* Parasites: Fleas, ticks, and other parasites can irritate their skin.
* Dirt and debris: Monkeys live in environments where they come into contact with dirt, leaves, and other substances that can get stuck in their fur.
* Allergies: Some monkeys can be allergic to certain foods or substances.
* Skin conditions: Just like humans, monkeys can develop skin conditions like eczema or psoriasis that cause itchiness.
It's important to remember that scratching their butts is a natural behavior for monkeys. They don't do it for amusement or any other reason. It's simply a way to relieve an uncomfortable sensation.