Male sexually aggressive to other male

Question
I have a 5-year old male (not neutered) miniature schnauzer.  He is not considered good for breeding, having only one testicle.  Recently my son purchased a male Maltese-Shiatsu cross, now about 8 months old.  For the past week or so, whenever they are together, our schnauzer is behaving very sexually toward the puppy.  Tonight it hit an extreme, and our dog was completely fixated..when he wasn't allowed close enough to actually touch the puppy, he stared at him and went quite goofy when we put him in a position where he couldn't see the pup!  This is VERY out of character...what's going on?  And what can we do about it.  Neither dog has been neutered.

Answer
A couple things here, and I'll start with the more obvious one. The puppy has reached adolescence, and your schnauzer is not only trying to re-establish his ranking in the (family) pack because of the new arrival, the schnauzer is pretty much telling the "teenager" that he is nothing more than a punk and that HE, the schnauzer, is the boss. Humping rarely has anything to do with sex; it is a demonstration of dominance. While this is most likely the reason, there is one other possibility.

If the puppy should have a bladder infection, he could be giving off an aroma that is similar to a bitch in season. You might have a urinalysis done to make sure he is healthy as I don't believe I have ever heard of a dog being "fixated" on another once they were separated.

Finally, and this is the most important.... since your schnauzer only has one descended testicle, you really need to have him neutered as there is a high incidence of cancer in undescended testicles. Having him neutered could give him many more years with you.

Karen