Board Thread:Naruto Discussions/@comment-28861328-20140618060932/@comment-7862945-20140628112705

But it was during the clashes between Hashirama and Madara that probably made the difference (though this is just opinion)... Your right that the Senju were superior to Uchiha, but I think that might have been because of the difference between the aces, Madara and Hashirama (and seeing your ace get his ass kicked would definitely drop moral).....

Also, when did the Uzumaki and other clans take on surnames to hide themselves?

And since Mito married into the Senju clan, Tsunade is a part of the Senju clan, though she is Uzumaki decent.

Names are things that Kishi didn't give us a lot of info on & we kinda got to make up our own theories.