Quote:
In contrast to seekers of the sun, with their male-dominated tribes and harems, the fundamental unit of keeper of the moon society is the family, centered around a strong mother figure and children. Keepers are known to form small communities composed of two to three families who hunt together.
This is purely about culture and has no bearing on birth rate, and is also kind of irrelevant as this is disregarding integration into eorzean culture (compare yugiri to the tribes on the azim steppe as an example). I could potentially believe that seekers are rarer due to in-fighting for power, but keepers should at least be visible within society more than it is.