I think the developers understood that two animals were not enough for the game to have a longer life span, or maybe they had it all planned out... Who knows?

The only thing I know is that my copy of the game is taken off the shelf more often because of this expansion. The new yet non-complex little mechanics of these animals became the reason why my group would play this game often.

That said, I'm not too sure if this expansion increases the replay-ability part of the spectrum. There still really isn't much of a reason for you to go back to won animal scenarios unless a new player arrives, or maybe a certain scenario really stuck with you.

Overall, I would say this expansion is undoubtedly an upgrade through and through, and is worth it if you liked the taste of the base game and want other animals to save.