Then, out of the corner of his eye, Chen saw something remarkable. One monkey, rather than handing his coin over to the humans for a grape or a slice of apple, instead approached a second monkey and gave it to her. Chen had done earlier research in which monkeys were found to be altruistic. Had he just witnessed an unprompted act of monkey altruism? After a few seconds of grooming—bam!—the two capuchins were having sex. What Chen had seen wasn’t altruism at all, but rather the first instance of monkey prostitution in the recorded history of science.
