The easily detected behavior for a human observer is the approach, since the wolf can be both heard and seen by the experimenter. How often will wolves approach a howling stranger? Who is most likely to respond?

Reference: Howl in Reply Keep Silent
Approach Move Away or Hold Ground Approach Move Away or Hold Ground
Harrington, 1986 18 approaches: pups approached recorded adult howls 9 times and recorded pups once; adults approached recorded adult howls 6 times and recorded pups twice. 117 replies to either recorded adults or pups with no approach. Replies were made by adults and/or pups. There were 137 total responses in 243 attempts to elicit a response. 104 attempts with no vocal or other observed behavioral response.
Harrington and Mech, 1978 7 approaches by adults; 5 involved either an alpha male or alpha female. All responses were to live human howling. 487 responses to human howling with no approach. At non-rendezvous sites, wolves held their ground 90% of the time and retreated only 10% of the time. At rendezvous sites, wolves remained 95% of the time (this included sessions with no vocal response). 6 approaches from pups with no vocalizations. Wolves at non-rendezvous sites remained about 70% of the time. At rendezvous sites, wolves remained 95% of the time (this included sessions with a vocal response).