It's nice to see people rezzing older topics like this to add things on...
Anyway,
Already I'm surprised at why they domesticated animals like dogs and cats (you know, forest/plains dwellers) and not things like crundles as pets, and dralthas instead of cows.
Yaks and mountain goats yes, because they shave the same starting biome. But waterfowl ? Hell, cows, which require large amounts of grass, and not cave fungi, to eat ?
I presume it has mostly to do with the fact that in the 2d and 40d eras, there were no domesticated cavern creatures, besides maybe purring maggots.
It is, as Bumber says, a decent assumption that they simply bought dogs, horses, chickens, and so on from humans in trade.
In any event, I've been more interested in the idea of gas/air quality as a means of forcing some engineering challenges.
Draco18s had some detailed arguments back in 2010 that I still think are the best arguments, yet. (Although I still prefer not to use the exact term "phlogiston", and instead just use "fresh air" and "bad air".)
Currently, there's nothing stopping you from digging a vertical up/down stairway straight to the magma sea in the first Spring, and largely just bypassing all the caverns. Requiring of players that they build more elaborate ventilation systems the more vertical they build (or at least, setting up air-recycling lichen farms underground or something else that is interesting, an engineering challenge, and time-consuming,) would make the separate layers of caverns things to actually stop at, especially if they bring with them some extra fresh air.