Why is my intact male dog suddenly marking and dominating his brother?

I see that Zeus is not neutered - unless he is a show or breeding dogs, this is the first thing I would suggest doing. It's extremely common for intact male dogs to have issues with urine marking in the house once they reach social maturity, usually around 12-18 months of age - so it sounds like Zeus is right on time for this behavior. I would also recommend supervising him constantly when he is in the house so that you can interrupt him if you see him starting to lift his leg, and confining him to a crate or dog-proofed room when you aren't home to help prevent accidents. A belly band (essentially a diaper for male dogs) can also be helpful in the short term while you are working on the problem.

Is there a reason you are not planning on desexing the older dog? Unless you are planning on breeding him it is best to have him castrated. This behavior of urinating on the puppy is a dominance behavior and it will only get worse. It is best to "nip it in the bud" and the way to do that is to have him desexed. The snapping at the puppy is also worriesome and a sign of aggression/dominance. I'm glad he seems OK now but this behavior could escalate especially as the younger puppy gets older and comes into sexual maturity himself. Just something to think about. Hope this helps. Best wishes.

Given that Jake is not neutered and you have introduced a new male puppy into the home, he may be urinating on the new puppy to declare his status as top dog in the home. He is letting the new puppy known that he is top of the hierarchy in the home, and that the new puppy is below him in status. This reason is most likely if he physically dominating the puppy (standing tall over him) as he does this behavior. I would consider neutering Jake to help reduce hormone related behavior.

It could be that he may have caught the scent of an intact female that is now setting off his sexual behaviors. Some dogs don't show the hormonal behaviors unless a particular scent catches their interest. I acquired one of my dogs (a former show dog) who was entire, and he never showed any hormone driven behaviors while he was still intact (he was intact for about a month) despite being out in public. Every dog varies in how they display hormonally driven behaviors, and I would suspect this recent interest has been trigger by a specific scent.

This can be either a dominance issue, or a submissive behavior, as he is still a young dog. You can use gentle correction at the first sign he may try to mark, but it is not an indication of anything serious. All dogs perceive their world just a bit differently from one another. Just like people. Work with him to discourage the behavior & try not to put him in the position where he needs to mark other dogs. He will hopefully outgrow the behavior. Thank you for using Petco Pet Education Center, formerly Petcoach.

Neutering both dogs at 1 year of age may help if this is related to the female (male dogs may still try to claim possession of her even if she is fixed). If the younger dog is already exhibiting undesirable behaviors, you may look at neutering him now. There is a possibility this is not related to the female. Working with a professional trainer may be your best option. Also, after the neutering, the hormones will still be around for about a month so it’s not instantaneous. I hope this helps.

This is likely a behavioral issue that will be a hard habit to break. He is probably being territorial, and being intact will increase his behaviors. You can try neutering him, removing the female, or confining him when he cannot be monitored.