I think they just peered over the edge and were convinced when they saw sphincter after sphincter.

I’d say the silliness seen here goes hand-in-hand with curiosity and intelligence, which makes them clever and effective hunters. This type of derpiness also earns them a rent free spot in monkey houses. So, dumb as it may seem, this is actually a very fit behavior.

It’s good to question motives but in this case I get the feeling they’re the good guys.