We recently purchased two new female cats (aged 1 and 2) from the local human society and brought them home and introduced them to our resident male cat (aged 5) we saved from abandonement who is a very mellow guy and has been with us for about 6 months. One of the new cats is now terrorizing the male cat and looks for opportunities to chase him and fight with him. Even though he is twice the size of this new cat, he runs away and hides. He now is beginning to show some resentment to us and is very cautious when walking around his house. What can we do to reign in this new cat so that they can get along? Our male cat did not originally display any aggression toward the new cats, but now hisses at them and growls. There does not seem to be any real hostility between the other new female cat and the male beyond normal "getting to know you" interactions.