You pick them up. The skulls can promote life or ammo. They can also make it harder to hurt enemies but if you kill the enemies with the skull you can unlock glitches and other weird stuff.
Usually the term Harley skull refers to any skull used in a Harley-Davidson design that features a skull....Several of these designs were so popular that any similar skull used with out a H-D name...
A skull acts as armor to protect the brain. While the skull is capable of fractures and breakage, it protects the brain floating in Cerebrospinal fluid. However, because the brain is floating in this...