For humans, mental illness can manifest in a number of ways, but it's easiest to identify when a person communicates their thoughts and feelings to a doctor.
But what if there is a language barrier? More generally, what if there is a species barrier?
This video from ASAP Thought explores the question "Do dogs and other animals get depressed?" Without language and human expression, it is hard to understand an animals' experience, but some physiological similarities can help us draw conclusions about their mental state.
"With humans, many depressed people show a physical change in their hippocampus, which is the part of the brain that controls emotion," explains Gregory Brown in the video above. "Interestingly, studies on rats have found a similar thing. When the hippocampus is damaged, they start to have symptoms analogous to depression in humans."
Studies have also shown aggressive dogs have lower levels of serotonin and higher levels of the stress hormone cortisol.
Though there is no proven correlation between depression and these changes, in the clip Mitchell Moffit speculates the mystery could "open some doggy doors for diagnosing and treating all sorts of mental health issues in dogs."