Most mammalian vaginas are more or less the same in overall configuration: they basically comprise a smoothish lubricated channel to allow penile insertion and subsequent parturition. Some vaginas are however exceptional. One such is the vagina in the female hippopotamus, coming equipped as it does with 10 to 19 transverse interlocking fibrous ridges. The ridges at the upper end of the vagina are the most pronounced, resembling heavy corrugations. Similar, though less pronounced, corrugations are discernible in the vagina of pig and warthog. In all cases the function of the ridges is a mystery.