system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Can animals be mentally ill?
gpt
Yes.

Harry Harlow have used a apparatus called the "pit of despair" to create depressed and antisocial monkeys. Basically, he would lock a newly born monkey inside a cage completely devoid of contact from other monkeys. That monkey would eventually become extremely mentally disturbed, unable to cope when reintroduced into a population of healthy monkeys and would simply just sit rocking back and fourth in a corner.

As we can't actually talk to animals, we can only judge if an animal have a mental illness based on visible symptoms, and we have many cases of animals exhibiting pica (eating and craving non-food items like dirt or rocks), or OCD (where neglected/understimulated animals will start to groom themselves nonstop, until they start bleeding).

Furthermore, to facilitate the research of psychiatric drugs, various strains of mentally ill mice and rats have been created (e.g. rats with schizophrenia, depression, OCD). It's not known if these animals are *actually* experiencing what humans with these diseases would experience (after all, we can't interview a rat to see if it's depressed), but these animals all act abnormally in one way or an other compared to normal animals (they are less socially adept, have trouble finding mates, have trouble dealing with new situations, have trouble learning, etc).