I usually hear the outdoor cats calling to my cats, who are strictly indoor, in taunting howls. But, my cats also howl for other reasons. I noticed my female will howl if she is calling me to help her. For example, when my male cat is bothering her for the cat tree position or wanting to clean her (which is actually another territorial act) to claim her, she howls for me. She will actually get up and find me. The whole time she howls until she finds me. My male cat has tried to howl with my Rott. That's new and wierd. I'm not sure if the dog is howling to scare off the outdoor cats who are terrorizing my indoor cats; or, if the cat is howling to chime in with the dog just being a dog.