How Does a Small Animal Swap Work?

My Honey and I were driving to Midland over the weekend. On M-46, as you're leaving St. Louis, there's this sign. I've driven by this sign for over 20 years in the trek back and forth between Lansing and Midland and this message appears at least once a year.

Do you suppose you could swap a guinea pig for a hamster? A bunny for a Chincilla? How does this work?