In the wild, animals live in vast, complex, ever-changing environments. Their world is unpredictable, and no two days are the same. Their days are filled with finding food, seeking a mate, looking after young, interacting with others, finding safe places to rest, defending territories, and so many other engaging activities.
The sights, sounds and smells of their natural environments constantly stimulate their senses. Their minds are active, their bodies are exercised.
In a captive environment, their living conditions are very different. Their living environments are far less complex and more static, and life is far more predictable. They have less space, less objects in their surroundings, less sights, sounds and smells. Their companions are more or less constant.
Added to this, they usually have to cope with being in close proximity to humans- something they usually wouldn’t have to face in the wild. They are frequently subjected to intense viewing pressure from zoo visitors, often all day every day, which can be very stressful.