Quote Originally Posted by amon View Post
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?
From the link I provided http://www.ratbehavior.org/WildRats.htm

They each have their own nest chamber, but they may share the burrow and may raise their young together (called communal nesting). When the offspring are weaned, the young males disperse.

The organization of male rats and the rat mating system changes depending on the population density of the colony. At low densities, a male rat monopolizes a burrow of females. He defends a territory, keeping other males away from the burrow and the surrounding area, and he mates only with the females of his group. One male mating with multiple females is a polygynous mating system. At low densities, Norway rats are therefore territorial and polygynous.

At high densities, males can no longer defend a burrow against intruders. There are simply too many intruders. At high densities, the social system becomes despotic, with one male becoming socially dominant while others become socially subordinate. Males no longer defend female burrows. Instead, when a female comes into heat a group of males rushes her and mates with her sequentially, with little or no competition between themselves. Males may mate with multiple estrus females this way, and females mate with multiple males. This mating system is polygynandrous. At high densities, Norway rats are therefore despotic and polygynandrous.
Hope that helps.