Military sop tattoos?

My dad was in the marines, and also in some sort of special forces unit (he wont discuss it) he has 2 tattoos, one is a skull with a snake wrapping around it and the other is a skunk (smelling) a rose. He will not say what they "mean" but I've been told that they symbolize a certain group in a "black ops" unit. does anyone know the meaning of these tattoos?