While "cleanliness" is a human concept, the pig is often cited by biologists as one of the cleanest animals, despite their reputation. Pigs do not have functional sweat glands, so they roll in mud to regulate their body temperature and protect their skin from parasites and sunburn; once the mud dries and falls off, they are remarkably clean. Furthermore, pigs are one of the few farm animals that refuse to "soil" the area where they sleep or eat if given enough space. Another top contender is the cat, which spends up to 50% of its waking hours self-grooming with a tongue covered in tiny papillae that act like a natural comb and antiseptic. In the wild, the songbird is exceptionally clean, frequently engaging in water baths and "anting" (rubbing ants on feathers) to use formic acid as a natural pesticide. However, if we define cleanliness by the lack of bacteria, the polar bear is a strong candidate because the extreme Arctic cold and their constant snow-grooming keep their fur nearly sterile. Ultimately, the cat remains the most "visibly" clean to humans due to its obsessive daily hygiene rituals.