Why does Twain use a frog instead of another animal in his story "The Celebrated Jumping Frog of Calaveras County"?

Twain probably had several good reasons for using a frog
as the title character of his great short story. One point he was trying to make in the
story was that "thish-yer Smiley" would bet on absolutely anything, including any animal
contest imaginable. Unlike horses, dogs or other larger animals that might be likely to
compete in a race, a contest between frogs could be held in a small, confined area, not
unlike a poker game. Frogs were also in abundance in the nearby swamp, so they were a
ready commodity. But the choice of a frog probably better illustrates the comic
absurdity of Smiley's addiction, not to mention that its small size better allows for
the twist of the con--pouring bird-shot into Dan'l,'s belly to weight it
down.