In addition to the moon-gazing theory, there is another story which says that all animals which chew their food and have nine bodily cavities carry their young before birth (as opposed to hatching their young from eggs).
除瞭望月而孕、口中吐子之說，另有一說是：凡咀嚼的動物都九竅而胎生，唯獨兔子雌雄皆只八竅。