Why a tonsure?

Wikipedia claims the one we think of on monks came from Saint Peter. Other theories:

“This practice is not improbably connected with the idea that long hair is the mark of a freeman, while the shaven head marks the slave (in the religious sense: a servant of God). Other theories are that the tonsure mimics male pattern baldness in an attempt to lend artificial respectability to men too young to display the real thing, or that the tonsure is a ritual created by balding superiors in act of vanity and power over young non-bald subordinates.”

So much better than a tonnot.

Maybe it was an attempt to mimic a crown. Like the crown Jesus Christ wore on the crucifiction.