Hedgehogs do a strange/funny thing called, annointing. This is a behavior said to be done to protect themself from predators by 'mixing' their secnt with the scent they are smelling. When a hedgie smells something new & or interesting to them they lick or chew that area that they smell the scent on.  After doing this they create a frothy foam/slobber that they then spread ALL over their shoulders & backs. This is said to help them 'blend' in.*shrugs*  It's very strange to say the least!   

They go CRAZY when annointing & will tip over every which way in order to spread the froth all over themselves. They have been known to still annoint while tipped over onto their sides & or backs! =]

When you first see your hedge doing this you will probly be freaked out like, 'what is it doing?!' hehe but it is normal, let them go to town its not going to hurt anything. (unless they are chewing something unsafe, of course.)

Some people have claimed to be allergic to them after annointing, but I'm not sure if that is ture or not? I wouldn't think it would be very common if it is.

Here are a few photos of self Annointing I have gotten!: