When Marty was born it was custom in his tribe on every 5th generation baby born, the baby be immediatly shoved up an asshole upon birth.it is said that doing so would rid off any future polops that could form in an asshole.
Well Marty was born a 5th generation baby and was shoved deep up inside an asshole upon birth.
Whether or not the theory worked to rid off dangerous polops worked still remains a mystery.
I beleive legend has it that Marty was shoved up every asshole that was in his tribe, somewhere around 35 not including all the horses assholes which would add up to about 30 horses.see this is why Marty's head is shaped like it fits in an asshole.
On Marty's 13th birthday it was customed for him to eat Buffalo turds, Marty did so so and did sure enjoy it.