Why does tail biting occur? An evolutionary perspective

Tail biting, tail docking and the improper provision of enrichment materials are interrelated and are a major welfare concern in modern pig farming. Many other authors have reviewed the multifactorial causes of tail biting. Here, instead our focus is on the evolutionary origins of the problem. We discuss possible phylogenetic and functional explanations of tail biting in pigs. Tail biting is not a natural behaviour, not being seen in wild or feral pigs. Instead, it is best characterised as abnormal behaviour or a psychopathology, resulting from the thwarting of a motivated source behaviour. The most likely source behaviour is foraging, but there may be a role for other source behaviours which contribute to the expression of tail biting, including resource competition, thermoregulation, sickness behaviour and cannibalism. Since cannibalism is remarkably widespread throughout the animal kingdom, this touches on the question as to whether tail biting may in some sense not be as abnormal as it is generally perceived. Though much of our discussion is rather speculative in nature we believe it is worthwhile exploring hypotheses around the evolutionary origin of tail biting in pigs, and other welfare issues in livestock farming, in more detail in the future. Such an approach can encourage a re-examination of assumptions and generate new hypotheses.