The Coolidge Effect | Reuniting:
When you drop a male rat into a cage with a receptive female rat, you see an initial frenzy of copulation. Then, progressively, the male tires of that particular female. Even without an apparent change in her receptivity he reaches a point where he has little libido-and simply ignores her. However, if you replace the original female with a fresh one, the male immediately revives and begins copulating again. You can repeat this process with fresh females until the rat nearly dies of exhaustion.
The rat's renewed vigor does not reflect an increase in his wellbeing - although it will look (and temporarily feel to him) that way. His vigor comes from surges of a neurochemical called dopamine, which flood the reward center of his primitive brain... so that he gets the job done.
In short, animals do not choose their mates randomly. They identify and reject those with whom they have already had sex. Scientists know this reflex as the "Coolidge Effect." It earned its name many years ago when President Coolidge and his wife were touring a farm. While the President was elsewhere, the farmer proudly showed Mrs. Coolidge a rooster that "could copulate with hens all day long, day after day." Mrs. Coolidge coyly suggested that the farmer tell that to Mr. Coolidge, which he did.
The President thought for a moment and then inquired, "With the same hen?"
"No, sir," replied the farmer.President and Mrs. Coolidge
"Tell that to Mrs. Coolidge," retorted the President.