If he is 5 months old then i assume his siblings are as well. Females can come into heat as early as 6 months and he may know that is coming soon. MOVE HIM.
Well in 45 years of keeping dogs, I think ALL of my boys will hump anything that tickles their fancy. It is simply a puppy thing, I just distract them and take them out for a walk or give them a different toy or wait until the older girls teach this youngster that this behavior is not appreciated. It is simply natural, something tickles and they scratch it, do not reprimand him at all, he will learn to mind his manners until the appropriate time.
Humping is not always sexual behavior - many times it is simply a dominance behavior. He may be asserting his dominance in the pack. Many times a dog will hump a person's leg, or his bed, or whatever he/she feels he can be dominant over. It's not necessarily sexual. And it's not always just the boys. We call our girls the "humpty dumpty sisters". They all think they are in charge, when in reality, it's me! HA! HA!
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Replying to:
I have a 5 month old boy that has just learned to hump.
Do I let him do this on his siblings, ususally on their heads or do I stop him or isolate him?
He has no idea what he's doing but has an itch he wants scratched
I had a breeder tell me I did not train my dog right and that I created a bad temperament with my male dog because he tried to hump. Give me a break he is a dog! When he was a puppy I did my best to discourage him from humping by telling him to stop or separating him from the pack for a little while. He still starts to hump and now that he is older when I tell him "no" he does stop and then 5 minutes later he is at it again. Good luck with that one.
my young men get the "hint" from the older girls, this quickly cures them of the puppy humps.
but it may be necessary to remove him from the older girls as he matures so he can get some of the self machoness back and preform with confidence when needed.