Accompanied by his trusty guide, he seeks out a very remote locale for researching the mating behaviour of the giant rat of Sumatra. Around dusk of the first day, he's sitting by the campfire with his guide when in the distance, he hears tribal drums. They get louder. The dusk turns evening. The drums get louder.
Evening turns to dead of night.
The drums get louder and louder, until it is obvious that the drummers must be quite close. An anthropologist decides to investigate the natives of a far-flung tropical island. He flew there, found a guide with a canoe to take him up the river to the remote site where he would make his collections. About noon on the second day of travel up the river they began to hear drums. Being a city boy by nature, the anthropologist was disturbed by this.