I came across a nature story over at damninteresting.com, and I felt actual sympathetic pain while I read it.
Briefly, there is this tiny toothpick-sized carnivorous fish in the Amazon River whose attack method is horrifying. It tastes, and follows, the urea stream found in the waste of a larger fish breathing out through it's gills. It strikes with lightening quickness, burying itself in the urea source, where it sticks out sharp anchoring prongs and then proceeds to bite into the wall of an artery or vein and gorges itself on the blood of the host animal.
Given that humans expel their urea through liquid waste, imagine you are a native, swimming in the Amazon and you need to tinkle....I'll let your imagination take it from there.