Bill Silvester and his son, Kim, had just finished some irrigation chores on their 2,000-acre family farm in Champion, Nebr., one morning in July of 1984 when they saw something that gave them pause. Krause, one of their 12 mules, had foaled. "Well, I'll tell you," says Bill, 64, "we just had to sit down and think about that one," and with good reason. Mules, the offspring of donkeys and horses, are supposed to be sterile. Yet, out there in the pasture, as plain as a jackass's laugh, was evidence to the contrary—a newborn colt, since named Blue Moon (as in "once in a..."). Oh, sure, Bill had seen Krause getting bigger, but he assumed it was "grass belly" from too much grazing.

Now she's done it again. Three months ago Krause produced another colt, White Lightning (as in "lightning doesn't strike twice"). "I figure the odds of a mule becoming a mother are one in five billion," says Oliver Ryder, a geneticist at the San Diego Zoo, whom the Silvesters consulted after calling the American Donkey and Mule Society. Tissue and blood samples of the mother, the father (a donkey named Chester) and the babies have been sent to Ryder, who's coined a name for the hybrids—donkules. As far as anyone knows, Krause is the first mule ever to be subjected to rigorous genetic study. (In the 1920s, a mule named Old Beck reproduced twice, but since genetic technology was in its infancy, scientists expressed bafflement.) "We don't expect this kind of thing to happen," says Ryder, "so when it does, it suggests that our knowledge of reproduction is incomplete."

Eventually, Ryder hopes, Krause's achievement will shed light on the causes of birth defects. Meanwhile, Bill, his wife, Oneta, their three kids and six grandchildren are trying to get Krause to breed again. "It's the family hobby," says Oneta, 58. "We don't go fishing anymore, just mess around with the mules." Folks in Champion are delighted, since Krause's first foal drew several thousand tourists to the farm. "It took Bill Silvester's ass," goes the local joke, "to put Champion on the map."