There's dark areas in the zone around the totem (I have yet to place any lighting) and I'm now up to 10 dwarves.

Early on, like at 3 dwarves, I opened up a tunnel that happened to have an ant nest, and it spawned a rats nest close enough that ants took it out, so I closed it off. When I became strong enough, I went after the ants. But like I said, despite plenty of places a rats nest could spawn, and no lights, I'm not getting a nest.

Is there a limit to how late in the game they spawn? Anyone know what the rules are?

Oh, and I forgot to mention, I'm at 29 comfort.

The only metric I am aware of is comfort level 50, but I understand they dropped that some time ago to mid 30s? Otherwise its just the light level as you mentioned, which cant be measured in game. Try placing some beds and food if you have not already done so, perhaps that draws them.

Also, I think only one nest will spawn at a time. Are you sure the ants destroyed the nest and not just the rats? Maybe another thing to check.

EIT: Actually just dawned on me... I dont think light is as much as a factor for rats as it is spiders and such. Try placing a torch or two as well. Couldn't hurt you at this point.