Well, the whole species came to be specifically because of the magics from the Keepers, casted specifically to give the Aspects their helpers and consorts. Maybe the only reason they were able to reproduce from the beginning was because of that magic were always there. Also, What Florena said :P Although, just a reminder that it was just headcanon. Lore-wise, I don't think we ever got any explanations.
Je veux le sang, sang, sang, et sang
Donnons le sang de guillotine
Pour guerir la secheresse de la guillotine
Je veux le sang, sang, sang, et sang.