They are self aware but not other aware. They cannot understand “you lose, I win.” All they relate to is the present (this feels good) and the past (this was reinforcing and felt good before).
Dogs live in the present, but bring their past experiences along with them to determine how they will respond to present circumstances and situations. What they cannot perceive is the future or how to control or influence the future.
Some portions of this information found on vetinfo.com.