Ha! So when adult critters have taken care of their hunger, they'll seek out mates. If you've got one selected and it's mood is "Romantic" that's what's going on. If two romantic critters see each other, they'll move into each other and a new critter will be spawned near them, taking one chromosome from each parent, with random mutations on each gene mediated by the terrarium's mutation rate.
By making lots of bushes, you gave them lots of food so it was easy for them to be well fed enough to turn romantic.