Canvas models, about the size of a pig's tail, were impregnated with pigs' blood or left plain, and were presented to pigs for 12 days in a 2-choice preference test. The pigs showed large, consistent, individual differences in response: some pigs chewed the models continuously while others chewed only slightly; some chewed much more on the blood-covered model, while others showed no preference. On average, the pigs chewed considerably more on the blood-covered model than on the plain one. In a second experiment, pigs presented with a choice test involving a blood-covered and a plain model showed a significant increase in chewing over a baseline level seen with plain models only. It is suggested that this strong but highly variable response to blood could explain how a relatively minor tail injury can stimulate a large but unpredictable increase in tail-biting among pigs.
Fraser, D. (1987). Attraction to blood as a factor in tail-biting by pigs. Applied Animal Behaviour Science, 17(1-2), 61-68.