Apes are peaceably creatures, but the humans destroy their living space by cutting down woods of the Cedar Forest. As they don't have really brains, they start to believe to become smarter and more powerful by eating humans.
The humans destroy their living space. So they need to forest up their home, but Lady Eboshi hinder them in their working. Still San encourage them to continue to work. However, more trees get cut down than they can plant new ones.