What was Lowell's first step in becoming an animal activist in the novel, We Are All Completely Beside Ourselves?

After Fern’s experiment is considered a failure, Rose’s father started to do more basic experiments with lab mice. The night that Lowell decided to leave one of the last things that he did was release all of the mice that their father had in the house. For Lowell, this was his first step into becoming an animal activist.