The bioadhesive glues used by mammals, plants, and mussels for adherance to mucosal surfaces (mucoadhesion) are made up of sticky proteins.
"Bioadhesion may be defined as the state in which two materials, at least one of which is biological in nature, are held together for extended periods of time by interfacial forces. In the pharmaceutical sciences, when the adhesive attachment is to mucus or a mucous membrane, the phenomenon is referred to as mucoadhesion." (Smart 2005:1557)
Learn more about this functional adaptation.