Speaking of rats...
Coolidge effect
In biology and psychology, the Coolidge effect is a phenomenon—seen in nearly every mammalian species in which it has been tested—whereby males (and to a lesser extent females) exhibit continuous sexual activity if they are introduced to receptive sexual partners,[1][2][3][4] but will eventually stop having sex if the partner has already been copulated with.
The original experiments with rats applied the following protocol:[6] A male rat was be placed into an enclosed large box with four or five female rats in in heat. He immediately began to mate with all the female rats again and again until eventually, he became exhausted. The females continued nudging and licking him, yet he did not respond. When a novel female was introduced into the box, he became alert and began to mate once again with the new female. This phenomenon is not limited to common rats.[7] The Coolidge effect is attributed to an increase in dopamine levels and the subsequent effect upon an animal's limbic system.[8]
Human males experience a post-ejaculatory refractory period after sex. They are temporarily incapable of engaging in sex with the same female after ejaculation and require time to recover full sexual function. In popular reference, the Coolidge effect is the well-documented phenomenon that the post-ejaculatory refractory period is reduced or eliminated if a novel female becomes available.[9] This effect is cited by evolutionary biologists as one reason why males are more likely to desire sex with a greater number and variety of partners than females,[9] though of course sometimes human females are known to copulate with multiple and novel partners as well.
While the Coolidge effect is usually seen demonstrated by males—that is, males displaying renewed excitement with a novel female—Lester and Gorzalka developed a model to determine whether or not the Coolidge effect also occurs in females. Their experiment, which used hamsters instead of rats, found that it does occur to a lesser degrees in females.[3][4]
