Non-invasive acoustic detection of wolves

Monitoring wolves (Canis lupus) is a difficult and often expensive task due to high mobility, pack dynamic, shyness and nocturnal activity of this species. Wolves communicate acoustically through howling, within pack and with packs of the neighbourhood. A wolf howl is a low-frequency vocalization that can be transmitted over long distances and thus it can be used for monitoring. Elicited howling survey is a current method to monitor wolves in different areas all over the world. Elicited howling, however, may be invasive to residential wolf packs and could create possible negative reactions from the human population. Here we show that it is possible to detect wolves by recording spontaneous howling events. We measured the sound pressure level of wolf howls by captive individuals and we further found that elicited howling may be recorded and clearly identified up to a distance of 3 km. We finally conducted a non-invasive acoustic detection of wolves in a free-ranging population. The use of passive sound recorders may provide a powerful non-invasive tool for future wolf monitoring and could help to establish sustainable management plans for this species.