In addition to howls, wolves can also produce whimpers, growls, barks and squeaks (Mech, 1970). Whimpering tends to serve as either a submissive or friendly greeting sound, since young wolf pups and wolves attempting to appear submissive often whimper. Wolves growl when they are attempting to threaten another wolf or are behaving aggressively. Wolves rarely bark, but may do so as an alarm call or during play. Captive wolves who have been exposed to domestic dogs may bark more often than wild wolves or captive wolves who have not been exposed to domestic dogs.