Why are the animals locked on zoo tycoon for the ds?

In "Zoo Tycoon" for the DS, animals are often locked to encourage players to progress through the game by completing specific tasks or challenges. This design choice adds an element of strategy, as players must effectively manage their zoo and meet certain criteria to unlock new animals. It also enhances the gameplay experience by motivating players to explore different aspects of zoo management. Unlocking animals can provide a sense of achievement and satisfaction as players build and expand their zoos.