The original experiments with rats applied the following protocol: A male rat was placed into an enclosed large box with four or five female rats in heat.[13] He immediately began to mate with all the female rats repeatedly until he eventually became exhausted.[13] The females continued nudging and licking him, yet he did not respond.[13] When a novel female was introduced into the box, he became alert and began to mate once again with the new female.[13] This phenomenon is not limited to common rats.[14]