Many people bite their fingernails or skin when they feel stressed or bored. Dogs and other animals also bite themselves when stressed or bored.

Some cats commonly show signs of obsessive-compulsive disorder.

These cats repeat behaviours again and again when only harm will result.

For instance, some cats lick themselves so much that they rub their fur off in spots.

This behaviour is similar to how some humans create bald spots by pulling out hair after hair on their head.

Many humans ruin their life with repeated use of alcohol or other drugs. Some animals also abuse psychoactive chemicals.

Cats get so stoned on catnip that they hallucinate and think they are stalking a prey when there is none.

Dolphins squeeze puffer fish (do not try this at home) to get them to release a neurotoxin that has a pleasing effect. The dolphins then pass the fish to another dolphin like a human passing a joint.

Humans develop phobias to all sorts of non-dangerous stimuli – elevators, mice, fruit.

Animals also develop phobias – of lightning, strangers, heights.

If an animal has been abused by a man who wears a black hat, black hats may become the phobic item.

Military dogs in war zones sometimes develop post-traumatic stress disorder. 

Seeing its beloved handler get shot dead would be stressful indeed.

Some animals show signs of depression. These signs include not eating and low general activity. Some primates also show depressed facial expressions.

Veterinarians often prescribe human anti-depressant drugs for dogs who have behavioural problems.

The drugs seem to help, but there are no large studies showing that they are more effective than a placebo.

Do you think that only humans intentionally kill themselves? Think again.

Highly stressed dolphins and dogs have been observed intentionally drowning themselves. Some dogs starve themselves to death after their human dies. 

I like to tell people about animal psychological disorders to make the point that humans with disorders are not weak willed or lacking in character.

Psychological disorders typically result from a combination of biological and social influences.

We and our animal friends do not choose to have a psychological disorder any more than we choose to have kidney disease.

On the bright side, humans and animals seem content most of the time.

You might say that we are usually as happy as a lark.