Review of wallowing in pigs: description of the behaviour and its motivational basis.
Wallowing, i.e. coating the body surface with mud, is a natural behaviour of pigs, commonly observed in feral pigs and wild boar, but rarely provided for in current housing systems for domestic pigs. Furthermore, in welfare science the subject has not been receiving much attention. This paper reviews wallowing in pigs and related species. The behaviour is described and its motivational basis is examined. Underlying the review was a literature search for scientific citations. In total 48 papers were identified containing citations about wallowing behaviour in pigs and wild boar, and 12 papers contained citations about wallowing in related species. Wallowing is observed in many related species including rhino's, elephants, bovids (e.g. American bison) and deer. Pigs also share several taxonomic characteristics with water-loving mammals such as water buffalo's, hippo's and whales. The common perception is that pigs wallow mainly for cooling, sunburn protection and the removal of ecto-parasites. Little scientific evidence exists for other functions than thermoregulation. Pigs lack functional sweat glands and wallowing in mud is an effective behavioural control mechanism in pigs to prevent hyperthermia. Wallowing, however, may also serve other functions, e.g. in scent-marking and sexual behaviour. In addition, wallowing in pigs, like dustbathing in poultry, may be indicative of positive welfare and, perhaps, the performance of the behaviour is 'hardwired' and rewarding in itself. If so, wallowing could be an important element of a good life in pigs.