[Futaba-chan]

Originally Posted by Yui from Okinawa

- hypothesis A- (a lot of theories)
When they goes to fountain and sex is fixed, and it starts and the birth ability is caused. It is not possible to give birth if sex is not fixed.
Only the effect of Estorojen as the girl.

-hypothesis B- (small number of theories)
The Fountain, and the change to the man is begun only by the person who selects a man.
Therefore, it matures as it is as a woman, and birth is also possible if it leaves the society when the fountain is refused and it lives.

That's another very good unanswered question. I think that hypothesis A is probably correct. Among other things,

Spoiler for episode 2:

when the Spring has turned Eri into Erif, Onashia tells him that the Spring has granted him adulthood. That's still ambiguous enough to allow for hypothesis B, but it sounds like it's saying that visiting the Spring makes one an adult... and therefore fertile. The Daikuurikans certainly seem to have developed as female before going to the Spring, including secondary sexual characteristics, and Eri explicitly says that she has been always female up to the point of her conversation on the train with Neville... but that doesn't necessarily include fertility.