When someone has drunk a lot in an evening, orders that one drink too far, takes a sip, places it on the bar and walks out without any goodbyes or acknowledgement to anyone.
Person 1: Where's Steve?
Person 2: I don't know, maybe he left. He was pretty far gone.
Person 1: Probably answered the Call of the Wild.
The urge one sometimes experiences to masterbate. Some people when they have to use the bathroom say they need to answer the call of nature, so when one gets an "urge", they can say they need to answer the call of the wild.