When a female horse meets a male donkey, the pitter-patter of little mule hooves often follows. Yet the offspring have an odd number of chromosomes, which nearly always means that they are sterile and can't reproduce.
But a female mule in Colbran, Colo., has recently become a mother, and her owners are trying to figure out how it happened.
In late April, Laura and Larry Amos discovered that a mule called Kate had a newborn foal. The Amos family runs a wilderness outfitter and owns a large herd of mules.
Laura Amos tells Robert Siegel that she and her husband realized the birth was a rare event and knew that there would be skeptics.
Hair samples sent to the University of Kentucky and blood work submitted to the University of California, Davis, verified yielded the same results: verifying that the samples came from a mule and her offspring.
Amos says that further genetic testing will provide more answers about the unnamed foal's origins, and much will depend on what genetic information his mother passed on.
Horses have 64 chromosomes, and donkeys have 62. Amos says the baby could be a mule (with 63 chromosomes), a donkey (with 62 chromosomes) or a chimera — an animal that has genetic material from different species.
Until the mystery of the foal's birth is resolved, Amos says that mother Kate will no longer work as a pack animal.
"Maybe I'll look for an agent for her," she says.