It all depends on how you shape your world. In my world there are two types of "shape changers"; one is shifters, and these are loosely based on the genetic muckification caused (in my world) by all the grafting and such (yes, there are long term effects in my world from using fish genes in tomatoes). In these cases, the change is DNA based, and not magical ('cause it's my rules) and always (though not always obviously) traces back to something to do with heritage (i.e. Brits turn to lions, Native Americans per their totem, Poles to eagles and so on). These shapes have no superpowers, nor catch. Just a version of the animal as an animal.
Then there is the magical/catch-ridden version, which means you have to have been bitten (or whatever).
dian