I have heard that the markers represent the traits of animals, I have also heard that it has to do with where they give birth, then I have heard the ducks are related to the flock in christian ideas, then there is the idea that the owl has to do with the Greek gods, also in Christian ideals the markers are relating to Jesus Christ and the resurrection, and of course the Bible scriptures relating to the heart etc.
I believe that the only way to know which of these (if any) are true is to know how old the markers are and who originally made the markers that lead to caches, but here is the problem, the Japanese used many of the same markers in the Philippines and the Spanish used the markers out here, so if this is an ancient system, who really started it in the first place would really go along ways towards understanding their belief systems and why certain markers are used to go to caches, especially the turtles, ducks, owls and hearts as they are absolutely used to mark treasure caches.
I do know the Spanish made many of the markers you see in the Southwest that lead to caches however they could very well have just used a much older system in which case it totally changes the meaning of the markers from a Christian base to something else entirely, by the way it is my belief that the turtle itself is not really what matters when it comes to treasure it's what's on its shell that matters, so when you see a turtle with a Heart for a shell it is going to a treasure.