Q. Our 5-year-old cat Gus weighs 19 pounds and insists on dominating his sweet little sister. After several years of watching Gus bully his sister, we are at our wits end with his behavior.

We adopted the littermates 5 years ago from a local shelter. A couple of years ago Gus had urinary crystals and was in a lot of pain. That is when his behavior became obvious and it escalated from that point on. Though he is no longer in pain and is quite healthy, he continues to jump his sister and bite her backside until she has welts on her back.

Our veterinarian suggested an antidepressant-type medication. Because Gus is naturally inactive, we hesitate to give him the medication, in case it makes him more sluggish.

My husband and I work during the day and hate to leave her home with Gus. Our vet insists that she will eventually begin to protect herself, but she never fights back. She actually runs to him when we discipline him as if to say, "Are you OK?"

We are at a loss and hope you can offer some help. 

A. This is a complicated and common behavioral problem. Medication alone will not solve the problem. Gus needs intense behavioral modification, which may require medication, temporarily.  I highly recommend that you consult a board-certified behaviorist for help with behavioral-modification techniques. Here are some suggestions:

1. Start keeping a diary, logging Gus' attacks and other behavior issues. This will help the behaviorist assess the situation. Such behavior requires much detective work to identify what triggers the aggression and how to deal with it.

2. Separate these cats when they are not being strictly and specifically observed.

3. Get both cats thoroughly checked out by your veterinarian, including routine laboratory tests to identify any abnormalities that may be contributing factors to the problem. Sometimes there is a physical reason for animal aggression.

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