Wallowing in animals is comfort behaviour during which an animal rolls about or lies in mud, water or snow. Some definitions include rolling about in dust, however, in ethology this is usually referred to as dust bathing. Wallowing is often combined with other behaviours to fulfill its purpose; for example, elephants will often blow dirt over themselves after wallowing to create a thicker "coating", or pigs will allow the mud to dry before rubbing themselves on a tree or rock to remove ectoparasites stuck in the mud.