By: Kristina Jacobsson

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Have you ever wondered about the mental health of your pet or the animals you see at the zoo? What about those in the wild? I never really thought about these concepts until recently, during a class entitled, Evolutionary Medicine. 

“Before I begin, I am going to ask you to check your human exceptionalism at the door.” It was the first day of class and this was Professor Natterson-Horowitz opening remark. 

After pausing to let this comment sink in, she continued: “What you will learn over the course of this class, is that hardly any disease, illness, or disorder is specific to humans. They are prevalent throughout the animal kingdom, and having this viewpoint can allow us to better understand etiology, manifestations, and treatments.”

A former cardiologist, Professor Natterson-Horowitz is dedicated to synthesizing evolutionary and cross-species perspectives on illness, believing that this can yield new insights in medicine. 

What do we know about behaviors related to mental illness in non-human species?

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Believe it or not, mental illness is prevalent in the animal kingdom. Cats over groom, viciously licking all their fur off. Birds commonly have feather-pecking disorders. Grasshoppers binge-eat high sugar foods when they are stressed. Both in the wild and in the lab, some animals and insects even abuse drugs and alcohol. 

And all of these behaviors seem to have an evolutionary basis. For example, grooming allows for cleanliness and sociability. Binge-eating foods that are high in sugar provides animals with energy, preparing them for danger. While largely maladaptive in humans, these behaviors have the potential to be adaptive in other species.

Well, what about depression? 

What about extreme forms of behavior that might appear to mimic mental illnesses, like depression? How does depression manifest in non-human species? Is there an adaptive value of depression? Even more, can you actually tell if an animal is depressed? 

Surprisingly, there are a variety of ways scientists study depression in animals. A lot of this work has focused on the effects of maltreatment or adverse rearing conditions on different symptoms of depression. 

The hallmark symptoms of depression in humans include: anhedonia (loss of interest or pleasure in activities) and difficulty concentrating. These symptoms can be measured in animals. Rats who are raised in isolation decrease the amount of sucrose they drink in adulthood. They also experience loss of concentration and have more difficulty completing a maze. The same goes for monkeys, who experience changes in posture and locomotor difficulty after isolation in childhood. And female baboons socially isolate themselves after adverse childhood experiences.

Depression may hold adaptive value in animals. In fact, animals who experience depressive symptoms after being bullied tend to be safer; they submit to the bully and stay away from further danger. 

Like Professor Natterson-Horowitz, I think that it could be highly beneficial to “check one’s human exceptionalism at the door” and look for evolutionary similarities and differences across species. Doing so may give us some important new clues about the causes of depression in humans, which could lead to new approaches to prevent this disorder.

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