After a male mammal mates with a female mammal, he passes through a refractory period during which he cannot mate or ejaculate again. Several biologists observed that the refractory period decreases considerably if the male encounters a new female instead of the female with which he just mated. This effect is known as the Coolidge effect.
The effect takes its name from an apocryphal story about our 30th President. Once upon a time, Coolidge and his wife were visiting a chicken farm for whatever reason. Coolidge's wife was surprised to see that the farm contained several dozen hens but only one rooster. She asked the farmer about it.
"Well, ma'am," said the farmer, "begging your pardon for mentioning it in a lady's presence, but a single rooster can mate many times a day!"
Mrs. Coolidge raised her eyebrows and pursed her lips. "Perhaps," she said, "you could point that out to Mr. Coolidge" (who was standing beside her).
Silent Cal blinked. "The male services a different hen each time, correct?"
Coolidge nodded. "Perhaps you could point that out to Mrs. Coolidge."