Here is the public protocol the mice and cheese use. It is subject to change at any time!
http://www.lilmikey.com/slwiki/index.asp?db=WikiData&o=EcosystemProject
If you want to make a predator you should send the mice a "health+//-25" message. That would cause the mouse to lose 25 health.
If you want to make something that gets eaten, you should respond to the "eat" message with a "health+//25" message. This would cause whatever eat you to gain 25 health.
I'm looking forward to see some new critters out there.