User:Soap/UMB

The uterine microbiome bacterium (UMB) leads women along the north coast of Repilia to miscarry male babies shortly after conception, resulting in a birth sex ratio of nearly 100% female. The rare male births are typically preceded by abnormal menstruation. These males are enough to sustain the civilization, because each man mates with dozens of women, but nonetheless the Repilian women along the coastal colonies often seek men outside their colonies for husbands. This means that almost all men in the colonies are outsiders. Since the UMB is dietary (and thus environmental), rather than genetic, these men and women see each other as kin, rather than there being a coastal lineage and an inland lineage, or likewise a tribal or racial boundary. (UMB can also stand for Uterine MicroBiome.)