The random factoid comes from this book on maternal behaviors, and has to do with infanticide in mice. Researchers couldn't figure out why some male mice would commit infanticide sometimes, but not others. And then some brilliant soul, in 1994, discovered that some species of mice have a--brace yourself--"ejaculation clock." Once the earth moved for the male mouse, a timer started up in the 'ol hind brain that lasts twenty-three days, the length of gestation/nursing. At the end of that time, the male mouse becomes infanticidal, because there is absolutely no chance he could have fathered any of the babies he's killing. If the mouse ejaculates at any point during that, then wham! Timer resets, because a male mouse doesn't want to kill his own offspring. A bizarrely elegant solution to the question of how to let your own young live, while turning the young of rival males into handy protein.
I realize this is a little weird and disturbing, but god, ain't nature cool?