donkeys and elephants

Why is the Democratic symbol a donkey? Why are Republicans elephants? Why did they switch underwear in this picture? I have my own theories (on the symbolism, not the underwear)...but I thought a lesson in history might serve this Canadian girl well. For those of you who may also need a refresher course on animal symbolism... the buck stops here. Pun intended.

http://www.c-span.org/questions/week174.htm