Measuring Animal Welfare Part Six: Can Animals Have Mental Illnesses?

This is such an on-brand post

Photo by Francesco Ungaro on Unsplash

[1, 2, 3, 4, 5]

As we’ve learned more about animal cognition, we’ve realized that many things we thought of as innately human—tool use, flexible problem-solving, emotions, complex social dynamics—are common among many species. So what about mental illnesses?

To some extent, the answer to whether nonhuman animals can have mental illnesses has to be “yes.” Look at the original learned helplessness research. Dogs that have been repeatedly electrically shocked with no control over the situation—a traumatizing event—don’t try to escape electrical shocks when escape is available. Instead, they just lie there and whine. That… is obviously mental illness, right? If a human did that, it would be mental illness. The fact that people keep saying “learned helplessness causes depression” implies that dogs can be depressed. I’m not saying that shrimps can have OCD or bees can have schizophrenia,¹ but the obvious interpretation of what we know about animal cognition is that many species of animals can be mentally ill.

Ferdowsian et al (2011) adapts the DSM-V criteria for depression and PTSD to apply to chimpanzees. This is a somewhat quixotic thing to do—it’s far from obvious that the DSM-V correctly categorizes the mental illnesses of the species it was developed for—but still interesting. The chimpanzee PTSD criteria are:

A. The chimpanzee experienced, witnessed, or was confronted with an event or events that involved actual or threatened death or serious injury, or a threat to the physical integrity of self or others.

B. The traumatic experience is persistently reexperienced in at least one of these ways:
1. Emotionally upset by reminders of the traumatic event.
2. A physical reaction to reminders of the traumatic event (e.g. goosebumps, heavy or irregular breathing).

C. Persistent avoidance of stimuli associated with the trauma and a numbing of general responsiveness, as indicated by at least three of the following:
1. Avoidance of certain activities, places or types of places, or certain individuals or groups (human or chimpanzee) that may arouse recollections of the trauma.
2. Lack of interest in play, food, other individuals, or grooming.
3. Social withdrawal.
4. Less variability in social expressions compared to other chimpanzees.

D. Persistent symptoms of increased arousal, as indicated by at least two of the following:
1. Awake or easily awakened during evening observations, difficulty falling asleep, or excessive sleep.
2. Excessive outbursts, or easily irritated or angered.
3. Poor attention to tasks or difficulty concentrating.
4. Extremely watchful or “on guard.”
5. Easily startled or jumpy.

Chimpanzee Depression Criteria:

Five or more of the following, including at least (1) or (2):

1. Depressed hunched posture, social withdrawal, or easily irritated or angered.
2. Loss of interest in food, play, other individuals, or grooming.
3. Unexpected weight loss, failure to gain weight, hoarding or gorging food, or excessive weight gain.
4. Awake or easily awakened during evening observations, difficulty falling asleep, or excessive sleep.
5. Restlessness or sluggishness.
6. Poor attention to tasks or difficulty concentrating.

3% of wild chimpanzees meet criteria for depression and 0.5% for PTSD. Conversely, 58% of chimpanzees living in sanctuaries² meet criteria for depression and 44% for PTSD.

These criteria closely mimic the human criteria for depression and PTSD. Most of the modifications are simply to account for the fact that you can’t ask a chimpanzee “do you have nightmares?” or “would you say you feel like a worthless person?” Nevertheless, they can be meaningfully applied to chimpanzees, with a reasonable degree of interrater reliability, and with results that basically track the intuitive result that being a subject of nonconsensual medical experimentation is bad for your mental health.

Bradshaw et al (2008) go beyond this research and discuss complex post-traumatic stress disorder in chimpanzees. The paper is difficult reading for anyone who loves these intelligent and beautiful animals, but I think the case studies inside it make the case for chimpanzee trauma in a way that a quantitative study can’t. The chimpanzees experience dissociation, anorexia, phobias, inappropriate anger, and self-harm. Setting aside whether CPTSD carves reality at the joints for any primate, it’s hard to read the paper without going “yes, the thing that these animals have is the same that traumatized and mentally ill humans have.”

Chimpanzees are not the only animal that has been so studied. Rizzolo and Bradshaw (2016) examine CPTSD in Asian elephants. While you can’t ask elephants whether they experience posttraumatic stress, you can observe their behavior, as you do with chimpanzees. Traumatized elephants behave similarly to traumatized humans: poor impulse control; flinching when they expect violence; being easily startled; self-injury; social withdrawal; even agoraphobia.

As strange as it might seem to say that a nonhuman animal can have a mental illness, I believe the most reasonable interpretation of the evidence is that some nonhuman species experience depression and trauma. The research I’m aware of has mostly been done on rather “sophisticated” species; chimpanzees are one of humans’ closest living relatives. I am very interested in research on less sophisticated species, as well as on a wider variety of mental illnesses. Which species, if any, can be psychotic? Do some chimpanzees have OCD? Can, perhaps, the zebrafish actually have autism after all?

1. Although they do keep trying to give the zebrafish autism.

2. Chimpanzees living in sanctuaries were experimented on, orphaned, illegally traded, or attacked by humans.