I seem to remember reading somewhere that dogs who howl (unless of course they are of a breed whose natural communication is howling, such as huskies and akitas) do so because it is the ultimate way of drawing the pack leader's attention to a threat. From what I remember, there is a sort of hierarchy of sounds dogs use to warn of threats, starting from growls, progressing through barks and finally getting to howls - depending upon how serious they view the threat. Trouble is, I can't remember wher I read it, or who wrote it - has anyone else on here come across this info? If it's true, Claire, it would seem that Cassie felt he was a serious threat and was trying to warn you.