I think some of mine are using the birth tie to protect themselves; essentially, they try to reproduce every turn, but can't if they don't have two body or if they already have something right in front of them. So, by keeping your child, and your child's child right next to you in a chain for a while, you can build up around 10 body, and actually stockpile enough resources to survive a volley of stray shots. Also, that allows the bot to have somewhat more powerful shots, yet still be able to reproduce once the birthtie is gone. Apparently, evolution favors NOT severing the birth tie. And, of course, they don't shoot their children since they move backwards and fire backwards, yet reproduce forwards.
Also, I use fixed bot radii.