In my opinion, the idea of dog depression is a byproduct of the outdated biogenic amine theory of depression. It is the idea that depression is caused by a lack of certain neurotransmitters. It would follow that if this were true for humans, and animals use the same neurochemicals, then animals could also suffer from chemical depression. This is the theory that underlies the design of all the new antidepressants on the market, even though the theory doesn't hold much water when considered from a comprehensive perspective.
However, when it appears to you that your dog is depressed and you are wondering, "What are the signs of dog depression?", you should instead wash the idea of depression from your mind and think about nutrition.
If you are in a position to ask yourself if your dog is depressed, then you are almost certainly a caring enough pet owner to not have a depressed dog. I will not rule out the idea that a dog can suffer from grief, despair, or sadness, but you must also consider that the canine psychology is far simpler than human psychology. To call a dog's state of mind "depressed" is even sloppier than the already-sloppy applications of the term "depression" in human beings. I feel this way about all types of pet psychology and most so-called "pet psychologists."
To put a dog on antidepressants is simply nonsensical. If you want to know the causes of dog depression, I will tell you the primary one: the kibble that you feed your dog is comprised primarily of grains. A dog's system has not evolved in the presence of large amounts of this type of food, and so your dog is probably suffering from chronic fatigue secondary to metabolic syndrome. In 9 out of 10 cases, you will be able to completely reverse this condition by finding a dog food that is lower in carbohydrate and higher in fat and protein. Replace a meal a day with heart, liver, or other organ meat from your butcher (usually very cheaply!) and your dog with most likely surge with energy.
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