My tests in the arena yesterday: five cats versus five chickens (cats win), five cats versus five raccoons (raccoons win), five raccoons versus five peahens (raccoons win), five raccoons versus one alligator (gator wins). It's an invaluable little feature because occasionally a bunch of billy goats or raccoons will come traipsing by your fortress and you're so low on food that the dwarfs are looking at their pets hungrily with knife and fork in hand....

It's the original Sim City meets Rogue meets Fallout, kinda.