Board Thread:Naruto Discussions/@comment-24847491-20140921145042/@comment-1512913-20140921182512

Yes, I believe the physical differences are more of a "symbolic" or "artistic" representation of from whom did the character learn Senjutsu. It's simply a way to tell: "oh, this guy learned it at frog mountain or snake cave" by looking at their eyes.

My theory is that Hashirama was a self-taught Sage Mode user, so no animal training stuff.