Last night I caught another guest speaker, who is actually my biological anthropology teacher. The lecture was on adaptive behavior in capuchin monkeys, and in particular their unique habit of washing themselves. In most cases a group of capuchins will look out for piper leaves, which they gather together and chew into a gooey paste. This paste is then rubbed all over their bodies for either therapeutic purposes, to remove bacteria and pathogens, or possibly both. Capuchins have also been observed using millipede species for the same cleaning purposes. A group of capuchins in eastern Ecuador has been cohabiting with humans in a local town for the past 35 years (the only such occurance recorded in the Americas), and over the summer my instructor - a Ph.D student at the university - travelled through the town and found the capuchins out in the rain, washing themselves with soap. After talking with the town's residents he found that the monkeys have a habit of running into stores to gather (or, from a human perspective, "steal") soap, and in most every instance they'll target soap over other products like food during their forays indoors.
Nobody knows why the capuchins do it, but it's suspected that further research might explain the behavior and eventually show evolutionary connections to our own custom of washing as often as we do.