STRINGS.CODEX.CRITTERSTATUS.FERTILITY.FERTILITYRATE.CONTAINER1
Each time a critter completes their reproduction cycle (i.e. at 100 percent), it lays an egg and restarts its cycle.
A critter's environment greatly impacts its base reproduction rate. When a critter is feeling cramped, it will wait until all eggs in the room have hatched or been removed before laying any of its own.
Critters will also stop reproducing when they feel confined, which happens when their space is too small or they are stuck in a door or tile.
Mood and domestication also impact reproduction: happy critters reproduce more regularly, and happy tame critters reproduce the fastest.