It is partly a matter of appearances, with zoo animals less active at peak visiting hours, and partly a matter of normal rhythms of carnivore life.
Lions in the wild are normally inactive for twenty to twenty-two hours a day because they need to conserve their energy for hunting, never being quite sure where their next meal is coming from.
As for the house cat, although it does know when the next can will be opened, the basic behavior pattern is the same. Even a cat that has never seen real prey will stalk a butterfly through the window.
Young animals can afford to do that, but from an evolutionary point of view, old lions especially must conserve their energy for the business of surviving. At some point in life, when the metabolism does begin to slow down, a sedentary cat may become overweight.
Of zoo animals, many are from hot climates and are most active in early morning and late afternoon, zoo-keepers say. They sensibly lie low from eleven to three on hot days.
Zoo animals have the benefit of a nutritional staff that prepares diets as close to natural as possible. Modern zookeepers also make sure that animals mimic natural behavior, living in groups, with plenty of space, and foraging and competing for food.
Some do tend to get chunky, like the dominant animal in a group that always gets its fill, but most stay lean.