The symbol of the Democratic Party is the donkey and the symbol of the Republican Party is the elephant why do they have these symbols and when did this tradition first begin?

The donkey started when Andrew Jackson was running for President in 1882. All of the campainers called hm a jackass (a donkey) so he used that as his symbol. Later a man used that in a cartoon and that is what started the Democratic Symbol.
The symbols were invented by Thomas Nast, the 19th century's most persuasive political cartoonist. Why? Only Thomas Nast seems to know.