I think it's a sign of him trying to show who is alpha with the cats....suggestion of something to try:
You show more alpha behavior with him for awhile, and when he goes for the cats, step inbetween him and the cats, to show that they are "yours" and handled and that he doesn't need to worry about them.
You may want to have him leashed for awhile to get more control over it for a few days.
And also what another poster said, provide a place for the cats to escape - perhaps a tall cat tree or something.
My two cents - take what you like and leave the rest. |