Dogs can't articulate feelings of jealousy like humans do, so when they think they're getting the short end of the stick, they act out. A dog's jealousy may take the form of aggression -- so when you sense that your pet feels left out, you need to step in and resolve the situation. To prevent your dog from acting jealously around other pets, adult humans or babies, you have to gently condition him to share that which is his and make sure he always feels he is getting the attention he deserves.
Introduce environmental changes gradually. Changes to a dog's environment, like rearranging the furniture, can upset him. If you make moderate changes in the home at the same time as you introduce a new housemate -- a person or a pet -- he may associate the disruption with the new person or pet.
Bring a new housemate into your home gradually. If you plan on bringing home another pet, or if another person or baby is moving in, start having guests over and invite them to bring pets. For example, if you're about to have a baby, invite guests with babies and young children. If a significant other is moving in, make sure he spends plenty of time in your home before they do, so your dog doesn't feel threatened by his sudden constant presence.
Slow down the introduction process. Dogs can be territorial, and a newcomer suddenly having access to your dog's favorite areas, he may become jealous and protective. Introduce a new pet slowly by keeping the two separated but in sight of one another. Gradually allow them to spend supervised time together, lengthening the play sessions until they play and coexist without any territorial fighting.
Maintain your dog's routine as much as possible. Dogs thrive on routine; when their schedules are interrupted, they can become agitated at whatever they perceive as the cause. Keep taking walks, brushing, feeding and doting on the original pooch at the usual times.
Give your dog equal treatment with other pets. For example, if your dog and another pet are in the same room, don't give the other pet a treat without also rewarding your dog. If you have two dogs, make them do a trick for affection or a treat, so that they always know how to earn your attention and don't mistakenly interpret any of your actions as favoritism.
Ignore "bad behavior" attempts to get your attention. When your dog acts in a so-called jealous manner to get your attention -- even negative attention -- he may see your giving in as a victory. If you're paying attention to one of your pets and he or another lashes out, such as by barking or chasing away the other, give him a firm "no" and walk away for a few minutes. The only way that he'll stop acting out this way is to learn that doing so deprives him of what he's after: Your attention.
If your dog continually acts aggressively toward another human or animal, consult a trainer or behaviorist.
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