The Coolidge Effect vs. The Bruce Effect

A term used in biology and psychology, the Coolidge effect describes a phenomenon whereby males show continuously high sexual performance given the introduction of new receptive females.

The term comes from an old joke, according to which President Calvin Coolidge and his wife allegedly visited a poultry farm. During the tour, Mrs. Coolidge inquired of the farmer how his farm managed to produce so many fertile eggs with such a small number of roosters. The farmer proudly explained that his roosters performed their duty dozens of times each day.

"Perhaps you could point that out to Mr. Coolidge," pointedly replied the First Lady.

The President, overhearing the remark, asked the farmer, "Does each rooster service the same hen each time?"

"No," replied the farmer, "there are many hens for each rooster."

"Perhaps you could point that out to Mrs. Coolidge," replied the President.

The Bruce effect is a strange phenomenon, whereby if you take a recently impregnated female mouse and place her in a cage with an unfamiliar male, the female, upon smelling the new male's urine, will often spontaneously abort her pregnancy.

What happens is the mother's body  drastically reduces its production of prolactin, a hormone responsible for progesterone secretion and thus essential to maintaining a pregnancy. The embryo doesn't implant and the female begins ovulating again. The male mouse can then get busy putting his move on the female mouse.