A mare mule--dam or foster mother?

A female, horse-like foal was apparently born to a mare mule from mating to a Shetland stallion. The presence of three pregnant Shetland mares in the same pasture, however, raised some questions about parentage, even though all three mares apparently gave birth to single foals within 6 weeks after the birth of the propositus. The transferrin system indicated that the foal was not born to the mare mule but was the offspring of one of the Shetland mares. Apparently one Shetland mare gave birth to twins 3 to 6 weeks apart, and the mare mule came into lactation spontaneously and adopted the first twin. The offspring was determined to be a horse by appearance, karyotype, blood type, and voice.