Im not sure how many animals there are that can experience "depression," but I know that dogs do. A few years ago we had two dogs that had grown up with each other. One of them got sick and was put to sleep because she had a terminal illness and was suffering. We decided at the time not to replace her, but just keep our one remaining dog. Over a period of weeks we began to notice a change in our dog. She no longer had a happy "spirit" if you will. She was not excited easily. She actually began to lose weight and was not eating as often. She layed around more often, her motivation to walk around, run, jump on people became non existent. She was essentially dying.

Realizing that she was suffering from depression because her friend was no longer with her and knowing her health was directly affected by this, we finally decided to get a new dog. At first she did not like the young pup. She was a 10 year old dog who liked to lie around, while this fiesty young pup climbed, jumped and ran circles around her as she slept. Over a few weeks the pup began to grow on her and she became "happy" again. She was not necessarily active as she was old, but the depression she had seemed to be lifted.

Our older dog lived for about 3 more years until she became ill and died. I truly believe she would have only lived a few more weeks had we not got a new friend for her. There are a lot of stories where grandparents die within 6 months of each other, usually because they lose the will to live when their companion for so many years dies. I think dogs are the same when they grow up with a companion.