Previously, I wrote about human research applied to parrot diets. This insight greatly opens up the much more prevalent research on humans. We can then supplement the much sparser research on animals, especially those kept as pets.
But how about mental disorders? There is a branch of psychology that tackles this question called trans-personal psychology. It’s not as crazy as it sounds! The article below has 40 bibliographic references so it’s not exactly a fringe line of thinking.
Trans-species psychology is the field of psychology that states that humans and nonhuman animals share commonalities in cognition (thinking) and emotions (feelings).Trans-personal psychology—Wikipedia
There strong evidence that animals suffer from mental disorders, especially in captivity. Here is a long quote on post-traumatic stress disorder (PTSD).
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PTSD, a pervasive psychological disorder, common to human prisoners of war, genocide survivors, and victims of domestic violence. It is characterized by difficulties with severe mood dysregulation, impaired interpersonal functioning, loss of sense of safety and security, and disruption in the sense of self.