Back to back postpartum breeding?

[amon]

The female rats are all breeding with one male. I don't remember the number of females per male, but this is very true. The females when pregnant will go to their OWN chamber and nurse their own litter. They only share nursing duties when the rat population is severly over crowded in that area.

Just curious, what happens to other adult males?

What happens when it gets crowded? Do they dig more rooms to live or separate into more colonies?