Owning a large dog is a responsibility. A doberman, is a big responsibility. Two dobermans, and you have your work cut out for you.
Trust me, I know what I'm talking about. My family has raised Akitas, and I've been attacked and had surgery & rehab. I will never be the same. I would not own one of these agressive dogs, without it being trained. As dogs mature & get older, they get more agressive & less able to be retrained. The fact that the 4 year old is now a bully fits in.
If you do not trust the dog, the dog will sense it. You could be in danger. Some dogs will attack only other dogs. Other dogs will attack anything or anyone, given the right circumstances. This is especially true if he thinks that he has to protect something (you, a bone, himself). This is also true if he thinks he can "alpha" over the victim. If you are the least bit afraid, the dog will "alpha" over you. To lower the dog, you need to physically "bring the dog down" by putting yourself physically over the dog & just about laying on him (bearing teeth & all). If the dog does not allow this or you are afraid to do it, every person (and dog) is in danger.
Dogs that are not walked alot, get excited about going for a walk. This is a HUGE outing for them. Dobies are high energy dogs, and need to go for walks (for their brain & their body). Being cooped up in the same surroundings is not good. Sometimes when dogs get VERY excited, they forget their brains and act on basic instinct. It could be that he was "GEE, I'M going for a WALK!!!!" in his head, and then lost all control. By not taking him for a walk, you could be compounding the problem for him. I agree that you need a choke chain, and you need to be trained on how to use it. A short, quick jerk is how it is used. A long, slow letting them "strangle" themselves is not how to use it. He needs to learn that you are the lead.
Dobbies are VERY smart dogs. They are used in police work, which requires brains & muscles. They need constant stimulations - different walks, different routines, different tricks. A dobbie mind is a terrible thing to waste. If the dog is bored, then anything & everything will become exciting. When dobbies are in an excited state, they may attack. So, while today it's a long awaited walk....tomorrow it could be that the neighbor's kid is running around or visitors dropped by.
Fixing a male dog, lowers its instinct to be protective and to bite. I am surprised that the nuetered male bit! You may have a really big problem.
I do not know what state you live in, but a vicious dog can leave the owner open to lawsuits should it attack a person or another animal. If it harms a child or a person, God forbid, they could sue your pants off & take away your house, your bank account, and garnish your wages for several years into the future. How much house liability insurance do you have? Some insurance companies are not covering these "known" bad breeds of dogs, namely Pits, Dobermans, and Akitas. Second, the house liability insurance will likely not cover the dog if the dog attacks while not on your property. Do you have a personal umbrella policy? Does it cover Dobermans. A dead kid is worth several million dollars in court, and a maimed one is worth much, much more (future medical bills, lost wages, pain & suffering, loss of companionship, etc $, etc $, etc $).
In short, get this dog some o-b-e-d-i-e-n-c-e lessons with a qualified dog trainer. He will tell you if he thinks the dog can become an honest citizen or a liability/loaded gun. Together, you, the dog trainer, and the dobie may be able to learn how to go for a proper walk. Dobermans are bred to be protectors. When a daschund digs a hole, everyone says, "Daschunds dig, that's normal." When a doberman is agressive, it's the doberman's nature. Dogs whose ears stand up are, generally, more agressive. It's a proven, hormone driven behaviour. Hormones make the ears stand up. Gentler dogs have less of this hormone. That's why Basset Hounds are very docile.
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