Hello Everyone

I've been reading a book about safari.
There's one sentence I can figure out what it mean.
"Some monkeys call out "Kwo!" as if to show how funny theey thought I was, or maybe to point out the silver trail on my chin."

I googled, googled, and googled, but i still can't find out what "silver trail on my chin" mean.

anyone got a clue?
plz help me.
thanks!