The dog tries to interrupt your interactions with its rival

Jealous Dog : 5 Astounding Signs Your Dog is Jealous
Jealous Dog : 5 Astounding Signs Your Dog is Jealous

If, as soon as you try to pet your new dog or cat, your doggie interposes or asks you too for a caress, this is the obvious sign that your dog is jealous.

The dog is aggressive

Dog Jealousy Aggression : Your dog's jealousy can cause it to adopt negative behaviors and become aggressive. Dog ressiveness will then be directed towards whomever it considers its rival.
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Dog Jealousy Aggression : Your dog’s jealousy can cause it to adopt negative behaviors and become aggressive. Jealous dog behavior and its aggressiveness will then be directed towards whomever it considers its rival.

For example, it may growl if the object of dog jealousy tries to sit next to you on the sofa.