Ever since the 19th century, when disease was first linked to sewage-contaminated water, humans have gone to great lengths to escape their own filth. Meanwhile, animals have gone on revelling in the stuff - eating it, strategically dropping it, flinging it around just to pass the time, and so on. Same goes for mud, piss, vomit, blood and rotting carcasses of every make and vintage. Most creatures just don't have our hang-ups.