Half Brother by Kenneth Oppel

Have you ever thought that your brother or sister wasn't even human?  Do your siblings act like animals?  Ben Tomlin can relate to that - his half-brother doesn't just act like an animal, he IS an animal.  He's a chimpanzee who has been adopted by the Tomlin family as part of an experiment being run by Ben's parents who work for a university.  Ben isn't sure he wants his quiet life disturbed by an animal who quickly becomes a celebrity as he learns to communicate.  It isn't long before a special bond develops between Ben and Zan, with Ben becoming very protective of his first "brother".  When Zan's behavior becomes too difficult to manage, Ben's parents make a decision about a new life for Zan without taking into account Ben's close relationship with him.  Ben, disagreeing with their decision, takes matters into his own hands to save Zan.