Actually, if you check carefully, you can see them in the fog of war to proliferate. Or their reproduction depends on the map makers? Your choice.
Ursadons were probably wiped out during Resurrection IV (the little known map that took place on Braxis, and gave us no-longer-infested-but-still-alive Alexei Stukov). I presume they mate like any mammal. (There's a weird ursadon in Dark Origin that "gives birth" to a broodling. Don't believe me? Kill all the ursadons (there's only three) in that map, and you'll see a broodling come out. Mind control it for more fun!)
They're aliens so they shouldn't do it the Earth way. My theory is that giant kakarus bring baby ursadons.
Global warming on braxis? :O damn colonists! Rule 34 people, i'm sure you could find out if you really wanted...
I like the earlier idea that, like nerve cells in the human body, the universe was created with a finite number of ursadons, spread out among the cosmos. Once one is gone, the total number of ursadons ticks down a notch forever.
